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Chemical Reaction and Equation: Class 10 Science answers, notes
Get summaries, questions, answers, solutions, notes, extras, PDF and guides for Chapter 1 Chemical Reaction and Equation: Class 10 Science textbook, which is part of the syllabus for students studying under SEBA (Assam Board), NBSE (Nagaland Board), TBSE (Tripura Board), CBSE (Central Board), MBOSE (Meghalaya Board), BSEM (Manipur Board), WBBSE (West Bengal Board), and all other boards following the NCERT books. These solutions, however, should only be treated as references and can be modified/changed.
If you notice any errors in the notes, please mention them in the comments
Summary
Chemical reactions occur when substances change to form new ones, like milk souring or iron rusting. We use chemical equations to show these changes. An equation lists reactants, which are the starting substances, and products, which are the new substances formed. An arrow in the equation points from the reactants to the products, showing the direction of the change. For example, when magnesium metal burns in air, it combines with oxygen to create a white powder called magnesium oxide.
Chemical equations must be balanced. This means the number of tiny particles, called atoms, for each element must be the same on both the reactant side and the product side of the arrow. Atoms are not lost or gained during a reaction; they are simply rearranged. We balance equations by placing numbers in front of the chemical formulas of the substances. Special symbols can also be used to show if a substance is a solid, a liquid, a gas, or dissolved in water. Sometimes, conditions needed for the reaction, like heat or light, are written above or below the arrow.
There are several types of chemical reactions. In a combination reaction, two or more simple substances join together to make one more complex product. Some reactions release heat, making things around them warm; these are called exothermic reactions. Burning magnesium is an example of this. A decomposition reaction is the opposite: one complex substance breaks down into two or more simpler ones. These reactions often need energy, such as heat, light, or electricity, to happen. Reactions that absorb heat and make their surroundings feel cold are called endothermic reactions.
In a displacement reaction, a more active element takes the place of a less active element in a compound. For instance, if you put an iron nail into a blue copper sulphate solution, the iron pushes out the copper, and the solution’s color changes. In a double displacement reaction, parts of two different compounds swap with each other, like partners changing in a dance. Often, one of the new compounds formed in such a reaction is a solid that does not dissolve in water; this solid is called a precipitate.
Oxidation happens when a substance gains oxygen or loses hydrogen during a reaction. Reduction is the opposite: it occurs when a substance loses oxygen or gains hydrogen. These two processes, oxidation and reduction, always happen together in what is called a redox reaction. One substance gets oxidised while another gets reduced. Oxidation also affects us in daily life. Corrosion is when metals get damaged by reacting with substances in their environment, like air and moisture, causing iron to rust, for example. Rancidity is when fats and oils in food spoil due to oxidation, which changes their smell and taste. Keeping food in airtight containers or using gases like nitrogen in chip bags helps to prevent this.
Textbook solutions
Intext Questions and Answers I
1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
2. Write the balanced equation for the following chemical reactions.
(I) Hydrogen + Chlorine → Hydrogen chloride
Answer: H₂(g) + Cl₂(g) → 2HCl(g)
(II) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
Answer: 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(ii) Sodium + Water Sodium hydroxide + Hydrogen
Answer: 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)
3. Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
Answer: The balanced chemical equation with state symbols is: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) Sodium hydroxide solution in water reacts with hydrochloric acid solution in water to produce sodium chloride solution and water.
Answer: The balanced chemical equation with state symbols is: NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Intext Questions and Answers II
1. A solution of a substance X is used for whitewashing
i. Name the substance ‘X’ and write its formula.
Answer: The substance ‘X’ used for whitewashing is calcium oxide, and its formula is CaO.
ii. Write the reaction of the substance ‘X’ named in (i) above with water.
Answer: The reaction of the substance ‘X’, calcium oxide, with water is that calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat. The chemical equation for this reaction is CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.
Answer: The activity 1.7 is known as electrolysis of water. Water (H₂O) is a molecule made up of two hydrogen atoms and one oxygen atom. Electrolysis uses electricity to decompose water into its constituent elements, hydrogen and oxygen. Since water has a 2:1 ratio of hydrogen to oxygen, the electrolysis process will produce hydrogen and oxygen in the same 2:1 ratio. Therefore, the test tube collecting hydrogen gas will have twice the volume of gas compared to the test tube collecting oxygen gas.
The gas collected in double volume is Hydrogen.
Intext Questions and Answers III
1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Answer: The colour of copper sulphate solution changes when an iron nail is dipped in it because a chemical reaction takes place where iron displaces or removes another element, copper, from the copper sulphate solution. This reaction is known as a displacement reaction.
2. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Answer: An example of a double displacement reaction other than the one given in Activity 1.10 is the reaction between potassium bromide solution and barium iodide solution which produces potassium iodide solution and a precipitate of barium bromide. The balanced chemical equation for this reaction is 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s).
3. Identify the substances that are oxidised and the substances that are reduced in the following reactions.
Answer: In the given reactions, one can identify the substances that are oxidised and the substances that are reduced.
(i) 4Na(s) + O2(g) → 2Na2O(s)
(i) In the reaction 4Na(s) + O₂(g) → 2Na₂O(s), sodium is oxidised because it gains oxygen, and oxygen is reduced.
(ii) CuO(s) + H2(g) → Cu(s) + H2O(l)
(ii) In the reaction CuO(s) + H₂(g) → Cu(s) + H₂O(l), copper(II) oxide is reduced because it loses oxygen, and hydrogen is oxidised because it gains oxygen.
Exercise Questions and Answers
1. Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(ii) (a), (b) and (c)
(iv) all
Answer: (i) (a) and (b)
2. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe
The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.
Answer: (d) displacement reaction.
3. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.
Answer: (a) Hydrogen gas and iron chloride are produced.
4. What is a balanced chemical equation? Why should chemical equations be balanced?
Answer: A balanced chemical equation is a chemical equation in which the number of atoms of each element is equal on both the reactant and product side of the equation.
Chemical equations should be balanced because of the law of conservation of mass, which states that mass can neither be created nor destroyed in a chemical reaction. This means the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
5. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulpur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
Answer: (a) The balanced chemical equation is:
3H₂(g) + N₂(g) → 2NH₃(g)
(b) The balanced chemical equation is:
2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) The balanced chemical equation is:
3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) The balanced chemical equation is:
2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)
6. Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl
Answer: (a) The balanced chemical equation is:
2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) The balanced chemical equation is:
2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) The balanced chemical equation is:
NaCl + AgNO₃ → AgCl + NaNO₃
(This equation is already balanced.)
(d) The balanced chemical equation is:
BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl
7. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
Answer: (a) The balanced chemical equation is:
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
(b) The balanced chemical equation is:
Zn(s) + 2AgNO₃(aq) → Zn(NO₃)₂(aq) + 2Ag(s)
(c) The balanced chemical equation is:
2Al(s) + 3CuCl₂(aq) → 2AlCl₃(aq) + 3Cu(s)
(d) The balanced chemical equation is:
BaCl₂(aq) + K₂SO₄(aq) → BaSO₄(s) + 2KCl(aq)
8. Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate(s) → Zinc oxide(s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
Answer: (a) The balanced chemical equation is:
2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(s)
Type of reaction: This is a double displacement reaction (and also a precipitation reaction as BaBr₂(s) is formed). There is an exchange of ions between the reactants.
(b) The balanced chemical equation is:
ZnCO₃(s) → ZnO(s) + CO₂(g)
Type of reaction: This is a decomposition reaction. A single reactant, zinc carbonate, breaks down to give simpler products, zinc oxide and carbon dioxide.
(c) The balanced chemical equation is:
H₂(g) + Cl₂(g) → 2HCl(g)
Type of reaction: This is a combination reaction. Two substances, hydrogen and chlorine, combine to form a single product, hydrogen chloride.
(d) The balanced chemical equation is:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
Type of reaction: This is a displacement reaction. Magnesium displaces hydrogen from hydrochloric acid.
9. What does one mean by exothermic and endothermic reactions? Give examples.
Answer: Reactions in which heat is released along with the formation of products are called exothermic chemical reactions. An example of an exothermic reaction is the reaction of calcium oxide with water to produce slaked lime, where a large amount of heat is evolved. Other examples include the burning of natural gas and respiration.
Reactions in which energy is absorbed are known as endothermic reactions. Decomposition reactions, such as the decomposition of ferrous sulphate or lead nitrate upon heating, require energy in the form of heat to break down the reactants, and are examples of endothermic reactions. Other decomposition reactions may require energy in the form of light or electricity, like the decomposition of silver chloride by sunlight or the electrolysis of water.
10. Why is respiration considered an exothermic reaction? Explain.
Answer: Respiration is considered an exothermic reaction because it is a process that provides energy, which is needed for us to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances like glucose. This glucose combines with oxygen in the cells of our body and provides energy. Since energy is released during this process, respiration is an exothermic reaction.
11. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: Decomposition reactions are called the opposite of combination reactions because they involve the breaking down of a single reactant into two or more simpler products, whereas combination reactions involve two or more substances (elements or compounds) combining to form a single product. In a combination reaction, two or more reactants combine to form a new single substance. In a decomposition reaction, a single substance decomposes to give two or more substances. For example, a combination reaction can be represented by the equation: CaO(s) + H₂O(l) → Ca(OH)₂(aq). A decomposition reaction can be represented by the equation: CaCO₃(s) Heat→ CaO(s) + CO₂(g).
12. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
Answer: Decomposition reactions require energy either in the form of heat, light, or electricity for breaking down the reactants. For example, when calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide; this is a thermal decomposition reaction where energy is supplied by heat, represented by the equation CaCO₃(s)
→Heat CaO(s) + CO₂(g). Silver chloride decomposes into silver and chlorine when exposed to sunlight; this decomposition reaction is caused by light, represented by the equation 2AgCl(s) →Sunlight 2Ag(s) + Cl₂(g). Water can be decomposed into hydrogen and oxygen gases when electricity is passed through it, as demonstrated in the electrolysis of water; this reaction uses electrical energy, represented by the equation 2H₂O(l)
→Electricity 2H₂(g) + O₂(g)
13. What Is the difference between displacement and double displacement reactions? Write equations for these reactions.
Answer: The difference between displacement and double displacement reactions is explained. In a displacement reaction, an element displaces another element from its compound. An equation for this reaction is given as Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). In contrast, in double displacement reactions, two different atoms or groups of atoms, which are ions, are exchanged between the reactants. An equation for a double displacement reaction is Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).
14. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
Answer: The reaction involved in the recovery of silver from silver nitrate solution by displacement by copper metal is copper reacting with silver nitrate solution to form copper nitrate solution and silver. The balanced chemical equation for this reaction is Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s).
15. What do you mean by a precipitation reaction? Explain by giving examples.
Answer: A white substance, which is insoluble in water, formed during a reaction is known as a precipitate. Any reaction that produces a precipitate can be called a precipitation reaction. An example is the reaction between sodium sulphate solution and barium chloride solution, where barium sulphate is formed as a white precipitate: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq).
16. Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction.
Answer: (a) Oxidation: If a substance gains oxygen during a reaction, it is said to be oxidised. For example, in the reaction 2Cu + O₂ → 2CuO, copper gains oxygen to form copper(II) oxide. In the reaction ZnO + C → Zn + CO, carbon gains oxygen to form carbon monoxide.
(b) Reduction: If a substance loses oxygen during a reaction, it is said to be reduced. For example, in the reaction CuO + H₂ → Cu + H₂O, copper(II) oxide loses oxygen to form copper. In the reaction ZnO + C → Zn + CO, zinc oxide loses oxygen to form zinc.
17. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Answer: The shiny brown coloured element ‘X’ is copper. The black coloured compound formed on heating copper in air is copper(II) oxide.
18. Why do we apply paint on iron articles?
Answer: Iron articles are painted because corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Painting helps to prevent this corrosion, which is a serious problem leading to significant costs for replacing damaged iron.
19. Oil and fat containing food items are flushed with nitrogen. Why?
Answer: Oil and fat containing food items are flushed with nitrogen because when fats and oils are oxidised, they become rancid and their smell and taste change. Nitrogen is used as a gas to prevent the chips from getting oxidised, thereby preventing rancidity.
20. Explain the following terms with one example each. (a) Corrosion (b) Rancidity.
Answer: (a) Corrosion is the process when a metal is attacked by substances around it such as moisture, acids, etc. An example is the rusting of iron, where iron articles get coated with a reddish brown powder when left for some time.
(b) Rancidity occurs when fats and oils are oxidised, causing their smell and taste to change. This happens, for example, with oil and fat containing food materials left for a long time.
Extras
Additional MCQs (Knowledge Based)
1. What is the chemical formula for quicklime?
A. Ca(OH)₂
B. CaO
C. CaCO₃
D. CaCl₂
Answer: B. CaO
2. The process of a substance gaining oxygen during a reaction is known as ________.
A. Reduction
B. Displacement
C. Oxidation
D. Decomposition
Answer: C. Oxidation
3. Which gas is typically produced when a dilute acid reacts with zinc granules?
A. Oxygen
B. Carbon dioxide
C. Hydrogen
D. Nitrogen
Answer: C. Hydrogen
4. A solution of substance ‘X’ is used for whitewashing walls. Identify ‘X’.
A. Calcium carbonate
B. Calcium oxide
C. Calcium hydroxide
D. Calcium sulphate
Answer: B. Calcium oxide
5. Which of the following is NOT a common indicator that a chemical reaction has occurred?
A. Change in colour
B. Change in mass
C. Evolution of a gas
D. Change in temperature
Answer: B. Change in mass
6. What is the colour of the fumes produced when lead nitrate is heated?
A. Colourless
B. Green
C. Brown
D. White
Answer: C. Brown
7. When an iron nail is placed in a copper sulphate solution, which observation is correct?
A. Solution turns green, nail remains unchanged.
B. Solution colour fades, brown deposit on nail.
C. No reaction occurs.
D. Solution turns red, nail dissolves.
Answer: B. Solution colour fades, brown deposit on nail.
8. Combination Reaction: Two or more reactants form a single product :: Decomposition Reaction: ________.
A. Two reactants exchange ions
B. A single reactant breaks into simpler products
C. One element displaces another
D. Heat is evolved
Answer: B. A single reactant breaks into simpler products
9. What is the solid product formed when calcium oxide reacts vigorously with water?
A. Calcium carbonate
B. Calcium chloride
C. Slaked lime
D. Quicklime
Answer: C. Slaked lime
10. Identify the characteristics of the reaction where magnesium ribbon is burnt in air:
P. It forms magnesium sulphate.
Q. It burns with a dazzling white flame.
R. A black powder is formed.
S. It is an oxidation reaction.
A. P and R
B. Q and S
C. P and Q
D. R and S
Answer: B. Q and S
11. The chemical formula of rust is primarily considered as hydrated:
A. Ferrous oxide
B. Ferric sulphate
C. Ferric oxide
D. Ferrous carbonate
Answer: C. Ferric oxide
12. Reactions in which heat is released along with the formation of products are called ________ reactions.
A. Endothermic
B. Exothermic
C. Displacement
D. Decomposition
Answer: B. Exothermic
13. In the electrolysis of water, which gas is collected in a larger volume?
A. Oxygen
B. Nitrogen
C. Hydrogen
D. Chlorine
Answer: C. Hydrogen
14. Identify the odd one out in terms of energy change during the reaction.
A. Respiration
B. Burning of natural gas
C. Decomposition of silver chloride by light
D. Reaction of quicklime with water
Answer: C. Decomposition of silver chloride by light
15. What is the notation used to represent a substance dissolved in water in a chemical equation?
A. (s)
B. (l)
C. (g)
D. (aq)
Answer: D. (aq)
16. What type of reaction occurs when silver bromide is exposed to sunlight?
A. Combination
B. Displacement
C. Photolytic decomposition
D. Double displacement
Answer: C. Photolytic decomposition
17. The reaction 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) is an example of:
A. Combination
B. Thermal decomposition
C. Displacement
D. Redox
Answer: B. Thermal decomposition
18. The substances that undergo chemical change in a reaction are known as ________.
A. Products
B. Catalysts
C. Reactants
D. Intermediates
Answer: C. Reactants
19. What is the colour of copper(II) oxide?
A. Green
B. Blue
C. Reddish-brown
D. Black
Answer: D. Black
20. Which of the following are examples of redox reactions?
P. CuO + H₂ → Cu + H₂O
Q. CaO + H₂O → Ca(OH)₂
R. ZnO + C → Zn + CO
S. Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
A. P and Q
B. Q and S
C. P and R
D. R and S
Answer: C. P and R
21. What gas is used to flush bags of chips to prevent rancidity?
A. Oxygen
B. Nitrogen
C. Carbon dioxide
D. Hydrogen
Answer: B. Nitrogen
22. A reaction in which an insoluble solid is formed is called a ________ reaction.
A. Combination
B. Decomposition
C. Precipitation
D. Displacement
Answer: C. Precipitation
23. What is the chemical name for CaCO₃, which gives a shiny finish to walls after whitewashing?
A. Calcium oxide
B. Calcium hydroxide
C. Calcium carbonate
D. Calcium sulphate
Answer: C. Calcium carbonate
24. Which of the following is NOT typically required for a decomposition reaction to occur?
A. Heat
B. Light
C. Catalyst
D. Electricity
Answer: C. Catalyst
25. The process of iron articles getting coated with a reddish-brown powder is known as:
A. Galvanization
B. Rancidity
C. Rusting
D. Alloying
Answer: C. Rusting
26. Gain of oxygen: Oxidation :: Loss of oxygen: ________.
A. Combination
B. Reduction
C. Displacement
D. Precipitation
Answer: B. Reduction
27. What is the approximate ratio of hydrogen to oxygen gas produced during the electrolysis of water?
A. 1:1
B. 1:2
C. 2:1
D. 3:1
Answer: C. 2:1
28. A student observes that a green coloured crystalline substance changes to a reddish-brown solid upon heating, along with the evolution of gases smelling like burning sulphur. The green substance is likely:
A. Copper sulphate
B. Ferrous sulphate
C. Lead nitrate
D. Magnesium carbonate
Answer: B. Ferrous sulphate
29. The reaction Zn + H₂SO₄ → ZnSO₄ + H₂ is an example of which type of equation if all atoms are counted?
A. Skeletal
B. Unbalanced
C. Balanced
D. Word
Answer: C. Balanced
30. The method of balancing chemical equations by making trials to equalize atoms using the smallest whole number coefficients is called the ________ method.
A. Algebraic
B. Oxidation number
C. Ion-electron
D. Hit-and-trial
Answer: D. Hit-and-trial
31. What is the physical state of water when it is indicated as H₂O(g) in a chemical reaction?
A. Liquid
B. Solid (ice)
C. Steam (gas)
D. Aqueous solution
Answer: C. Steam (gas)
32. When carbon dioxide gas is passed through lime water (calcium hydroxide solution), what is the insoluble substance formed?
A. Calcium oxide
B. Calcium bicarbonate
C. Calcium carbonate
D. Calcium chloride
Answer: C. Calcium carbonate
33. Which metal is more reactive than copper and can displace it from its salt solution?
A. Silver
B. Gold
C. Zinc
D. Mercury
Answer: C. Zinc
34. Identify the reaction that is NOT a combination reaction.
A. C(s) + O₂(g) → CO₂(g)
B. 2H₂(g) + O₂(g) → 2H₂O(l)
C. CaO(s) + H₂O(l) → Ca(OH)₂(aq)
D. CaCO₃(s) → CaO(s) + CO₂(g)
Answer: D. CaCO₃(s) → CaO(s) + CO₂(g)
35. What is the common name for CaO?
A. Slaked lime
B. Limestone
C. Quicklime
D. Marble
Answer: C. Quicklime
36. Reactions in which energy is absorbed are known as ________ reactions.
A. Exothermic
B. Combustion
C. Endothermic
D. Neutralization
Answer: C. Endothermic
37. The black coating formed on silver articles over time is an example of:
A. Rancidity
B. Alloying
C. Corrosion
D. Electroplating
Answer: C. Corrosion
38. A reaction where an element displaces another element from its compound is known as:
A. Decomposition
B. Double displacement
C. Displacement
D. Combination
Answer: C. Displacement
39. What is the colour of silver chloride (AgCl) before it is exposed to sunlight?
A. Grey
B. Black
C. White
D. Yellow
Answer: C. White
40. Which of the following is NOT a product when lead nitrate (Pb(NO₃)₂) is heated strongly?
A. Lead oxide (PbO)
B. Nitrogen dioxide (NO₂)
C. Oxygen (O₂)
D. Lead metal (Pb)
Answer: D. Lead metal (Pb)
41. The chemical process of digestion in our body is an example of:
A. Combination reaction
B. Decomposition reaction
C. Displacement reaction
D. Redox reaction
Answer: B. Decomposition reaction
42. The symbolic representation of a chemical reaction using formulae is called a ________.
A. Word equation
B. Chemical equation
C. Reaction mechanism
D. Stoichiometric map
Answer: B. Chemical equation
43. What is the purpose of rubbing a magnesium ribbon with sandpaper before burning it?
A. To make it shiny
B. To remove the protective layer of magnesium oxide
C. To increase its mass
D. To make it brittle
Answer: B. To remove the protective layer of magnesium oxide
44. If a substance loses hydrogen during a reaction, it is said to be:
A. Reduced
B. Oxidised
C. Neutralised
D. Hydrolysed
Answer: B. Oxidised
45. The reaction between sodium sulphate and barium chloride solutions results in the formation of a white precipitate of:
A. Sodium chloride
B. Barium sulphate
C. Sodium barium sulphate
D. Barium sodium chloride
Answer: B. Barium sulphate
46. Respiration: Exothermic :: Photosynthesis: ________.
A. Exothermic
B. Endothermic
C. Combination
D. Displacement
Answer: B. Endothermic
47. What is the chemical formula for magnesium oxide?
A. Mg₂O
B. MgO
C. MgO₂
D. Mg(OH)₂
Answer: B. MgO
48. Which of the following statements are true for a balanced chemical equation?
P. The number of atoms of each element is the same on both sides.
Q. The total mass of reactants equals the total mass of products.
R. Physical states of reactants and products must always be shown.
S. It represents the law of constant proportions.
A. P and R
B. Q and S
C. P and Q
D. R and S
Answer: C. P and Q
49. The decomposition of vegetable matter into compost is an example of an:
A. Endothermic reaction
B. Exothermic reaction
C. Photolytic reaction
D. Electrolytic reaction
Answer: B. Exothermic reaction
50. Identify the substance that does NOT undergo decomposition by heat.
A. Ferrous sulphate (FeSO₄)
B. Calcium carbonate (CaCO₃)
C. Lead nitrate (Pb(NO₃)₂)
D. Sodium chloride (NaCl)
Answer: D. Sodium chloride (NaCl)
Additional MCQs (Competency Based)
1. Assertion (A): When calcium oxide is added to water, the container feels warm.
Reason (R): The reaction between calcium oxide and water is an endothermic process.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: C. A is true but R is false.
2. Assertion (A): In the reaction ZnO + C → Zn + CO, carbon is undergoing oxidation.
Reason (R): Oxidation is defined as the gain of oxygen by a substance.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: A. Both A and R are true and R is the correct explanation of A.
3. Assertion (A): A magnesium ribbon should be cleaned with sandpaper before burning it in air.
Reason (R): The ribbon is often coated with a layer of magnesium oxide or magnesium carbonate which can hinder burning.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: A. Both A and R are true and R is the correct explanation of A.
4. Assertion (A): Respiration is considered an exothermic process.
Reason (R): During respiration, glucose combines with oxygen to produce carbon dioxide, water, and release energy.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: A. Both A and R are true and R is the correct explanation of A.
5. Assertion (A): The reaction 2AgCl(s) –Sunlight–> 2Ag(s) + Cl₂(g) is an example of a thermal decomposition reaction.
Reason (R): Decomposition reactions that require heat energy are called thermal decomposition reactions.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: D. A is false but R is true.
6. Assertion (A): The reaction BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) is an example of a precipitation reaction.
Reason (R): In this reaction, an insoluble substance, barium sulphate, is formed.
A. Both A and R are true and R is the correct explanation of A.
B. Both A and R are true but R does not explain A.
C. A is true but R is false.
D. A is false but R is true.
Answer: A. Both A and R are true and R is the correct explanation of A.
7. A student observes that when a few granules of a metal ‘X’ are added to a blue coloured solution ‘Y’, the solution gradually turns pale green and a reddish-brown deposit forms on the metal granules. What could metal ‘X’ and solution ‘Y’ be?
A. X: Copper, Y: Iron sulphate solution
B. X: Iron, Y: Copper sulphate solution
C. X: Zinc, Y: Silver nitrate solution
D. X: Silver, Y: Copper sulphate solution
Answer: B. X: Iron, Y: Copper sulphate solution
8. During an experiment, a white solid ‘A’ is heated strongly in a dry test tube. A brown gas ‘B’ is evolved, and a yellow residue ‘C’ is left behind, which turns white on cooling.
What could solid ‘A’ be?
A. Ferrous sulphate
B. Calcium carbonate
C. Lead nitrate
D. Copper carbonate
Answer: C. Lead nitrate
9. A student adds a few drops of barium chloride solution to sodium sulphate solution in a test tube. A white substance immediately forms and settles at the bottom.
What is the nature of this white substance and the type of reaction?
A. Soluble salt, Displacement reaction
B. Insoluble precipitate, Double displacement reaction
C. Soluble base, Combination reaction
D. Insoluble acid, Decomposition reaction
Answer: B. Insoluble precipitate, Double displacement reaction
10. Potato chips are often sold in bags flushed with a particular gas to prevent them from becoming rancid. This is done to slow down a chemical process.
What is this process and which gas is commonly used?
A. Reduction, Oxygen gas
B. Oxidation, Nitrogen gas
C. Hydration, Carbon dioxide gas
D. Dehydrogenation, Hydrogen gas
Answer: B. Oxidation, Nitrogen gas
11. A shiny brown coloured element ‘X’ when heated in air turns into a black coloured compound ‘Y’. When hydrogen gas is passed over this heated compound ‘Y’, it turns back to ‘X’ and water is formed. Identify ‘X’ and ‘Y’.
A. X: Iron, Y: Iron(II,III) oxide
B. X: Silver, Y: Silver oxide
C. X: Copper, Y: Copper(II) oxide
D. X: Aluminium, Y: Aluminium oxide
Answer: C. X: Copper, Y: Copper(II) oxide
12. Consider the following chemical equation: 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g).
Based on this equation, which statement is correct?
A. Iron reacts with liquid water to form iron oxide and hydrogen.
B. Steam is passed over iron to produce iron(II,III) oxide and hydrogen gas.
C. This reaction is an example of a combination reaction.
D. For every 3 moles of iron, 1 mole of hydrogen gas is produced.
Answer: B. Steam is passed over iron to produce iron(II,III) oxide and hydrogen gas.
13. “During the electrolysis of acidified water, two gases are collected in separate test tubes over the electrodes. The volume of gas collected over one electrode is double the volume of gas collected over the other.” Which statement correctly identifies the gases and their respective electrodes?
A. Oxygen (double volume) at anode, Hydrogen at cathode.
B. Hydrogen (double volume) at cathode, Oxygen at anode.
C. Hydrogen (double volume) at anode, Oxygen at cathode.
D. Carbon dioxide (double volume) at cathode, Oxygen at anode.
Answer: B. Hydrogen (double volume) at cathode, Oxygen at anode.
14. “When fats and oils are oxidised, they become rancid and their smell and taste change. Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.” Based on this information, what is the primary role of an antioxidant in food?
A. To enhance the flavour of fats and oils.
B. To speed up the process of rancidity.
C. To slow down or prevent the oxidation of fats and oils.
D. To change the colour of the food product.
Answer: C. To slow down or prevent the oxidation of fats and oils.
15. Consider the reaction: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
Which of the following statements accurately describes this reaction?
A. It is an endothermic combination reaction.
B. It is an exothermic decomposition reaction.
C. It is an exothermic combination reaction.
D. It is an endothermic displacement reaction.
Answer: C. It is an exothermic combination reaction.
16. “Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Every year an enormous amount of money is spent to replace damaged iron.” What can be inferred about the economic impact of corrosion, particularly for iron?
A. It is a minor issue with negligible financial implications.
B. It primarily affects non-metallic objects, saving costs on metals.
C. It leads to significant financial expenditure due to the need for replacement.
D. It enhances the durability of iron objects, reducing long-term costs.
Answer: C. It leads to significant financial expenditure due to the need for replacement.
17. Match the type of chemical reaction in Column A with its characteristic in Column B.
| Column A (Reaction Type) | Column B (Characteristic) |
| (i) Combination | P. A single reactant breaks down |
| (ii) Decomposition | Q. Exchange of ions occurs |
| (iii) Displacement | R. Two or more reactants form a single product |
| (iv) Double Displacement | S. A more reactive element displaces a less reactive one |
A. (i)-R, (ii)-P, (iii)-S, (iv)-Q
B. (i)-P, (ii)-R, (iii)-Q, (iv)-S
C. (i)-R, (ii)-S, (iii)-P, (iv)-Q
D. (i)-Q, (ii)-P, (iii)-S, (iv)-R
Answer: A. (i)-R, (ii)-P, (iii)-S, (iv)-Q
18. Match the substance in Column A with its common observation or use in Column B.
| Column A (Substance) | Column B (Observation/Use) |
| (i) Calcium oxide | P. Turns grey in sunlight |
| (ii) Silver chloride | Q. Used in whitewashing |
| (iii) Copper sulphate solution | R. Brown fumes on heating |
| (iv) Lead nitrate | S. Blue coloured solution |
A. (i)-P, (ii)-Q, (iii)-R, (iv)-S
B. (i)-Q, (ii)-P, (iii)-S, (iv)-R
C. (i)-S, (ii)-R, (iii)-Q, (iv)-P
D. (i)-Q, (ii)-S, (iii)-P, (iv)-R
Answer: B. (i)-Q, (ii)-P, (iii)-S, (iv)-R
19. Match the chemical change in Column A with the primary indicator observed in Column B.
| Column A (Chemical Change) | Column B (Primary Indicator) |
| (i) Burning of magnesium ribbon | P. Evolution of gas |
| (ii) Reaction of zinc with dilute acid | Q. Formation of precipitate |
| (iii) Reaction of lead nitrate with potassium iodide | R. Change in temperature |
| (iv) Reaction of quicklime with water | S. Dazzling white flame |
A. (i)-S, (ii)-P, (iii)-Q, (iv)-R
B. (i)-P, (ii)-S, (iii)-R, (iv)-Q
C. (i)-R, (ii)-Q, (iii)-P, (iv)-S
D. (i)-S, (ii)-R, (iii)-Q, (iv)-P
Answer: A. (i)-S, (ii)-P, (iii)-Q, (iv)-R
20. Match the reactant(s) in Column A with the type of reaction they primarily undergo as described in Column B.
| Column A (Reactant(s)) | Column B (Primary Reaction Type) |
| (i) FeSO₄(s) on heating | P. Combination |
| (ii) CaO(s) + H₂O(l) | Q. Displacement |
| (iii) Zn(s) + CuSO₄(aq) | R. Decomposition |
| (iv) AgNO₃(aq) + NaCl(aq) | S. Double Displacement |
A. (i)-R, (ii)-P, (iii)-Q, (iv)-S
B. (i)-P, (ii)-R, (iii)-S, (iv)-Q
C. (i)-S, (ii)-Q, (iii)-P, (iv)-R
D. (i)-R, (ii)-S, (iii)-Q, (iv)-P
Answer: A. (i)-R, (ii)-P, (iii)-Q, (iv)-S
21. Arrange the following steps for balancing the chemical equation Fe + H₂O → Fe₃O₄ + H₂ in the generally recommended order.
(i) Balance the number of iron atoms.
(ii) Start balancing with the compound containing the maximum number of atoms (e.g., Fe₃O₄) and balance the element with the maximum atoms first (oxygen).
(iii) Balance the number of hydrogen atoms.
(iv) List the number of atoms of different elements present in the unbalanced equation.
A. (iv) → (ii) → (iii) → (i)
B. (i) → (ii) → (iii) → (iv)
C. (iv) → (i) → (ii) → (iii)
D. (ii) → (iv) → (i) → (iii)
Answer: A. (iv) → (ii) → (iii) → (i)
22. A student performs an experiment involving adding iron nails to copper sulphate solution. Arrange the following observations/steps in the correct sequence as they would occur or be performed.
(i) The blue colour of the copper sulphate solution fades.
(ii) Iron nails are cleaned by rubbing with sandpaper.
(iii) A brownish deposit forms on the iron nails.
(iv) Iron nails are immersed in the copper sulphate solution.
A. (ii) → (iv) → (i) → (iii)
B. (iv) → (ii) → (iii) → (i)
C. (ii) → (i) → (iv) → (iii)
D. (i) → (ii) → (iii) → (iv)
Answer: A. (ii) → (iv) → (i) → (iii)
23. A solution of slaked lime is used for whitewashing walls. The process involves several steps leading to a shiny finish. Arrange the following events in the correct chronological order.
(i) Calcium carbonate forms on the walls.
(ii) Slaked lime (calcium hydroxide) is applied to the walls.
(iii) Calcium hydroxide reacts slowly with carbon dioxide from the air.
(iv) A shiny finish appears on the walls after two to three days.
A. (ii) → (iii) → (i) → (iv)
B. (i) → (ii) → (iii) → (iv)
C. (ii) → (i) → (iii) → (iv)
D. (iii) → (ii) → (iv) → (i)
Answer: A. (ii) → (iii) → (i) → (iv)
24. In an experiment involving the electrolysis of water, the following volumes of gases were collected over a period:
Gas over Cathode: 10 mL
Gas over Anode: 5 mL
Which conclusion can be drawn?
A. The gas over the cathode is oxygen and over the anode is hydrogen.
B. The formula of water is H₂O₂.
C. The gas over the cathode is hydrogen and over the anode is oxygen.
D. The reaction is not proceeding correctly as volumes should be equal.
Answer: C. The gas over the cathode is hydrogen and over the anode is oxygen.
25. The equation for the formation of ammonia is N₂(g) + 3H₂(g) → 2NH₃(g).
If 1 mole of nitrogen gas reacts completely, how many moles of ammonia are formed?
A. 1 mole
B. 2 moles
C. 3 moles
D. 0.5 moles
Answer: B. 2 moles
26. The reaction for the decomposition of lead nitrate is: 2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g).
If 2 moles of lead nitrate are completely decomposed, what is the total number of moles of gaseous products formed?
A. 2 moles
B. 4 moles
C. 5 moles
D. 7 moles
Answer: C. 5 moles
27. A student is balancing the equation: _Mg + _O₂ → _MgO.
After balancing, the coefficients for Mg, O₂, and MgO are respectively:
A. 1, 1, 1
B. 2, 1, 2
C. 1, 2, 1
D. 2, 2, 2
Answer: B. 2, 1, 2
28. A chemical equation is given as: a Fe + b H₂O → c Fe₃O₄ + d H₂.
For the equation to be balanced, what are the correct integer values for a, b, c, and d?
A. a=1, b=1, c=1, d=1
B. a=3, b=4, c=1, d=4
C. a=2, b=3, c=1, d=3
D. a=3, b=2, c=1, d=2
Answer: B. a=3, b=4, c=1, d=4
Additional Questions and Answers
1. What is meant by a chemical reaction?
Answer: A chemical reaction is a process where a chemical change occurs. Chemical reactions involve the breaking and making of bonds between atoms to produce new substances.
2. List four observations that help determine that a chemical reaction has taken place.
Answer: Four observations that help determine that a chemical reaction has taken place are:
(i) change in state
(ii) change in colour
(iii) evolution of a gas
(iv) change in temperature.
3. What is a word-equation in the context of chemical reactions?
Answer: In the context of chemical reactions, a word-equation shows the change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products and shows the direction of the reaction.
4. Write the word-equation for the reaction between zinc and dilute sulphuric acid.
Answer: The word-equation for the reaction between zinc and dilute sulphuric acid is: Zinc + Sulphuric acid → Zinc sulphate + Hydrogen
5. How is a skeletal chemical equation different from a balanced chemical equation?
Answer: A skeletal chemical equation is an unbalanced chemical equation where the mass is not the same on both sides of the equation, meaning the number of atoms of each element is not the same on both sides. In contrast, a chemical equation is balanced if the number of atoms of each element is the same on both sides of the arrow.
6. State the law of conservation of mass as applied to chemical reactions.
Answer: As applied to chemical reactions, the law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
7. Which symbols are used to denote the physical states (solid, liquid, gas, aqueous) in a chemical equation?
Answer: In a chemical equation, the gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water.
8. Where are reaction conditions such as temperature, pressure or catalyst indicated in a chemical equation?
Answer: Reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the chemical equation.
9. Define a combination reaction.
Answer: A combination reaction is one where two or more substances (elements or compounds) combine to form a single product.
10. Give the chemical equation for the formation of slaked lime by a combination reaction.
Answer: The chemical equation for the formation of slaked lime (calcium hydroxide) by the combination reaction of calcium oxide (quick lime) with water is: CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
11. Define a decomposition reaction.
Answer: In a decomposition reaction, a single substance decomposes to give two or more substances.
12. Define a displacement reaction.
Answer: When an element displaces another element from its compound, a displacement reaction occurs.
13. Write the chemical equation for the displacement reaction between iron and copper sulphate.
Answer: The chemical equation for the displacement reaction between iron and copper sulphate is: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
14. What is a double displacement reaction?
Answer: A double displacement reaction is one in which two different atoms or groups of atoms (ions) are exchanged.
15. What precipitate is formed when barium chloride reacts with sodium sulphate? Write its balanced equation.
Answer: When barium chloride reacts with sodium sulphate, the white precipitate of BaSO₄ is formed. The balanced equation is: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
16. Define oxidation in terms of oxygen and hydrogen changes.
Answer: Oxidation is the gain of oxygen or loss of hydrogen.
17. Define reduction in terms of oxygen and hydrogen changes.
Answer: Reduction is the loss of oxygen or gain of hydrogen.
18. What is a redox reaction?
Answer: Oxidation-reduction reactions or redox reactions are reactions in which one reactant gets oxidised while the other gets reduced during the reaction.
19. Describe the step-by-step method of balancing the equation Fe+H₂O→Fe₃O₄+H₂?
Answer: The step-by-step method for balancing the chemical equation Fe + H₂O → Fe₃O₄ + H₂ is as follows:
Step I: To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.
Fe + H₂O → Fe₃O₄ + H₂
Step II: List the number of atoms of different elements present in the unbalanced equation.
| Element | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
Step III: It is often convenient to start balancing with the compound that contains the maximum number of atoms. It may be a reactant or a product. In that compound, select the element which has the maximum number of atoms. Using these criteria, we select Fe₃O₄ and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS. To balance the oxygen atoms –
| Atoms of oxygen | In reactants | In products |
| (i) Initial | 1 (in H₂O) | 4 (in Fe₃O₄) |
| (ii) To balance | 1×4 | 4 |
To equalise the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions. For example, to balance oxygen atoms we can put coefficient ‘4’ as 4 H₂O and not H₂O₄ or (H₂O)₄. Now the partly balanced equation becomes – Fe + 4 H₂O → Fe₃O₄ + H₂ (partly balanced equation)
Step IV: Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms in the partly balanced equation. To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.
| Atoms of hydrogen | In reactants | In products |
| (i) Initial | 8 (in 4 H₂O) | 2 (in H₂) |
| (ii) To balance | 8 | 2 × 4 |
The equation would be – Fe + 4 H₂O → Fe₃O₄ + 4 H₂ (partly balanced equation)
Step V: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.
| Atoms of iron | In reactants | In products |
| (i) Initial | 1 (in Fe) | 3 (in Fe₃O₄) |
| (ii) To balance | 1 × 3 | 3 |
To equalise Fe, we take three atoms of Fe on the LHS.
3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂
Step VI: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.
3Fe + 4H₂O → Fe₃O₄ + 4H₂ (balanced equation)
The numbers of atoms of elements on both sides of the equation are equal. This equation is now balanced. This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.
20. Explain why the reaction between calcium oxide and water is both a combination and an exothermic reaction.
Answer: The reaction between calcium oxide and water is a combination reaction because in this reaction, calcium oxide and water combine to form a single product, calcium hydroxide.
CaO(s) + H₂O(l) → Ca(OH)₂(aq)
This reaction is also an exothermic reaction because a large amount of heat is evolved, which makes the reaction mixture warm. Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
21. Compare combination and decomposition reactions, highlighting their defining characteristics and examples.
Answer: Combination reactions and decomposition reactions are essentially opposite to each other. A combination reaction is one in which two or more substances (elements or compounds) combine to form a single new substance. The defining characteristic is the formation of a single product from multiple reactants. Examples include:
(i) Burning of coal: C(s) + O₂(g) → CO₂(g)
(ii) Formation of water from H₂(g) and O₂(g): 2H₂(g) + O₂(g) → 2H₂O(l)
A decomposition reaction is one in which a single substance decomposes to give two or more simpler substances. The defining characteristic is the breakdown of a single reactant into multiple products. Decomposition reactions require energy in the form of heat, light, or electricity. Examples include:
(i) Thermal decomposition of ferrous sulphate: 2FeSO₄(s) Heat→ Fe₂O₃(s) + SO₂(g) + SO₃(g)
(ii) Thermal decomposition of calcium carbonate: CaCO₃(s) Heat→ CaO(s) + CO₂(g)
(iii) Decomposition of silver chloride by light: 2AgCl(s) Sunlight→ 2Ag(s) + Cl₂(g)
22. Explain the significance of physical state symbols and the representation of reaction conditions in chemical equations.
Answer: To make a chemical equation more informative, physical state symbols and reaction conditions are used.
The significance of physical state symbols is that they indicate the physical state of each reactant and product. The gaseous, liquid, aqueous, and solid states are represented by the notations (g), (l), (aq), and (s), respectively. The word aqueous (aq) is written if the reactant or product is present as a solution in water. Usually, physical states are not included in a chemical equation unless it is necessary to specify them. For example, the symbol (g) is used with H₂O to indicate that in a reaction water is used in the form of steam.
The significance of representing reaction conditions is that they provide information about the specific conditions under which a reaction occurs. Sometimes the reaction conditions, such as temperature, pressure, catalyst, etc., for the reaction are indicated above and/or below the arrow in the equation. For example:
CO(g) + 2H₂(g) 340 atm→ CH₃OH(l)
6CO₂(aq) + 12H₂O(l) Sunlight, Chlorophyll→ C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l)
23. Discuss how oxidation and reduction occur simultaneously in a single redox reaction, using the reaction between copper(II) oxide and hydrogen as an example.
Answer: In the reaction between copper(II) oxide and hydrogen, CuO + H₂ Heat→ Cu + H₂O, oxidation and reduction occur simultaneously.
During this reaction, the copper(II) oxide (CuO) is losing oxygen and is being reduced to copper (Cu). Simultaneously, hydrogen (H₂) is gaining oxygen and is being oxidised to water (H₂O).
In other words, one reactant (CuO) gets reduced while the other reactant (H₂) gets oxidised during the reaction. Such reactions, where oxidation and reduction happen at the same time, are called oxidation-reduction reactions or redox reactions. If a substance gains oxygen during a reaction, it is said to be oxidised. If a substance loses oxygen during a reaction, it is said to be reduced.
24. Describe the three types of decomposition reactions based on energy input, giving an example of each.
Answer: Decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Based on the form of energy input, there are three types:
(i) Thermal decomposition: When a decomposition reaction is carried out by heating, it is called thermal decomposition.
Example: Decomposition of calcium carbonate on heating.
CaCO₃(s) Heat→ CaO(s) + CO₂(g)
(ii) Decomposition by light (Photodecomposition): When a decomposition reaction is caused by light energy.
Example: Decomposition of silver chloride in sunlight.
2AgCl(s) Sunlight→ 2Ag(s) + Cl₂(g)
(iii) Decomposition by electricity (Electrolysis): When a decomposition reaction is carried out by passing electricity.
Example: Electrolysis of water, where water decomposes into hydrogen gas and oxygen gas.
25. Explain the process of corrosion and describe the appearance of corrosion products on iron, copper and silver.
Answer: The process of corrosion occurs when a metal is attacked by substances around it such as moisture, acids, etc. When this happens, the metal is said to corrode, and this process is called corrosion. The appearance of corrosion products on different metals is as follows:
Iron: Iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.
Copper: The green coating on copper is an example of corrosion.
Silver: The black coating on silver is an example of corrosion.
26. Define precipitation reactions and illustrate with the reaction between sodium sulphate and barium chloride.
Answer: A precipitation reaction is any reaction that produces an insoluble substance, known as a precipitate. When solutions of reactants are mixed and a solid substance, which is insoluble in water, is formed, this insoluble substance is the precipitate. An illustration of a precipitation reaction is the reaction between sodium sulphate solution and barium chloride solution:
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
In this reaction, when solutions of sodium sulphate and barium chloride are mixed, a white substance, barium sulphate (BaSO₄), is formed. This barium sulphate is insoluble in water and thus forms a precipitate. The white precipitate of BaSO₄ is formed by the reaction of SO₄²⁻ and Ba²⁺.
27. How does the law of conservation of mass govern the balancing of chemical equations?
Answer: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. This means that the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation to ensure that the number of atoms of each element is the same on both the reactant and product sides, thereby upholding the law of conservation of mass.
for the question ” Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.”
The correct answer should be ” The activity 1.7 is known as electrolysis of water.
Water (H₂O) is a molecule made up of two hydrogen atoms and one oxygen atom. Electrolysis uses electricity to decompose water into its constituent elements, hydrogen and oxygen. Since water has a 2:1 ratio of hydrogen to oxygen, the electrolysis process will produce hydrogen and oxygen in the same 2:1 ratio. Therefore, the test tube collecting hydrogen gas will have twice the volume of gas compared to the test tube collecting oxygen gas.”
Editor’s Comment
Thank you. We have updated the answer.