CBSE Class 10 Chemistry Chemical Reactions and Equations MCQs Set 10

Answer: (b) (i), (iii) and (vi)

Explanation:
1. In digestion, ripening and electrolysis of water, new substances form, so they are chemical changes.
2. Liquefaction, dissolving sulphur and freezing only change the state or mix substances, so they are physical changes.

Teacher's Note:
1. Ask whether a new substance with new properties is formed.
2. Changes of state and dissolving are physical.

Answer: (d) Decay of matter

Explanation:
1. Melting, vaporisation and liquefaction are changes of state with no new substance.
2. Decay produces new substances, so it is a chemical change.

Teacher's Note:
1. Changes of state are physical.
2. Decay, rusting and burning are chemical.

Answer: (a) Melting of ice

Explanation:
1. Melting of ice only changes the state of water.
2. Carbon cycle, dehydration and fermentation all form new substances.

Teacher's Note:
1. In a physical change the composition stays the same.
2. Ice and water are both \( \ce{H2O} \).

Answer: (a) temporary

Explanation:
1. A physical change can usually be reversed by changing the conditions, so it is temporary.
2. For example, water vapour condenses back to water on cooling.

Teacher's Note:
1. Chemical changes are permanent.
2. Physical changes can be either endothermic or exothermic.

Answer: (c) permanent, irreversible and a new substance is produced

Explanation:
1. A chemical change gives new substances and is generally permanent and irreversible.
2. Light is not always involved, but heat usually is.

Teacher's Note:
1. Chemical changes are accompanied by energy changes in general.
2. Light is emitted only in some.

Answer: (a) Solubility in water

Explanation:
1. Dissolving a substance in water does not form a new substance, so it is a physical change.
2. Combustion, aerial oxidation and reaction with water form new substances.

Teacher's Note:
1. Dissolving is a physical change.
2. The dissolved substance can be recovered by evaporation.

Answer: (c) 4 and 2

Explanation:
1. Balance hydrogen: 2 water molecules have 4 H, so x = 4.
2. Balance nitrogen: 4 N on the left = 2 in the nitrate + y, so y = 2.
3. Oxygen check: 12 on the left and 6 + 4 + 2 = 12 on the right.

Teacher's Note:
1. Balance one element at a time and verify oxygen at the end.
2. Atoms of each element must be equal on both sides.

Answer: (b) \( \ce{2C4H10 + 12O2 -> 8CO2 + 10H2O} \)

Explanation:
1. In (b) the right side has 16 + 10 = 26 oxygen atoms but the left has 24, so it needs 13 \( \ce{O2} \).
2. The other three equations are balanced.

Teacher's Note:
1. Count atoms of each element on both sides.
2. Balance oxygen last when it appears in several substances.

Answer: (c) 1, 3, 2, 3

Explanation:
1. \( \ce{Fe2O3 + 3H2 -> 2Fe + 3H2O} \) balances Fe (2 and 2), O (3 and 3) and H (6 and 6).
2. So a, b, c, d = 1, 3, 2, 3.

Teacher's Note:
1. Start with the element in the most complex substance.
2. Always check hydrogen and oxygen at the end.

Answer: (b) \( \ce{FeCl2} \)

Explanation:
1. Ferrous means iron(II), so the salt is \( \ce{FeCl2} \).
2. \( \ce{Fe(OH)2 + 2HCl -> FeCl2 + 2H2O} \).

Teacher's Note:
1. Ferrous is +2 and ferric is +3.
2. Neutralisation gives salt and water.

Answer: (c) Burning of petrol

Explanation:
1. Combustion is burning in oxygen with release of heat and light.
2. Burning of petrol is combustion, while boiling and melting are physical changes.

Teacher's Note:
1. Combustion is always exothermic.
2. Fuel plus oxygen gives carbon dioxide and water.

Answer: (c) a combination reaction

Explanation:
1. Two elements combine to form a single compound, so this is a combination reaction.

Teacher's Note:
1. Combination: A + B gives AB.
2. The reaction is also a redox reaction in which hydrogen is oxidised.

Answer: (a) electrolytic decomposition

Explanation:
1. Electric current breaks water into hydrogen and oxygen.
2. This is electrolysis, so the process is electrolytic decomposition.

Teacher's Note:
1. Energy used: heat gives thermal, light gives photolytic, electricity gives electrolytic.
2. Water gives hydrogen and oxygen in a 2 to 1 volume ratio.

Answer: (c) \( \ce{2KClO3 -> 2KCl + 3O2} \) (on heating)

Explanation:
1. A decomposition reaction breaks one compound into two or more simpler substances.
2. Heating potassium chlorate gives potassium chloride and oxygen.
3. The other three are neutralisation, rearrangement and combination.

Teacher's Note:
1. One reactant, two or more products.
2. Decomposition needs heat, light or electricity.

Answer: (d) \( \ce{H2 + Cl2 -> 2HCl} \)

Explanation:
1. Heating calcium carbonate, heating potassium chlorate and digestion of food all break a compound into simpler substances.
2. \( \ce{H2 + Cl2 -> 2HCl} \) forms one compound from two elements, which is combination.

Teacher's Note:
1. Digestion breaks big molecules into smaller ones.
2. Combination is the reverse type of change.

Answer: (d) All of these

Explanation:
1. \( \ce{2Pb(NO3)2 -> 2PbO + 4NO2 + O2} \) on heating.
2. All three named products are formed.

Teacher's Note:
1. Lead nitrate decomposes with a crackling sound and brown fumes.
2. The brown gas is nitrogen dioxide.

Answer: (c) Both (a) and (b)

Explanation:
1. \( \ce{CaCO3 -> CaO + CO2} \) on heating.
2. Quicklime and carbon dioxide are both formed.

Teacher's Note:
1. Limestone is calcium carbonate.
2. This is a thermal decomposition used in making cement.

Answer: (a) black copper oxide

Explanation:
1. \( \ce{2Cu + O2 -> 2CuO} \).
2. Copper(II) oxide is black.

Teacher's Note:
1. Brown copper turns black on heating in air.
2. Hydrogen reduces the black oxide back to copper.

Answer: (a) \( \ce{CaO + CO2 -> CaCO3} \)

Explanation:
1. In (a) the two compounds CaO and \( \ce{CO2} \) join to form one compound.
2. In (b) and (c) a compound combines with an element, and (d) is not a combination.

Teacher's Note:
1. Look at the nature of both reactants.
2. Both reactants must be compounds here.

Answer: (d) (ii) and (iv)

Explanation:
1. \( \ce{BaCl2 + (NH4)2SO4 -> BaSO4 + 2NH4Cl} \).
2. Insoluble barium sulphate is formed, so it is a precipitation reaction.
3. Ions are exchanged between the two compounds, so it is double displacement as well.

Teacher's Note:
1. A double displacement reaction that forms an insoluble solid is also a precipitation reaction.
2. Barium sulphate is a white precipitate.

Answer: (a) \( \ce{2H2O -> 2H2 + O2} \) (electric current)

Explanation:
1. Water decomposes on passing electric current, which is electrolysis.
2. The others are decomposed by heat, so they are thermal decomposition reactions.

Teacher's Note:
1. Name the decomposition by the energy supplied.
2. Thermal means heat is the energy used.

Answer: (a) (i) and (ii)

Explanation:
1. Carbon takes oxygen, so carbon is oxidised: (iii) is correct.
2. PbO loses oxygen, so lead oxide is reduced: (iv) is correct.
3. Lead is a product and carbon dioxide is also a product, so (i) and (ii) are incorrect.

Teacher's Note:
1. The reactant that loses oxygen is reduced and the one that gains oxygen is oxidised.
2. Products are not oxidised or reduced in the reaction.

Answer: (a) Hydrogen gas and iron chloride are produced

Explanation:
1. Iron is above hydrogen in the reactivity series.
2. \( \ce{Fe + 2HCl -> FeCl2 + H2} \).

Teacher's Note:
1. Metal plus dilute acid gives salt and hydrogen.
2. Iron forms iron(II) chloride, not iron(III).

Answer: (d) single displacement reaction

Explanation:
1. Aluminium, being more reactive, displaces iron from its oxide.
2. One element replaces another, so it is a single displacement reaction.

Teacher's Note:
1. This is the thermite reaction used to weld rails.
2. Aluminium is more reactive than iron.

Answer: (c) \( \ce{Fe + CuSO4 -> FeSO4 + Cu} \)

Explanation:
1. Iron displaces copper from copper sulphate solution.
2. The others are decomposition and neutralisation reactions.

Teacher's Note:
1. A more reactive metal displaces a less reactive one.
2. Blue copper sulphate solution turns pale green.