CBSE Class 10 Science Chemical Reactions and Equations MCQs Set 13

Answer: (b) a reducing agent. A reducing agent donates electrons or hydrogen (or accepts oxygen), so it is itself oxidised.

Teacher's Note:
1. Oxidation of the agent and reduction of the other substance always occur together.
2. Remember: the reducing agent is the one that gets oxidised.

Answer: (b) 4Na + O₂ → 2Na₂O. Only Na and O₂ are elements here; in the other reactions at least one reactant is a compound.

Teacher's Note:
1. Count the reactants and check whether each is an element or a compound.
2. A combination reaction forms a single product from two or more reactants.

Answer: (d) a double displacement reaction. CuSO₄ + H₂S → CuS↓ + H₂SO₄: the ions of the two compounds are exchanged and a precipitate forms.

Teacher's Note:
1. Look for exchange of ions and a precipitate to identify double displacement.
2. The black solid is copper sulphide.

Answer: (c) Obtaining salt from sea water. Salt is recovered by evaporation, which can be reversed by dissolving it again; no new substance forms.

Teacher's Note:
1. A physical change does not produce a new substance.
2. The other three changes form new substances.

Answer: (c) No reaction takes place. Copper is less reactive than iron, so it cannot displace iron from its salt solution.

Teacher's Note:
1. A metal displaces another only if it is higher in the reactivity series.
2. Copper lies below iron.

Answer: (b) A gas evolves that burns with a pop sound. Zn + 2HCl → ZnCl₂ + H₂↑; hydrogen burns with a pop sound.

Teacher's Note:
1. The pop sound with a burning splinter is the standard test for hydrogen.
2. The resulting zinc chloride solution is colourless.

Answer: (c) a colourless gas in A. Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂↑; there is no reaction with NaOH.

Teacher's Note:
1. Carbon dioxide is colourless and turns lime water milky.
2. A carbonate does not react with a base.

Answer: (b) law of conservation of mass. The number of atoms of each element is the same on both sides, so mass is conserved.

Teacher's Note:
1. Balancing an equation means making atoms equal on both sides.
2. This is a direct application of conservation of mass.

Answer: (c) 4 and 2. Balancing H gives x = 4; balancing N gives 4 = 2 + y, so y = 2.

Teacher's Note:
1. Balance hydrogen first as it appears in only two species.
2. Then check N and O to confirm.

Answer: (b) single displacement reaction. Zinc displaces hydrogen from the acid, an example of a displacement reaction.

Teacher's Note:
1. A more reactive metal replaces a less reactive element from its compound.
2. Here zinc replaces hydrogen.

Answer: (c) double displacement reaction. This is the definition of a double displacement reaction, for example BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl.

Teacher's Note:
1. Two compounds on each side signals double displacement.
2. Precipitation and neutralisation reactions belong to this type.

Answer: (d) one substance is oxidised while the other is reduced. Oxidation and reduction always occur simultaneously.

Teacher's Note:
1. Redox is short for reduction and oxidation.
2. One cannot happen without the other.

Answer: (b) a reducing agent. H₂S loses hydrogen (S is formed from H₂S), so it is oxidised and acts as a reducing agent.

Teacher's Note:
1. Loss of hydrogen is oxidation.
2. The substance undergoing it is the reducing agent.

Answer: (b) 2. Two Na atoms give 2NaCl, which needs 2 Cl, so x = 2.

Teacher's Note:
1. Balance the metal first, then the chlorine.
2. Finally check hydrogen and oxygen.

Answer: (d) displacement reaction. Aluminium displaces iron from its oxide; it is also called the thermite reaction.

Teacher's Note:
1. Aluminium is more reactive than iron.
2. The reaction releases so much heat that iron is formed in the molten state.

Answer: (b) grey. 2AgCl → 2Ag + Cl₂ in sunlight; the silver metal formed is grey.

Teacher's Note:
1. This is a photochemical decomposition reaction.
2. Silver salts are stored in dark bottles for this reason.

Answer: (b) Decomposition of silver bromide. Light decomposes silver bromide on the film into silver and bromine.

Teacher's Note:
1. The film is coated with silver bromide.
2. The silver formed gives the dark parts of the image.

Answer: (a) black copper(II) oxide. 2Cu + O₂ → 2CuO; copper(II) oxide is black.

Teacher's Note:
1. The reddish-brown copper turns black on heating in air.
2. Passing hydrogen over the hot black solid restores the brown colour.

Answer: (a) 2Pb₃O₄ → 6PbO + O₂. Red lead oxide (Pb₃O₄) decomposes to yellow lead(II) oxide, PbO.

Teacher's Note:
1. Note the colour change from red to yellow.
2. Pb₃O₄ is also called red lead or minium.

Answer: (b) brown fumes and a yellow residue. 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂; NO₂ is brown and PbO is yellow.

Teacher's Note:
1. The brown fumes are nitrogen dioxide.
2. This is a thermal decomposition reaction.

Answer: (b) 2, 2, 4, 1. The balanced equation is 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂.

Teacher's Note:
1. Balance nitrogen and oxygen after calcium.
2. Check that 12 oxygen atoms appear on both sides.

Answer: (d) Heating a magnesium ribbon in air at high temperature. 2Mg + O₂ → 2MgO forms a new substance; the other three are physical changes.

Teacher's Note:
1. Burning magnesium gives a dazzling white flame and a white powder.
2. A new substance with new properties marks a chemical change.

Answer: (a) 19. Adding the coefficients gives 2 + 7 + 4 + 6 = 19.

Teacher's Note:
1. Balance carbon, then hydrogen, then oxygen.
2. Add the coefficients only after the equation is balanced.

Answer: (b) exothermic and combination. CaO + H₂O → Ca(OH)₂ + heat; one product forms and heat is released.

Teacher's Note:
1. The beaker becomes hot, which shows that the reaction is exothermic.
2. Slaked lime is used for whitewashing.

Answer: (c) 2 : 1. 2H₂O → 2H₂ + O₂, so hydrogen is formed at twice the volume of oxygen.

Teacher's Note:
1. The gas at the cathode is hydrogen.
2. The ratio follows the mole ratio in the balanced equation.

Answer: (b) prevent oxidation of fats and oils. Nitrogen is unreactive and keeps oxygen away, so the oil does not become rancid.

Teacher's Note:
1. Rancidity is the oxidation of fats and oils.
2. Adding antioxidants is another method to prevent it.

Answer: (b) Fe₂O₃·xH₂O. Rust is hydrated iron(III) oxide formed when iron reacts with oxygen and moisture.

Teacher's Note:
1. Both air and water must be present for rusting.
2. Painting and galvanising keep them away from iron.

Answer: (b) reddish-brown. 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃; Fe₂O₃ is reddish-brown and the gases smell of burning sulphur.

Teacher's Note:
1. The crystals first lose water and turn white.
2. On stronger heating they decompose.

Answer: (c) Decomposition of calcium carbonate on heating. CaCO₃ → CaO + CO₂ needs continuous heat, so it absorbs energy.

Teacher's Note:
1. Decomposition reactions generally need energy as heat, light or electricity.
2. Combustion and respiration release energy.

Answer: (a) Both A and R are true and R is the correct explanation of A. Gain of oxygen is oxidation, so R correctly explains A.

Teacher's Note:
1. Oxidation can be defined as gain of oxygen or loss of hydrogen.
2. Both A and R follow the same definition.

Answer: (a) Both A and R are true and R is the correct explanation of A. Combination joins substances into one; decomposition splits one into several, so R explains A.

Teacher's Note:
1. The opposite nature is seen in the number of reactants and products.
2. Decomposition by heat, light and electricity are the three types.

Answer: (d) 4. 3Fe + 4H₂O → Fe₃O₄ + 4H₂; four oxygen atoms are needed to match Fe₃O₄.

Teacher's Note:
1. Balance oxygen first, then hydrogen, then iron.
2. Iron reacts with steam in this way.