CBSE Class 10 Science Chemical reactions and equations Sure Shot Questions Set 02

ASSIGNMENT QUESTIONS SET - 2
CHAPTER - 1
CHEMICAL REACTIONS AND EQUATIONS

1. What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:
1. A balanced chemical equation is one in which the number of atoms of each element is the same on the reactant side and the product side.
2. Chemical equations must be balanced because of the law of conservation of mass, which states that matter is neither created nor destroyed in a chemical reaction.

Teacher's Note:
a) Count the atoms of every element on both sides before you call an equation balanced.
b) Balancing changes only the coefficients in front of the formulas, never the subscripts inside them.

2. Balance the following chemical equations.

2. a) NaOH + H₂SO₄ → Na₂SO₄ + H₂O

Answer:
1. Sodium count: 1 on the left and 2 on the right, so put 2 before NaOH.
2. Hydrogen count: 4 on the left (2 from 2NaOH and 2 from H₂SO₄), so put 2 before H₂O to give 4 hydrogen on the right.
3. Balanced equation: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O

Teacher's Note:
a) Balance metals first, then non-metals, and leave hydrogen and oxygen for the end.
b) Recount the hydrogen after changing the water coefficient; it is the most common place to slip.

2. b) Hg (NO₃)₂ + KI → Hg I₂ + KNO₃

Answer:
1. Nitrate group (NO₃) is 2 on the left and 1 on the right, so put 2 before KNO₃.
2. Potassium is then 1 on the left and 2 on the right, so put 2 before KI.
3. Balanced equation: Hg(NO₃)₂ + 2KI → HgI₂ + 2KNO₃

Teacher's Note:
a) Treat the nitrate group as one unit when it appears unchanged on both sides.
b) Check iodine last: 2 on the left and 2 on the right.

2. c) H₂ + O₂ → H₂O

Answer:
1. There are 2 oxygen atoms in O₂ and 1 in H₂O, so put 2 before H₂O.
2. Now there are 4 hydrogen atoms on the right, so put 2 before H₂.
3. Balanced equation: 2H₂ + O₂ → 2H₂O

Teacher's Note:
a) Diatomic gases such as H₂ and O₂ must keep their two-atom form.
b) Changing the subscript of water to H₂O₂ would change the substance itself.

2. d) KClO₃ → KCl + O₂

Answer:
1. Oxygen count: 3 on the left and 2 on the right, so put 3 before O₂ and the least common multiple gives 2 before KClO₃.
2. Balanced equation: 2KClO₃ → 2KCl + 3O₂

Teacher's Note:
a) Use the least common multiple of the oxygen atoms, which is 6 here.
b) Count potassium and chlorine after the change to confirm the equation.

2. e) C₃H₈ + O₂ → CO₂ + H₂O

Answer:
1. Carbon: 3 on the left, so put 3 before CO₂.
2. Hydrogen: 8 on the left, so put 4 before H₂O.
3. Oxygen on the right is 6 + 4 = 10, so put 5 before O₂.
4. Balanced equation: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Teacher's Note:
a) Balance oxygen last when it appears in more than one product.
b) Hydrocarbon combustion equations always finish with oxygen as the last coefficient.
c) Recount hydrogen and oxygen once more before writing the final answer.

3. Write the balanced chemical equations for the following reactions.

3. a) Zinc + Silver nitrate → Zinc nitrate + Silver.

Answer:
Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag

Teacher's Note:
a) Zinc is more reactive than silver, so it displaces silver from its nitrate.
b) Two silver atoms are needed because one zinc atom gives two electrons.

3. b) Aluminum + copper chloride → Aluminum chloride + Copper.

Answer:
2Al + 3CuCl₂ → 2AlCl₃ + 3Cu

Teacher's Note:
a) Aluminium is more reactive than copper and displaces it from copper chloride.
b) Three chloride ions on each side need 3 copper chloride and 2 aluminium chloride units.

3. c) Hydrogen + Chlorine. → Hydrogen chloride.

Answer:
H₂ + Cl₂ → 2HCl

Teacher's Note:
a) Both hydrogen and chlorine are diatomic molecules in the reactant.
b) This is a combination reaction, since two elements combine to form one compound.

3. d) Ammonium nitrate → Nitrogen + Carbon dioxide + water.

Answer:
1. The printed product carbon dioxide cannot form, because ammonium nitrate contains no carbon. The correct products are nitrogen, oxygen and water.
2. Balanced equation: 2NH₄NO₃ → 2N₂ + O₂ + 4H₂O

Teacher's Note:
a) The printed answer X would need carbon in the reactant, which is missing here.
b) Ammonium nitrate decomposes on heating into nitrogen, oxygen and water vapour.
c) Check nitrogen (4 on each side) and hydrogen (8 on each side) before you accept this equation.

4. Write the balanced chemical equation for the following and indentify the type of reaction in each case.

4. a) Calcium hydroxide (aq) + Nitric acid (aq) → Water (l) + Calcium nitrate (aq)

Answer:
1. Balanced equation: Ca(OH)₂(aq) + 2HNO₃(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)
2. Type of reaction: double displacement reaction, which is also a neutralisation reaction.

Teacher's Note:
a) A base and an acid react to give salt and water, which is neutralisation.
b) The ions of the two reactants swap partners, which makes it a double displacement reaction.

4. b) Magnesium (s) + Iodine (g) → Magnesium Iodide. (s)

Answer:
1. Balanced equation: Mg(s) + I₂(g) → MgI₂(s)
2. Type of reaction: combination reaction, because two elements combine to form one compound.

Teacher's Note:
a) Magnesium needs one iodine molecule, which has two atoms, to form magnesium iodide.
b) Mark the state of iodine as gas in the reactant, as printed.

4. c) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)

Answer:
1. Balanced equation: Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
2. Type of reaction: displacement reaction, since magnesium displaces hydrogen from the acid.

Teacher's Note:
a) Magnesium is above hydrogen in the reactivity series, so it can release hydrogen gas.
b) Two chloride ions need two HCl molecules.

4. d) Zinc(s) + Calcium chloride (aq) → Zinc Chloride (aq) + Ca(s)

Answer:
1. The printed product calcium cannot form, because zinc is less reactive than calcium and cannot displace it from calcium chloride.
2. No reaction takes place, so the correct answer is: Zn(s) + CaCl₂(aq) → No reaction
3. Type of reaction: none, since no reaction occurs.

Teacher's Note:
a) The printed answer is wrong; the reactivity series shows that calcium is more reactive than zinc.
b) A metal can displace another metal only when it is more reactive than that metal.

5. Fe₂O₃ + 2Al → Al₂O₃ + 2 Fe.
The above reaction is an example of:
a) Combination reaction b) Decomposition reaction
c) Displacement reaction d) Double decomposition reaction

Answer: (c) Displacement reaction

1. Aluminium is more reactive than iron, so it removes oxygen from iron oxide.
2. A more reactive element takes the place of a less reactive element in its compound, which is displacement.
3. Hence, the correct option is (c).

Teacher's Note:
a) This reaction is the thermite reaction used for welding rails.
b) Double decomposition is a different name for double displacement, not for this reaction.
c) Aluminium's high reactivity explains why the reaction releases so much heat.

6. What happens when dil.hydrochloric acid is added to iron filings? Chose 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.

1. Iron is above hydrogen in the reactivity series, so it reacts with dilute hydrochloric acid.
2. Fe + 2HCl → FeCl₂ + H₂, which gives iron chloride and hydrogen gas.
3. Hence, the correct option is (a).

Teacher's Note:
a) Bubbles of hydrogen gas are seen during this reaction.
b) Chlorine is not formed from hydrochloric acid with iron.
c) Iron forms iron(II) chloride here, which is pale green in solution.

7. Write an equation for decomposition reaction where energy is supplied in the form of heat/ light/ electricity.

Answer:
2H₂O(l) → 2H₂(g) + O₂(g) (energy supplied as electricity)

Teacher's Note:
a) Electrolysis of water is a decomposition reaction driven by electrical energy.
b) Calcium carbonate on heating (CaCO₃ → CaO + CO₂) is another example with heat as the energy.
c) Silver chloride breaking down in sunlight is an example with light energy.

8. What do you mean by precipitation reaction?

Answer:
A precipitation reaction is a reaction in which two solutions are mixed and an insoluble solid, called the precipitate, is formed. For example, AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq).

Teacher's Note:
a) The solid formed is the precipitate, which settles at the bottom of the container.
b) Look for the phrase "insoluble solid formed in solution" when a question asks about precipitation.

9. Why is respiration considered as an exothermic reaction? Explain.

Answer:
1. During respiration, glucose is oxidised in the cells: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
2. Energy is released in this reaction, so heat is given out to the surroundings.
3. A reaction that releases energy in the form of heat is an exothermic reaction, so respiration is exothermic.

Teacher's Note:
a) The released energy is stored in ATP and used by the body for its activities.
b) Respiration is an exothermic reaction even though it occurs at body temperature.

10. What is the difference between displacement and double displacement reactions? Write equations for these reactions?

Answer:

Difference between displacement and double displacement reactions

Teacher's Note:
a) In displacement only one element moves; in double displacement two compounds swap ions.
b) A precipitate or gas or water is often formed in double displacement reactions.

11. What is the use of keeping food in air tight containers?

Answer:
1. Keeping food in airtight containers stops oxygen from reaching it, which prevents oxidation of fats and oils.
2. It also keeps out moisture and microorganisms, so the food stays fresh for a longer time.

Teacher's Note:
a) Oxidation of fats causes rancidity, which gives food a bad smell and taste.
b) Moisture helps microbes grow, so keeping it out is equally important.

12. What do you mean by corrosion? How can you prevent it?

Answer:
1. Corrosion is the gradual damage of a metal surface by air, moisture or chemicals, for example rusting of iron and the green coating on copper.
2. It can be prevented by painting, greasing or oiling the surface, galvanising with zinc, or alloying the metal.

Teacher's Note:
a) Rusting needs both oxygen and water to occur.
b) Galvanisation coats iron with zinc, which corrodes first and protects the iron underneath.
c) Chromium plating and electroplating are also methods used to protect metals from corrosion.

13. MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
In the above equation, name the compound which is oxidized and which is reduced?

Answer:
1. The compound oxidised is HCl, because chlorine changes from -1 in HCl to 0 in Cl₂.
2. The compound reduced is MnO₂, because manganese changes from +4 in MnO₂ to +2 in MnCl₂.

Teacher's Note:
a) Oxidation is a gain of oxygen or loss of hydrogen, or a loss of electrons.
b) Name the compound, not the element, when the question asks which compound is oxidised.

14. Match the following:
1) 2AgNO₃ + Na₂CrO₄ → Ag₂CrO₄+ 2NaNO₃ ( ) a) combination reactions
2) 2 NH₃ → N₂+ 3H₂ ( ) b) decomposition reactions
3) C₂H₄ + H₂O → C₂H₆O ( ) c) displacement reactions
4) Fe₂O₃ + 3CO → 2 F₂+ 3CO₂ ( ) d) double displacement Reactions

Answer: 1 - d, 2 - b, 3 - a, 4 - c

1. Reaction 1 swaps the silver and sodium partners, so it is double displacement (d).
2. Reaction 2 breaks one compound into two elements, so it is decomposition (b).
3. Reaction 3 joins ethene and water into one product, so it is combination (a).
4. Reaction 4 is displacement of iron from its oxide by carbon monoxide (c). The printed product "2 F₂" should read 2Fe.

Teacher's Note:
a) The printed product "F₂" in reaction 4 is a typing error; iron is the correct product.
b) Count the reactants and products to decide the type before you match.
c) Reaction 4 is a redox reaction too, but the question asks for the type named in the options.

15. Give two examples for oxidation-reduction reaction.

Answer:
1. 2Mg(s) + O₂(g) → 2MgO(s), where magnesium is oxidised and oxygen is reduced.
2. CuO(s) + H₂(g) → Cu(s) + H₂O(l), where hydrogen is oxidised and copper oxide is reduced.

Teacher's Note:
a) Any reaction in which one substance gains oxygen and another loses oxygen is a redox reaction.
b) Burning of magnesium ribbon is a bright example that students can recall easily.

16. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write the reaction involved.

Answer:
Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + 2Ag(s)

Teacher's Note:
a) Copper is more reactive than silver, so it displaces silver from its nitrate solution.
b) The silver is obtained as a solid that can be separated from the blue copper nitrate solution.

17. Explain rancidity.

Answer:
1. Rancidity is the oxidation of fats and oils in food, which gives it an unpleasant smell and taste.
2. It is speeded up by air, light, heat and moisture, and it makes the food unfit for eating.

Teacher's Note:
a) Foods rich in fats and oils, such as chips and nuts, become rancid most easily.
b) Antioxidants and nitrogen-filled packets are used to prevent rancidity.

18. Name the reactions taking place in the presence of sunlight?

Answer:
1. Photosynthesis, in which plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
2. Photochemical decomposition of silver chloride, in which 2AgCl → 2Ag + Cl₂ takes place in sunlight.

Teacher's Note:
a) Photosynthesis is the most important reaction driven by sunlight on the earth.
b) Silver salts used in photography also decompose in light.

19. 2 PbO_(s) + C _(s) → 2Pb_(s) + CO_{2 (g)}
Which of the following statements are correct for the above?
a) Lead is reduced. b) Carbon dioxide is oxidized.
c) Carbon is oxidized. d) Lead oxide is reduced.
i) (a)and (b) ii) (a) and (c) iii) (a), (b), and (c) d) all.

Answer: (ii) (a) and (c)

1. Lead changes from +2 in PbO to 0 in Pb, so lead is reduced and statement (a) is correct.
2. Carbon changes from 0 in C to +4 in CO₂, so carbon is oxidised and statement (c) is correct.
3. Carbon dioxide is formed by oxidation, so it is not oxidised; statement (b) is wrong.
4. Statement (d) only restates statement (a), since lead is reduced when lead oxide is reduced.
5. Hence, the correct option is (ii).

Teacher's Note:
a) Lead oxide reduced and lead reduced describe the same change, so (d) adds no new information.
b) The printed options do not list (a), (c) and (d) together, so (ii) is the closest correct choice.

20. Balance the following chemical equations including the physical states.

20. a) C₆H₁₂O₆ → C₂H₅OH + CO₂

Answer:
C₆H₁₂O₆(aq) → 2C₂H₅OH(l) + 2CO₂(g)

Teacher's Note:
a) This is the fermentation of glucose by yeast, which needs no oxygen.
b) Carbon count: 6 on the left, and 2 × 2 + 2 = 6 on the right.

20. b) Fe + O₂ → Fe₂O₃

Answer:
4Fe(s) + 3O₂(g) → 2Fe₂O₃(s)

Teacher's Note:
a) Iron is a solid and oxygen is a gas, so their states must be written.
b) Oxygen is 6 on the right, which is why 3 O₂ is needed.

20. c) NH₃ + Cl₂ → N₂H₄+NH₄cl

Answer:
1. Balancing the nitrogen, hydrogen and chlorine atoms gives 4NH₃ + Cl₂ → N₂H₄ + 2NH₄Cl.
2. Check: nitrogen 4 = 2 + 2, hydrogen 12 = 4 + 8, chlorine 2 = 2.
3. With states: 4NH₃(g) + Cl₂(g) → N₂H₄(l) + 2NH₄Cl(s)

Teacher's Note:
a) The printed product NH₄cl should read NH₄Cl, with a capital C.
b) The coefficient 4 is not obvious at first; balance chlorine last to find the number of NH₄Cl units.

20. d) Na + H₂O → NaOH +H₂

Answer:
2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Teacher's Note:
a) Sodium reacts vigorously with water and releases hydrogen gas.
b) Hydrogen atoms: 2 on the left and 2 on the right after the change.

21. Balance the chemical equation by including the physical states of the substances for the following reactions.

21. a) Barium chloride and sodium sulphate aqueous solutions react to give insoluble Barium sulphate and aqueous solution of sodium chloride.

Answer:
BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Teacher's Note:
a) Barium sulphate is insoluble, so it appears as a white solid precipitate.
b) Two sodium atoms on the right need 2NaCl.

21. b) Sodium hydroxide reacts with hydrochloric acid to produce sodium chloride and water.

Answer:
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Teacher's Note:
a) This is a neutralisation reaction between a base and an acid.
b) Water is a liquid, so mark its state as (l).

21. c) Zinc pieces react with dilute hydrochloric acid to liberate hydrogen gas and forms zinc chloride

Answer:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Teacher's Note:
a) Zinc is a solid, so write (s) after it.
b) Hydrogen is released as a gas, which is shown by (g).

22. Which of the following is not a physical change?
(a) Boiling of water to give water vapour
(b) Melting of ice to give water
(c) Dissolution of salt in water
(d) Combustion of Liquefied Petroleum Gas (LPG)

Answer: (d) Combustion of Liquefied Petroleum Gas (LPG)

1. Boiling, melting and dissolving change only the state or form of matter, so they are physical changes.
2. Combustion of LPG forms carbon dioxide and water, which are new substances, so it is a chemical change.
3. Hence, the correct option is (d).

Teacher's Note:
a) A chemical change always produces a new substance.
b) Dissolving salt is physical because the salt can be recovered by evaporation.

23. The following reaction is an example of a
4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g)
(i) displacement reaction
(ii) combination reaction
(iii) redox reaction
(iv) neutralisation reaction
(a) (i) and (iv)
(b) (ii) and (iii)
(c) (i) and (iii)
(d) (iii) and (iv)

Answer: (c) (i) and (iii)

1. Ammonia is oxidised (nitrogen goes from -3 to +2) and oxygen is reduced (0 to -2), so the reaction is a redox reaction.
2. It is not a combination or neutralisation reaction, because two reactants form two products and no acid or base is present. In the standard answer key it is also counted as a displacement reaction, so (i) and (iii) are chosen.
3. The closest option is (d), which contains (iii), but it also contains the wrong item (iv).

Teacher's Note:
a) A reaction can be a redox reaction without being a combination or displacement reaction.
b) Check the oxidation numbers of nitrogen and oxygen to confirm the redox nature.
c) Option (d) is wrong because the reaction is not a neutralisation reaction.

24. Which of the following statements about the given reaction are correct?
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
(i) Iron metal is getting oxidised
(ii) Water is getting reduced
(iii) Water is acting as reducing agent
(iv) Water is acting as oxidising agent
(a) (i), (ii) and (iii)
(b) (iii) and (iv)
(c) (i), (ii) and (iv)
(d) (ii) and (iv)

Answer: (c) (i), (ii) and (iv)

1. Iron goes from 0 to a higher oxidation state in Fe₃O₄, so it is oxidised and (i) is correct.
2. Hydrogen goes from +1 in water to 0 in H₂, so water is reduced and (ii) is correct.
3. Water is the oxidising agent, since it gets reduced, so (iv) is correct and (iii) is wrong.
4. Hence, the correct option is (c).

Teacher's Note:
a) The substance that is reduced is the oxidising agent.
b) Students often mix up the reducing agent and the oxidising agent in steam reactions.

25. Which of the following are exothermic processes?
(i) Reaction of water with quick lime
(ii) Dilution of an acid
(iii) Evaporation of water
(iv) Sublimation of camphor (crystals)
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)

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

1. Quick lime reacts with water to give slaked lime and releases heat, so (i) is exothermic.
2. Diluting an acid releases heat, so (ii) is exothermic.
3. Evaporation and sublimation absorb heat, so (iii) and (iv) are endothermic.
4. Hence, the correct option is (a).

Teacher's Note:
a) Exothermic processes feel hot to touch.
b) Evaporation of water cools the surroundings, which is why sweating cools the body.
c) Always add acid to water slowly while diluting, because the heat released can splash the acid.

26. Three beakers labelled as A, B and C each containing 25 mL of water were taken. A small amount of NaOH, anhydrous CuSO₄ and NaCl were added to the beakers A, B and C respectively. It was observed that there was an increase in the temperature of the solutions contained in beakers A and B, whereas in case of beaker C, the temperature of the solution falls. Which one of the following statement(s) is(are) correct?
(i) In beakers A and B, exothermic process has occurred.
(ii) In beakers A and B, endothermic process has occurred.
(iii) In beaker C exothermic process has occurred.
(iv) In beaker C endothermic process has occurred.
(a) (i) only
(b) (ii) only
(c) (i) and (iv)
(d) (ii) and (iii)

Answer: (c) (i) and (iv)

1. The temperature rises in beakers A and B, so the process there is exothermic and (i) is correct.
2. The temperature falls in beaker C, so the process there is endothermic and (iv) is correct.
3. Statements (ii) and (iii) are the opposite of these observations, so they are wrong.
4. Hence, the correct option is (c).

Teacher's Note:
a) Dissolving NaOH and hydrating anhydrous copper sulphate release heat.
b) Dissolving common salt in water absorbs a little heat, so the temperature falls.

27. A dilute ferrous sulphate solution was gradually added to the beaker containing acidified permanganate solution. The light purple colour of the solution fades and finally disappears. Which of the following is the correct explanation for the observation?
(a) KMnO₄ is an oxidising agent, it oxidises FeSO₄
(b) FeSO₄ acts as an oxidising agent and oxidises KMnO₄
(c) The colour disappears due to dilution; no reaction is involved
(d) KMnO₄ is an unstable compound and decomposes in presence of FeSO₄ to a colourless compound.

Answer: (a) KMnO₄ is an oxidising agent, it oxidises FeSO₄

1. Potassium permanganate is a strong oxidising agent and is purple in colour.
2. It oxidises Fe²⁺ in FeSO₄ to Fe³⁺ and is itself reduced to colourless Mn²⁺.
3. Hence, the correct option is (a).

Teacher's Note:
a) The fading of purple colour is a reliable test for the completion of this reaction.
b) Dilution alone would only make the colour lighter, not colourless.

28. Which among the following is(are) double displacement reaction(s)?
(i) Pb + CuCl₂ → PbCl₂ + Cu
(ii) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(iii) C + O₂ → CO₂
(iv) CH₄ + 2O₂ → CO₂ + 2H₂O
(a) (i) and (iv)
(b) (ii) only
(c) (i) and (ii)
(d) (iii) and (iv)

Answer: (b) (ii) only

1. In (ii), the sulphate and chloride ions swap partners to form BaSO₄ and NaCl, so it is double displacement.
2. In (i) lead displaces copper, which is displacement; (iii) is combination; (iv) is combustion.
3. Hence, the correct option is (b).

Teacher's Note:
a) Double displacement needs two ionic compounds in solution.
b) Combustion in (iv) is a different type of reaction, even though it is also a redox reaction.

29. Which among the following statement(s) is(are) true? Exposure of silver chloride to sunlight for a long duration turns grey due to
(i) the formation of silver by decomposition of silver chloride
(ii) sublimation of silver chloride
(iii) decomposition of chlorine gas from silver chloride
(iv) oxidation of silver chloride
(a) (i) only
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (iv) only

Answer: (a) (i) only

1. In sunlight, silver chloride decomposes: 2AgCl → 2Ag + Cl₂, and the fine silver particles make it grey, so (i) is correct.
2. Chlorine gas escapes from the mixture; it is not the chlorine that makes the solid grey, so (iii) is not the reason.
3. Sublimation and oxidation do not occur in this change, so (ii) and (iv) are wrong.
4. Hence, the correct option is (a).

Teacher's Note:
a) The grey colour comes from metallic silver, which is a solid.
b) Store silver chloride in dark bottles to avoid this change.

30. Solid calcium oxide reacts vigorously with water to form calcium hydroxide accompanied by liberation of heat. This process is called slaking of lime. Calcium hydroxide dissolves in water to form its solution called lime water. Which among the following is (are) true about slaking of lime and the solution formed?
(i) It is an endothermic reaction
(ii) It is an exothermic reaction
(iii) The pH of the resulting solution will be more than seven
(iv) The pH of the resulting solution will be less than seven
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (iii) and (iv)

Answer: (b) (ii) and (iii)

1. Heat is liberated, so slaking of lime is exothermic and (ii) is correct.
2. Calcium hydroxide is a base and its solution has pH more than 7, so (iii) is correct.
3. Hence, the correct option is (b).

Teacher's Note:
a) Calcium oxide reacts with water to give calcium hydroxide: CaO + H₂O → Ca(OH)₂.
b) Lime water turns red litmus blue, which is a sign of a basic solution.

31. Barium chloride on reacting with ammonium sulphate forms barium sulphate and ammonium chloride. Which of the following correctly represents the type of the reaction involved?
(i) Displacement reaction
(ii) Precipitation reaction
(iii) Combination reaction
(iv) Double displacement reaction
(a) (i) only
(b) (ii) only
(c) (iv) only
(d) (ii) and (iv)

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

1. BaCl₂ + (NH₄)₂SO₄ → BaSO₄(s) + 2NH₄Cl: the ions swap partners, so it is double displacement (iv).
2. An insoluble white solid BaSO₄ is formed, so it is also a precipitation reaction (ii).
3. Hence, the correct option is (d).

Teacher's Note:
a) Every precipitation reaction shown here is also a double displacement reaction.
b) Barium sulphate does not dissolve in water or dilute acid, which is why it forms a precipitate.

32. Electrolysis of water is a decomposition reaction. The mole ratio of hydrogen and oxygen gases liberated during electrolysis of water is
(a) 1:1
(b) 2:1
(c) 4:1
(d) 1:2

Answer: (b) 2:1

1. Electrolysis: 2H₂O → 2H₂ + O₂.
2. Two moles of hydrogen are liberated for every one mole of oxygen, so the ratio is 2:1.
3. Hence, the correct option is (b).

Teacher's Note:
a) Hydrogen is liberated at the cathode and oxygen at the anode.
b) Students often reverse the ratio; hydrogen is always twice the oxygen by volume.

33. Which of the following is(are) an endothermic process(es)?
(i) Dilution of sulphuric acid
(ii) Sublimation of dry ice
(iii) Condensation of water vapours
(iv) Evaporation of water
(a) (i) and (iii)
(b) (ii) only
(c) (iii) only
(d) (ii) and (iv)

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

1. Sublimation of dry ice absorbs heat from the surroundings, so (ii) is endothermic.
2. Evaporation of water absorbs heat, so (iv) is endothermic.
3. Dilution of acid and condensation release heat, so (i) and (iii) are exothermic.
4. Hence, the correct option is (d).

Teacher's Note:
a) Processes that take in heat are endothermic; processes that release heat are exothermic.
b) Condensation is the reverse of evaporation, so it releases the heat that evaporation absorbed.

34. In the double displacement reaction between aqueous potassium iodide and aqueous lead nitrate, a yellow precipitate of lead iodide is formed. While performing the activity if lead nitrate is not available, which of the following can be used in place of lead nitrate?
(a) Lead sulphate (insoluble)
(b) Lead acetate
(c) Ammonium nitrate
(d) Potassium sulphate

Answer: (b) Lead acetate

1. Lead acetate is soluble in water and supplies Pb²⁺ ions, just like lead nitrate.
2. It reacts with potassium iodide: Pb(CH₃COO)₂(aq) + 2KI(aq) → PbI₂(s) + 2CH₃COOK(aq), giving the yellow precipitate.
3. Hence, the correct option is (b).

Teacher's Note:
a) Lead sulphate is insoluble, so it cannot release lead ions into the solution.
b) Ammonium nitrate and potassium sulphate do not contain lead, so no lead iodide forms.

35. Which of the following gases can be used for storage of fresh sample of an oil for a long time?
(a) Carbon dioxide or oxygen
(b) Nitrogen or oxygen
(c) Carbon dioxide or helium
(d) Helium or nitrogen

Answer: (d) Helium or nitrogen

1. Oil becomes rancid when it is oxidised by oxygen in air.
2. Helium and nitrogen are unreactive gases, so they protect the oil from oxidation.
3. Hence, the correct option is (d).

Teacher's Note:
a) Oxygen speeds up the spoiling of oil, so options containing oxygen are wrong.
b) Nitrogen is the gas commonly used to flush packets of chips and nuts.

36. The following reaction is used for the preparation of oxygen gas in the laboratory
2KClO₃(s) → 2KCl(s) + 3O₂(g) (heat and catalyst)
Which of the following statement(s) is(are) correct about the reaction?
(a) It is a decomposition reaction and endothermic in nature
(b) It is a combination reaction
(c) It is a decomposition reaction and accompanied by release of heat
(d) It is a photochemical decomposition reaction and exothermic in nature

Answer: (a) It is a decomposition reaction and endothermic in nature

1. One compound, KClO₃, breaks into two products and oxygen, so it is a decomposition reaction.
2. The reaction needs heating to start and absorbs heat, so it is endothermic.
3. Hence, the correct option is (a).

Teacher's Note:
a) Manganese dioxide is used as a catalyst to lower the temperature needed.
b) The reaction is not photochemical, since no light is used.

37. Which one of the following processes involve chemical reactions?
(a) Storing of oxygen gas under pressure in a gas cylinder
(b) Liquefaction of air
(c) Keeping petrol in a china dish in the open
(d) Heating copper wire in presence of air at high temperature

Answer: (d) Heating copper wire in presence of air at high temperature

1. Heating copper in air forms a new black substance, copper oxide: 2Cu + O₂ → 2CuO.
2. Storage under pressure and liquefaction of air change only the state of the gases, so they are physical changes.
3. Hence, the correct option is (d).

Teacher's Note:
a) Petrol in an open dish evaporates and burns, but that is not the reason it is chosen here.
b) Look for a new substance being formed when you decide if a process is chemical.

38. In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?
(a) 2H₂(l) + O₂(l) → 2H₂O(g)
(b) 2H₂(g) + O₂(l) → 2H₂O(l)
(c) 2H₂(g) + O₂(g) → 2H₂O(l)
(d) 2H₂(g) + O₂(g) → 2H₂O(g)

Answer: (d) 2H₂(g) + O₂(g) → 2H₂O(g)

1. Hydrogen and oxygen are gases at the reaction temperature, which is high.
2. At this high temperature the water formed is in the vapour (gas) state.
3. Hence, the correct option is (d).

Teacher's Note:
a) Water is a liquid only at room temperature; at the reaction temperature it is steam.
b) Options (a) and (b) wrongly show oxygen as a liquid.

39. Which of the following are combination reactions?
(i) 2KClO₃ → 2KCl + 3O₂ (heat)
(ii) MgO + H₂O → Mg(OH)₂
(iii) 4Al + 3O₂ → 2Al₂O₃
(iv) Zn + FeSO₄ → ZnSO₄ + Fe
(a) (i) and (iii)
(b) (iii) and (iv)
(c) (ii) and (iv)
(d) (ii) and (iii)

Answer: (d) (ii) and (iii)

1. In (ii) magnesium oxide and water combine to form one product, magnesium hydroxide.
2. In (iii) aluminium and oxygen combine to form one product, aluminium oxide.
3. Reaction (i) is decomposition and (iv) is displacement.
4. Hence, the correct option is (d).

Teacher's Note:
a) A combination reaction has two or more reactants forming a single product.
b) Reaction (i) is the reverse type of reaction, where one reactant breaks up.

40. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

40. a) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773K to form ammonia gas.

Answer:
1. Balanced equation: N₂(g) + 3H₂(g) → 2NH₃(g) (catalyst: iron, 773 K)
2. Type of reaction: combination reaction.

Teacher's Note:
a) This is the Haber process used to make ammonia for fertilisers.
b) Two elements combine to give one compound, which is the mark of a combination reaction.

40. b) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.

Answer:
1. Balanced equation: NaOH(aq) + CH₃COOH(aq) → CH₃COONa(aq) + H₂O(l)
2. Type of reaction: neutralisation reaction, which is also a double displacement reaction.

Teacher's Note:
a) A base reacting with an acid always gives salt and water.
b) The equation is already balanced with one atom of each element on both sides of the reactants and products.

40. c) Ethanol is warmed with ethanoic acid to form ethyl acetate in the presence of concentrated H₂SO₄.

Answer:
1. Balanced equation: CH₃CH₂OH(l) + CH₃COOH(l) → CH₃COOCH₂CH₃(l) + H₂O(l)
2. Type of reaction: esterification, a reaction of an alcohol with an acid that forms an ester and water.

Teacher's Note:
a) Concentrated sulphuric acid acts as a catalyst and also absorbs the water formed.
b) Ester formation is a kind of condensation reaction, and it smells sweet.

40. d) Ethene is burnt in the presence of oxygen to form carbon dioxide, water and releases heat and light.

Answer:
1. Balanced equation: C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l) + heat + light
2. Type of reaction: combustion, which is also a redox reaction.

Teacher's Note:
a) Burning of any hydrocarbon in excess oxygen gives carbon dioxide and water.
b) Combustion is exothermic, which is why it releases heat and light.

41. Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

41. a) Thermit reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

Answer:
1. Balanced equation: Fe₂O₃(s) + 2Al(s) → Al₂O₃(s) + 2Fe(l)
2. Type of reaction: displacement reaction, which is also a redox reaction.

Teacher's Note:
a) The reaction releases so much heat that the iron formed is molten.
b) Aluminium is more reactive than iron, so it takes the oxygen from iron oxide.

41. b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

Answer:
1. Balanced equation: 3Mg(s) + N₂(g) → Mg₃N₂(s)
2. Type of reaction: combination reaction.

Teacher's Note:
a) Three magnesium atoms are needed because each nitride ion carries a charge of 3-.
b) Magnesium continues to burn in nitrogen, while it burns in oxygen to form the white oxide.

41. c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

Answer:
1. Balanced equation: Cl₂(g) + 2KI(aq) → 2KCl(aq) + I₂(s)
2. Type of reaction: displacement reaction, since chlorine displaces iodine from potassium iodide.

Teacher's Note:
a) Chlorine is more reactive than iodine, which is why it can take its place.
b) The solid iodine formed gives a brown colour to the solution.

41. d) Ethanol is burnt in air to form carbon dioxide, water and releases heat.

Answer:
1. Balanced equation: C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l) + heat
2. Type of reaction: combustion, which is also a redox reaction.

Teacher's Note:
a) Ethanol has two carbon atoms, so it gives two molecules of carbon dioxide.
b) Combustion of ethanol is used as a fuel in many countries.

42. Complete the missing components/variables given as x and y in the following reactions

42. a) Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(x) + 2KNO₃(y)

Answer:
1. x = (s), because lead iodide is the yellow solid precipitate.
2. y = (aq), because potassium nitrate remains dissolved in the solution.

Teacher's Note:
a) The precipitate is always written with the solid symbol (s) in the equation.
b) Most nitrates and potassium salts stay dissolved, so they carry (aq).

42. b) Cu(s) + 2AgNO₃(aq) → Cu(NO₃)₂(aq) + x(s)

Answer:
x = 2Ag, because copper displaces two silver atoms from silver nitrate.

Teacher's Note:
a) The product x is solid silver, which is deposited on the copper strip.
b) Balance silver on both sides: two Ag on the right match 2AgNO₃ on the left.

42. c) Zn(s) + H₂SO₄(aq) → ZnSO₄(x) + H₂(y)

Answer:
1. x = (aq), because zinc sulphate is soluble in water.
2. y = (g), because hydrogen is released as a gas.

Teacher's Note:
a) Bubbles seen in this reaction are the hydrogen gas formed.
b) Zinc sulphate solution is colourless and can be crystallised.

42. d) CaCO₃(s) ^x→ CaO(s) + CO₂(g)

Answer:
x = heat (Δ), because calcium carbonate decomposes only when it is heated strongly.

Teacher's Note:
a) The arrow with x printed above it shows the condition that is needed.
b) Limestone is heated in a lime kiln to make quick lime.

43. Which among the following changes are exothermic or endothermic in nature?
(a) Decomposition of ferrous sulphate
(b) Dilution of sulphuric acid
(c) Dissolution of sodium hydroxide in water
(d) Dissolution of ammonium chloride in water

Answer:
1. (a) Decomposition of ferrous sulphate is endothermic, since heat is absorbed.
2. (b) Dilution of sulphuric acid is exothermic, since heat is released.
3. (c) Dissolution of sodium hydroxide is exothermic, since heat is released.
4. (d) Dissolution of ammonium chloride is endothermic, since heat is absorbed.

Teacher's Note:
a) Dissolving ammonium chloride in water feels cold, which shows heat absorption.
b) Always add concentrated acid slowly to water, since dilution releases much heat.

44. Write the balanced chemical equations for the following reactions

44. a) Sodium carbonate on reaction with hydrochloric acid in equal molar concentrations gives sodium chloride and sodium hydrogencarbonate.

Answer:
Na₂CO₃(aq) + HCl(aq) → NaCl(aq) + NaHCO₃(aq)

Teacher's Note:
a) Only one molecule of HCl is used, so only half of the sodium carbonate is converted.
b) The sodium hydrogencarbonate formed is the baking soda used in cooking.

44. b) Sodium hydrogencarbonate on reaction with hydrochloric acid gives sodium chloride, water and liberates carbon dioxide.

Answer:
NaHCO₃(aq) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

Teacher's Note:
a) Carbon dioxide released here makes baking soda fizz with vinegar or lemon juice.
b) Every atom is balanced in this equation with one molecule on each side.

44. c) Copper sulphate on treatment with potassium iodide precipitates cuprous iodide (Cu₂ I₂), liberates iodine gas and also forms potassium sulphate.

Answer:
2CuSO₄(aq) + 4KI(aq) → Cu₂I₂(s) + I₂(s) + 2K₂SO₄(aq)

Teacher's Note:
a) Copper(II) is reduced to copper(I), while iodide is oxidised to iodine.
b) The iodine liberated is a solid, not a gas, under these conditions, so its state is written as (s).

45. Identify the reducing agent in the following reactions

[Figure: Four equations labelled (a) 4NH₃ + 5O₂ → 4NO + 6H₂O; (b) H₂O + F₂ → HF + HOF; (c) Fe₂O₃ + 3CO → 2Fe + 3CO₂; (d) 2H₂ + O₂ → 2H₂O]

Answer:
1. (a) NH₃, because nitrogen goes from -3 to +2 and is oxidised.
2. (b) H₂O, because oxygen goes from -2 to 0 and is oxidised.
3. (c) CO, because carbon goes from +2 to +4 and is oxidised.
4. (d) H₂, because hydrogen goes from 0 to +1 and is oxidised.

Teacher's Note:
a) The reducing agent is the substance that is itself oxidised.
b) Oxidation numbers of each element on both sides decide which reactant is the reducing agent.
c) In reaction (b), water is the reducing agent even though fluorine is the more reactive partner.

46. Identify the oxidising agent (oxidant) in the following reactions

[Figure: Six equations labelled (a) Pb₃O₄ + 8HCl → 3PbCl₂ + Cl₂ + 4H₂O; (b) 2Mg + O₂ → 2MgO; (c) CuSO₄ + Zn → Cu + ZnSO₄; (d) V₂O₅ + 5Ca → 2V + 5CaO; (e) 3Fe + 4H₂O → Fe₃O₄ + 4H₂; (f) CuO + H₂ → Cu + H₂O]

Answer:
1. (a) Pb₃O₄, because lead is reduced from +8/3 to +2.
2. (b) O₂, because oxygen is reduced from 0 to -2.
3. (c) CuSO₄, because copper(II) is reduced to copper metal.
4. (d) V₂O₅, because vanadium is reduced from +5 to 0.
5. (e) H₂O, because hydrogen is reduced from +1 to 0.
6. (f) CuO, because copper is reduced from +2 to 0.

Teacher's Note:
a) The oxidising agent is the substance that gets reduced in the reaction.
b) Look for the element whose oxidation number goes down; that species is the oxidant.
c) Reaction (a) uses HCl as a reactant, but it is the oxidant that the question asks about.

47. A solution of potassium chloride when mixed with silver nitrate solution, an insoluble white substance is formed. Write the chemical reaction involved and also mention the type of the chemical reaction?

Answer:
1. KCl(aq) + AgNO₃(aq) → AgCl(s) + KNO₃(aq)
2. Type of reaction: double displacement reaction, which is also a precipitation reaction.

Teacher's Note:
a) Silver chloride is the white precipitate formed.
b) Silver chloride turns grey in sunlight, which is why it is stored in dark bottles.

48. Ferrous sulphate decomposes with the evolution of a gas having a characteristic odour of burning sulphur. Write the chemical reaction involved and identify the type of reaction.

Answer:
1. 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
2. Type of reaction: thermal decomposition reaction.

Teacher's Note:
a) The gas with the smell of burning sulphur is sulphur dioxide.
b) Sulphur trioxide is also formed and forms a fuming acid in water.

49. Why do fire flies glow at night?

Answer:
1. Fire flies glow because a chemical reaction takes place in their body in which the substance luciferin reacts with oxygen in the presence of the enzyme luciferase.
2. In this reaction, chemical energy is converted into light energy, and very little heat is released.

Teacher's Note:
a) This natural light production is called bioluminescence.
b) The light produced is called cold light because almost no heat is given out.

50. Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?

Answer:
1. Plucked grapes ferment when they are crushed and kept in the presence of yeast, without air, at a suitable temperature.
2. Fermentation is a chemical change, because the sugar in the grapes is converted into alcohol and carbon dioxide, which are new substances.

Teacher's Note:
a) Yeast from the skin of the grape or the air is responsible for fermentation.
b) Grapes on the plant are protected by the living plant tissue and do not ferment.
c) Wine is made by this same fermentation process.

51. Which among the following are physical or chemical changes?
(a) Evaporation of petrol
(b) Burning of Liquefied Petroleum Gas (LPG)
(c) Heating of an iron rod to red hot.
(d) Curdling of milk
(e) Sublimation of solid ammonium chloride

Answer:
1. (a) Evaporation of petrol is a physical change.
2. (b) Burning of LPG is a chemical change.
3. (c) Heating an iron rod to red hot is a physical change.
4. (d) Curdling of milk is a chemical change.
5. (e) Sublimation of ammonium chloride is a physical change.

Teacher's Note:
a) Changes in which no new substance forms are physical changes.
b) Curdling forms a new substance, lactic acid curd, so it cannot be reversed by cooling.

52. During the reaction of some metals with dilute hydrochloric acid, following observations were made.
(a) Silver metal does not show any change
(b) The temperature of the reaction mixture rises when aluminium (Al) is added.
(c) The reaction of sodium metal is found to be highly explosive
(d) Some bubbles of a gas are seen when lead (Pb) is reacted with the acid.
Explain these observations giving suitable reasons.

Answer:
1. (a) Silver is less reactive than hydrogen, so it cannot displace hydrogen from dilute hydrochloric acid.
2. (b) The reaction of aluminium with the acid is exothermic, so the temperature of the mixture rises.
3. (c) Sodium is very reactive; the heat from its reaction ignites the hydrogen gas released, causing an explosion.
4. (d) Lead reacts slowly with the acid and releases hydrogen gas, which appears as bubbles.

Teacher's Note:
a) The reactivity of a metal decides how fast it displaces hydrogen from acids.
b) Sodium must be handled with care because of this violent reaction.
c) Lead also forms a coating of lead chloride, which slows the reaction.

53. A substance X, which is an oxide of a group 2 element, is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify X and also write the chemical reactions involved.

Answer:
1. X is calcium oxide (quick lime, CaO).
2. CaO(s) + H₂O(l) → Ca(OH)₂(aq)
3. Ca(OH)₂(aq) → Ca²⁺(aq) + 2OH^-(aq), which makes the solution basic and turns red litmus blue.

Teacher's Note:
a) Calcium is the group 2 element present in bones and teeth.
b) Quick lime is used in cement and in the making of mortar.
c) Dissolution of calcium hydroxide releases hydroxide ions, which are the reason for basicity.

54. Write a balanced chemical equation for each of the following reactions and also classify them.

54. a) Lead acetate solution is treated with dilute hydrochloric acid to form lead chloride and acetic acid solution.

Answer:
1. Balanced equation: Pb(CH₃COO)₂(aq) + 2HCl(aq) → PbCl₂(s) + 2CH₃COOH(aq)
2. Type of reaction: double displacement reaction.

Teacher's Note:
a) Lead chloride is a white precipitate, so it is a precipitation reaction too.
b) Acetate ions and chloride ions exchange partners in this reaction.

54. b) A piece of sodium metal is added to absolute ethanol to form sodium ethoxide and hydrogen gas.

Answer:
1. Balanced equation: 2Na(s) + 2C₂H₅OH(l) → 2C₂H₅ONa(aq) + H₂(g)
2. Type of reaction: displacement reaction, since sodium displaces hydrogen from ethanol.

Teacher's Note:
a) The reaction is slower than with water because ethanol is a weaker source of hydrogen ions.
b) Sodium ethoxide is a strong base used in organic synthesis.

54. c) Iron (III) oxide on heating with carbon monoxide gas reacts to form solid iron and liberates carbon dioxide gas.

Answer:
1. Balanced equation: Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
2. Type of reaction: redox reaction, in which iron oxide is reduced and carbon monoxide is oxidised.

Teacher's Note:
a) This reaction is used in the blast furnace to extract iron from its ore.
b) Carbon monoxide is the reducing agent in this reaction.

54. d) Hydrogen sulphide gas reacts with oxygen gas to form solid sulphur and liquid water.

Answer:
1. Balanced equation: 2H₂S(g) + O₂(g) → 2S(s) + 2H₂O(l)
2. Type of reaction: redox reaction, in which hydrogen sulphide is oxidised and oxygen is reduced.

Teacher's Note:
a) Sulphur is deposited as a yellow solid in this reaction.
b) Hydrogen sulphide is a gas with a smell of rotten eggs.

55. Why do we store silver chloride in dark coloured bottles?

Answer:
1. Silver chloride decomposes in sunlight: 2AgCl(s) → 2Ag(s) + Cl₂(g).
2. Dark coloured bottles stop light from reaching the chemical, so it does not decompose.

Teacher's Note:
a) The grey colour seen on exposure is due to the silver formed.
b) This is an example of a photochemical decomposition reaction.

56. Balance the following chemical equations and identify the type of chemical reaction.

Answer:
No chemical equations are printed with this question in the paper, so there is nothing to balance here.

Teacher's Note:
a) Check the source for the missing equations before this question is used in a test.
b) Balancing an equation means equal numbers of each atom on both sides, as shown in question 2.

57. A magnesium ribbon is burnt in oxygen to give a white compound X accompanied by emission of light. If the burning ribbon is now placed in an atmosphere of nitrogen, it continues to burn and forms a compound Y.
(a) Write the chemical formulae of X and Y.
(b) Write a balanced chemical equation, when X is dissolved in water.

57. a) Write the chemical formulae of X and Y.

Answer:
1. X = MgO (magnesium oxide).
2. Y = Mg₃N₂ (magnesium nitride).

Teacher's Note:
a) Magnesium nitride is a yellowish solid.
b) Nitride ions carry a 3- charge, so the formula needs three magnesium ions.

57. b) Write a balanced chemical equation, when X is dissolved in water.

Answer:
MgO(s) + H₂O(l) → Mg(OH)₂(aq)

Teacher's Note:
a) Magnesium hydroxide is a weak base, so its solution turns red litmus blue slightly.
b) Each side has one magnesium atom, two oxygen atoms and two hydrogen atoms, so the equation is balanced.

58. Zinc liberates hydrogen gas when reacted with dilute hydrochloric acid, whereas copper does not. Explain why?

Answer:
1. Zinc is above hydrogen in the reactivity series, so it displaces hydrogen from dilute hydrochloric acid: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g).
2. Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid.

Teacher's Note:
a) Only metals above hydrogen in the reactivity series release hydrogen from acids.
b) Copper is less reactive, so it forms no bubbles in dilute acid.

59. A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

Answer:

59. a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

Answer:
1. Silver reacts with hydrogen sulphide present in the air to form a black coating of silver sulphide.
2. The phenomenon is called tarnishing, which is a form of corrosion.

Teacher's Note:
a) Air pollution and sulphur compounds speed up tarnishing.
b) Toothpaste contains a mild abrasive that rubs off the black layer.

59. b) Name the black substance formed and give its chemical formula.

Answer:
The black substance is silver sulphide, with the formula Ag₂S.

Teacher's Note:
a) Silver sulphide is a black solid that sticks to the surface of silver.
b) The formula has two silver atoms for every sulphur atom.

60. On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed
(a) Write a balanced chemical equation of the reaction.
(b) Identity the brown gas X evolved.
(c) Identity the type of reaction.
(d) What could be the pH range of aqueous solution of the gas X?

60. a) Write a balanced chemical equation of the reaction.

Answer:
2Cu(NO₃)₂(s) → 2CuO(s) + 4NO₂(g) + O₂(g)

Teacher's Note:
a) The four nitrogen atoms in 2Cu(NO₃)₂ become four NO₂ molecules.
b) Oxygen count: 12 on the left and 2 + 8 + 2 = 12 on the right.

60. b) Identity the brown gas X evolved.

Answer:
The brown gas X is nitrogen dioxide (NO₂).

Teacher's Note:
a) Nitrogen dioxide is a reddish-brown gas with a sharp smell.
b) It is a toxic gas that causes breathing problems.

60. c) Identity the type of reaction.

Answer:
The reaction is a thermal decomposition reaction, because one compound breaks down into three products on heating.

Teacher's Note:
a) Heat is needed to break the bonds in copper nitrate.
b) Decomposition reactions produce two or more products from one reactant.

60. d) What could be the pH range of aqueous solution of the gas X?

Answer:
The aqueous solution of nitrogen dioxide is acidic, because it forms nitric acid and nitrous acid in water, so its pH is less than 7.

Teacher's Note:
a) An acid solution turns blue litmus red and has pH below 7.
b) NO₂ dissolves in water to form acids, which causes acid rain.

61. Give the characteristic tests for the following gases
(a) CO₂
(b) SO₂
(c) O₂
(d) H₂

Answer:
1. CO₂: passing the gas through lime water turns it milky, due to formation of calcium carbonate.
2. SO₂: it turns acidified potassium dichromate solution from orange to green.
3. O₂: a glowing splinter bursts into flame when put in the gas.
4. H₂: a burning splinter produces a "pop" sound when brought near the gas.

Teacher's Note:
a) Lime water is the reagent used to test for carbon dioxide.
b) Sulphur dioxide also decolourises purple potassium permanganate solution.
c) Oxygen is the only one of these gases that relights a glowing splinter.
d) Hydrogen is highly flammable, so the test must be done carefully.

62. What happens when a piece of
(a) zinc metal is added to copper sulphate solution?
(b) aluminium metal is added to dilute hydrochloric acid?
(c) silver metal is added to copper sulphate solution?
Also, write the balanced chemical equation if the reaction occurs

62. a) Zinc metal is added to copper sulphate solution.

Answer:
1. The blue colour of the solution fades and reddish-brown copper is deposited on the zinc.
2. Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)

Teacher's Note:
a) Zinc is more reactive than copper, so it displaces copper from its salt.
b) The fading blue colour shows that copper ions are being used up.

62. b) Aluminium metal is added to dilute hydrochloric acid.

Answer:
1. Aluminium dissolves and hydrogen gas bubbles out of the solution.
2. 2Al(s) + 6HCl(aq) → 2AlCl₃(aq) + 3H₂(g)

Teacher's Note:
a) Aluminium is above hydrogen in the reactivity series, so it releases hydrogen.
b) The reaction is exothermic, so the mixture becomes warm.

62. c) Silver metal is added to copper sulphate solution.

Answer:
No reaction takes place, because silver is less reactive than copper and cannot displace it from copper sulphate.

Teacher's Note:
a) The blue colour of the copper sulphate solution remains unchanged.
b) No equation is written because no reaction occurs.

63. What happens when zinc granules are treated with dilute solution of H₂SO₄, HCl, HNO₃, NaCl and NaOH, also write the chemical equations if reaction occurs.

63. a) Zinc granules with dilute H₂SO₄.

Answer:
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g). Hydrogen gas bubbles out.

Teacher's Note:
a) Zinc is above hydrogen in the reactivity series, so it releases hydrogen from any dilute acid.
b) The bubbles can be tested with a burning splinter, which gives a pop.

63. b) Zinc granules with dilute HCl.

Answer:
Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). Hydrogen gas bubbles out.

Teacher's Note:
a) Two HCl molecules are needed to take up both hydrogens of the acid.
b) The zinc chloride formed stays dissolved in water.

63. c) Zinc granules with dilute HNO₃.

Answer:
3Zn(s) + 8HNO₃(aq) → 3Zn(NO₃)₂(aq) + 2NO(g) + 4H₂O(l). Hydrogen gas is not evolved; nitric acid is reduced to nitrogen monoxide instead.

Teacher's Note:
a) Nitric acid is a strong oxidising agent, so it does not release hydrogen like other dilute acids do.
b) The brown fumes seen in this reaction come from the nitrogen monoxide reacting with air.

63. d) Zinc granules with NaCl.

Answer:
No reaction takes place, because zinc is less reactive than sodium and cannot displace it from sodium chloride.

Teacher's Note:
a) Sodium is more reactive than zinc, so zinc cannot take its place.
b) No equation is written because no reaction occurs.

63. e) Zinc granules with NaOH.

Answer:
Zn(s) + 2NaOH(aq) → Na₂ZnO₂(aq) + H₂(g). Zinc reacts with hot concentrated sodium hydroxide, forming sodium zincate and hydrogen gas.

Teacher's Note:
a) Zinc is one of the few metals that react with a strong alkali, because it forms an amphoteric oxide.
b) The reaction is slow with dilute sodium hydroxide, so the answer is given for the concentrated form.

64. On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained.
(a) Write a balanced chemical equation of the reaction involved
(b) What other name can be given to this precipitation reaction?
(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?

64. a) Write a balanced chemical equation of the reaction involved.

Answer:
BaCl₂(aq) + Na₂SO₃(aq) → BaSO₃(s) + 2NaCl(aq)

Teacher's Note:
a) The white precipitate formed is barium sulphite.
b) Two chloride ions on the right come from the one barium chloride unit and two sodium ions.

64. b) What other name can be given to this precipitation reaction?

Answer:
It can also be called a double displacement reaction.

Teacher's Note:
a) Every precipitation reaction of this type exchanges ions between two compounds.
b) Barium and sodium swap their anions to form the two new compounds.

64. c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?

Answer:
1. The white precipitate is barium sulphite, which dissolves in dilute hydrochloric acid.
2. BaSO₃(s) + 2HCl(aq) → BaCl₂(aq) + H₂O(l) + SO₂(g), so the solid turns into a soluble salt and the gas escapes.

Teacher's Note:
a) Barium sulphate, by contrast, does not dissolve in dilute acid.
b) Sulphur dioxide is released as a gas, which has a suffocating smell.

65. You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO₃, ZnCl₂ and H₂O. In which of the above containers these solutions can be kept?

Answer:
1. Dilute HCl: copper container, because copper is below hydrogen and does not react with it; aluminium would react.
2. ZnCl₂: copper container, because copper cannot displace zinc; aluminium would displace zinc (2Al + 3ZnCl₂ → 2AlCl₃ + 3Zn).
3. H₂O: either container can hold water.
4. Dilute HNO₃: neither container, because both copper and aluminium react with dilute nitric acid.

Teacher's Note:
a) Aluminium is more reactive than both zinc and hydrogen, which is why it reacts with two of the solutions.
b) Copper is an unreactive metal and suits most solutions except nitric acid.
c) Nitric acid must be stored in glass or plastic containers instead.

Question. Which of the following statements is/are true ?
(A) The total mass of the system remains same in a chemical change.
(B) A chemical change is permanent and irreversible.
(C) A physical change is temporary and reversible.
(D) All of these
Answer : D

Question. Neutralization reaction is an example of -
(A) exothermic reaction
(B) endothermic reaction
(C) oxidation reaction
(D) none of these
Answer : A

Question. The term ‘rancidity‘ represents -
(A) acid rain
(B) oxidation of fatty food
(C) rottening of fruit
(D) fading of coloured clothes in the sun.
Answer : B

Question. What will happen on putting dilute HCl on iron powder ?
(A) Hydrogen gas and FeCl₂ will form
(B) Chlorine gas and FeCl₂ will form
(C) FeCl₂ and H₂O will form
(D) Will be no reaction
Answer : A

Question. The equation - Cu + xHNO₃ → Cu(NO₃)₂ + yNO₂ + 2H₂O
The values of x and y are -
(A) 3 and 5
(B) 8 and 6
(C) 4 and 2
(D) 7 and 1
Answer : C

Question. Which of the following statements is/are correct ?
(A) A chemical equation tells us about the substances involved in a reaction.
(B) A chemical equation informs us about the symbols and formulae of the substances involved in a reaction.
(C) A chemical equation tells us about the atoms or molecules of the reactants and products involved in a reaction.
(D) All are correct.
Answer : D

Question. Which of the following equations is representing combination of two elements ?
(A) CaO + CO₂ → CaCO₃
(B) 4Na + O₂ → 2Na₂O
(C) SO₂ + 1/2 O₂ → SO₃
(D) 2Na + 2H₂O → 2NaOH + H₂
Answer : B

Question. Which of the following statements is correct for oxidation reaction ?
(A) Gain or addition of electronegative radical or element
(B) Removal of hydrogen atom
(C) Removal or loss of electropositive radical or element
(D) All the above statements are correct
Answer : D

Question. In the process of burning of magnesium in air, magnesium undergoes -
(A) reduction
(B) sublimation
(C) oxidation
(D) all of these
Answer : C

Question. A substance which oxidises itself and reduces other is known as -
(A) an oxidising agent
(B) a reducing agent
(C) Both of these
(D) None of these
Answer : B

Question. A redox reaction is one in which -
(A) both the substances are reduced.
(B) both the substances are oxidised.
(C) an acid is neutralised by the base.
(D) one substance is oxidised, while the other is reduced.
Answer : D

Question. In the reaction xPb (NO₃)₂ →^Heat  yPbO + zNO₂ + O₂ x,y and z are -
(A) 1,1,2
(B) 2,2,4
(C) 1,2,4
(D) 4,2,2
Answer : B

Question. In the reaction FeSO₄ + x → Na₂SO₄ + Fe(OH)₂ , x is -
(A) Na₂SO₄
(B) H₂SO₄
(C) NaOH
(D) none of these
Answer : C

Question. In the reaction Mg⁺ Cl₂ → MgCl₂ Chlorine may be regarded as -
(A) an oxidising agent
(B) a reducing agent
(C) a catalyst
(D) providing an inert medium
Answer : A

Question. When crystals of FeSO₄ are strongly heated the residue obtained is:
(A) red in colour
(B) blue in colour
(C) green in colour
(D) colourless
Answer : A

Question.

From the above experimental setup, what precipiate we obtain and what is the colour of obtained precipitate ?
(A) Lead Iodide-Yellow
(B) Potassium Nitrate- Yellow
(C) Lead Iodide-Red
(D) Potassium Nitrate- White
Answer : A

Question. The antioxidant which is used to prevent rancidity in foods is
(A) butylated hydroxyl anisole.
(B) sodium hydroxide.
(C) sodium carbonate.
(D) methylated hydroxyl anisole
Answer : A

Question. Corrosion of silver occurs due to the formation of
(A) Ag₂O
(B) AgCl
(C) Ag₂CO₃
(D) Ag₂S
Answer : D

Question. Rancidity can be prevented by -
(A) adding antioxidants
(B) packging oily food in nitrogen gas
(C) both (A) and (B)
(D) none of these
Answer : C

Question. Why should a magnesium ribbon be cleaned before burning in air ?
Answer : To remove the layer of MgO.

Question. We need to balance a chemical equation. Give reason to justify the statement.
Answer : To obey law of conservation of mass.

Question. What is observed when a solution of potassium iodide solution is added to a solution of lead nitrate? Name the type of reaction. Write a balanced chemical equation to represent the above chemical reaction.
Answer : Yellow precipitate is formed. It is precipitation reaction.

Question. Which products will be obtained when lead nitrate is heated simply? Write balanced chemical equation for the reaction.
Answer : 

Question. Why does the color of copper sulphate solution change when an iron nail is dipped in it?
Answer : It is because displacement reaction takes place.

Question. In electrolysis of water, why is the volume of gas collected over one electrode double that of gas collected over the other electrode?
Answer : It is because water contains hydrogen and oxygen in the ratio of 2 : 1.

Question. Giving an example list two information which make a chemical equation more useful (informative).
Answer : (i) Physical state of reactants must be mentioned
(ii) Condition in which reaction takes place are written on the arrow head

Question. What does one mean by exothermic and endothermic reactions ? Give examples.
Answer : Exothermic reactions : heat is evolved
Example : (i) C (s) + O₂ (g) → CO₂ (g) + Heat
(ii) N₂ (g) + 3H₂ (g) → 2NH₃ (g) + Heat
Endothermic reactions : heat is absorbed
Examples :(i) C (s) + 2S (s) → CS₂ (l) – Heat
(ii) N₂ (g) + O₂ (g) → 2NO(g) – Heat

Answer : (i) Combination reaction
(ii) Double displacement reaction (Precipitation reaction)
(iii) Thermal Decomposition reaction.

Question. Ferrous sulphate crystals are heated in a dry boiling tube.
(i) List any two observations.
(ii) Name the type of chemical reaction taking place.
(iii) ‘Write the chemical equation for the reaction.
Answer : (i)Two observations are
(a) change in colour (b) Smell of burning sulphur
(ii) Decomposition reaction

Question. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer : In decomposition reaction, a compound is broken down into simpler compounds or elements, e.g.

Question. The following diagram displays a chemical reaction. Observe carefully and answer the following questions

(a) Identify the type of chemical reaction that will take place and define it.
(b) How will the color of the salt change? Write the chemical equation of the reaction that takes place.
(c) Mention one commercial use of this salt.
Answer : (a) Photochemical decomposition reaction.
(b) The colour of salt will change from white to grey.
(c) in photography

Question. What is rancidity? Mention any two ways by which rancidity can be prevented.
Answer : Spoilage of food containing oil.
Prevention from rancidity:-
(i) Antioxidants
(ii) airtight container in refrigerator.

Question. A solution of substance ‘X’ is used for white washing. What is the substance ‘X’? State the chemical reaction of ‘X’ with water.
Answer : ‘X’ is calcium oxide (CaO).

Answer :