science chapter 1 solution class 10 anwser

Class 10 Science — In-text Questions & Exercises (Pages 6–16)

Class 10 Science — Question Exercise Answers

In-text questions — Set 1 (Page 6)

  1. 1. Why should a magnesium ribbon be cleaned before burning in the air?
    Solution:

    Magnesium ribbon should be cleaned before burning in air because magnesium metal reacts with atmospheric oxygen to form a layer of magnesium oxide (MgO), which is very stable. To allow the magnesium to burn and react further with oxygen during the demonstration, the oxide layer must be removed by cleaning the ribbon.

  2. 2. Write a balanced equation for the following chemical reactions.
    1. i) Hydrogen + Chlorine → Hydrogen chloride
      H2 + Cl2 → 2HCl
    2. ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
      3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
    3. iii) Sodium + Water → Sodium hydroxide + Hydrogen
      2Na + 2H2O → 2NaOH + H2
  3. 3. Write a balanced chemical equation with state symbols for the following reactions
    1. i) Solutions of barium chloride and sodium sulphate react to give insoluble barium sulphate and sodium chloride solution.
      BaCl2(aq) + Na2SO4(aq) → BaSO4(s) + 2NaCl(aq)
    2. ii) Sodium hydroxide solution reacts with hydrochloric acid solution to produce sodium chloride solution and water.
      NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)

In-text questions — Set 2 (Page 10)

  1. 1. A solution of a substance, ‘X’, is used for whitewashing.
    (i) Name the substance ‘X’ and write its formula.
    (ii) Write the reaction of the substance ‘X’ named in (i) above with water.

    Solution:

    i) The substance X used for whitewashing is quicklime or calcium oxide — CaO.

    ii) Reaction with water:
    CaO + H2O → Ca(OH)2

  2. 2. Why is the amount of gas collected in one of the test tubes in Activity 1.7 double the amount collected in the other? Name this gas.

    Solution: During electrolysis of water, two molecules of hydrogen gas and one molecule of oxygen gas are produced for every 2H2O decomposed. Therefore the volume of hydrogen collected is double that of oxygen. The gas with the greater volume is hydrogen (H2).

In-text questions — Set 3 (Page 13)

  1. 1. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

    Solution: Iron displaces copper from copper sulphate because iron is more reactive. The blue copper sulphate solution loses Cu2+ ions as they are displaced, and iron sulphate (FeSO4) forms. The reaction is:
    Fe + CuSO4 → FeSO4 + Cu

  2. 2. Give an example of a double displacement reaction other than the one given in Activity 1.10.

    Solution: Reaction between silver nitrate and sodium chloride which produces white silver chloride precipitate:
    AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)

  3. 3. Identify the substances that are oxidised and that are reduced in the following equations.

    Solution:

    • 4Na(s) + O2(g) → 2Na2O(s) — Sodium (Na) is oxidised (it loses electrons / gains oxygen).
    • CuO(s) + H2(g) → Cu(s) + H2O(l) — Copper in CuO is reduced to Cu, hydrogen is oxidised to H2O.

Exercise Questions (Pages 14–16)

1.

Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO2(g)

Options:

  1. Lead is getting reduced
  2. Carbon dioxide is getting oxidised
  3. Carbon is getting oxidised
  4. Lead oxide is getting reduced

Answer: (i) (a) and (b) are incorrect.

Explanation: (a) is incorrect because oxygen is being removed from lead oxide to form lead (so lead oxide is reduced); (b) is incorrect because the oxygen removed from lead is added to carbon to form CO2, so CO2 is the oxidised form of carbon.

2.

Fe2O3 + 2Al → Al2O3 + 2Fe

Answer: Displacement reaction (single displacement).

Explanation: Aluminium (more reactive) displaces iron from its oxide to form aluminium oxide and elemental iron.

3.

What happens when dilute hydrochloric acid is added to iron filings?

Answer: Hydrogen gas and iron chloride are produced.

Explanation: 2HCl + Fe → FeCl2 + H2

4.

Question: What is a balanced chemical equation? Why should a chemical equation be balanced?

Solution: A balanced chemical equation has an equal number of each type of atom on both sides of the equation. Chemical equations must be balanced to obey the Law of Conservation of Mass. Balancing is usually done by inspection (trial-and-error).

5.

Translate the following statements into chemical equations and balance them.

  1. Hydrogen gas combines with nitrogen to form ammonia.
    Balanced: 3H2 + N2 → 2NH3
  2. Hydrogen sulphide gas burns in the air to give water and sulphur dioxide.
    Balanced: 2H2S + 3O2 → 2H2O + 2SO2
  3. Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
    Balanced: 3BaCl2 + Al2(SO4)3 → 2AlCl3 + 3BaSO4
  4. Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
    Balanced: 2K + 2H2O → 2KOH + H2

6.

Balance the following chemical equations.

  1. 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
  2. 2NaOH + H2SO4 → Na2SO4 + 2H2O
  3. NaCl + AgNO3 → AgCl + NaNO3
  4. BaCl2 + H2SO4 → BaSO4 + 2HCl

7.

Write the balanced chemical equation for the following reactions.

  1. Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
    2Ca(OH)2 + 2CO2 → 2CaCO3 + 2H2O
  2. Zinc + Silver nitrate → Zinc nitrate + Silver
    Zn + 2AgNO3 → Zn(NO3)2 + 2Ag
  3. Aluminium + Copper chloride → Aluminium chloride + Copper
    2Al + 3CuCl2 → 2AlCl3 + 3Cu
  4. Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride
    BaCl2 + K2SO4 → BaSO4 + 2KCl

8.

Write a balanced equation and identify the type of reaction.

  1. 2KBr + BaI2 → 2KI + BaBr2 — Double displacement
  2. ZnCO3 → ZnO + CO2 — Decomposition
  3. H2 + Cl2 → 2HCl — Combination
  4. Mg + 2HCl → MgCl2 + H2 — Displacement

9.

Question: What is meant by exothermic and endothermic reactions? Give examples.

Solution: Endothermic reactions absorb energy from the surroundings (e.g. photosynthesis, melting of ice). Exothermic reactions release energy to the surroundings (e.g. combustion, many oxidation reactions).

10.

Question: Why is respiration considered to be an exothermic reaction?

Solution: Respiration breaks down glucose in the presence of oxygen to produce carbon dioxide, water and energy (heat), which keeps body temperature stable. Equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

11.

Question: Why are decomposition reactions called the opposite of combination reactions? Give examples.

Solution: Combination forms a larger molecule from smaller ones; decomposition splits a larger molecule into smaller ones. Examples:
ZnCO3 → ZnO + CO2
CaCO3 + heat → CaO + CO2
2HgO → 2Hg + O2

12.

Question: Write one decomposition equation each where energy is supplied as heat, light or electricity.

Solution:

  1. Thermal (heat): 2KClO3 + heat → 2KCl + 3O2
  2. Electrolysis (electricity): molten NaCl → Na + Cl2
  3. Photolysis (light): 2H2O2 + light → 2H2O + O2

13.

Question: Difference between displacement and double displacement reactions with examples.

Solution: In displacement, a more reactive element displaces a less reactive one from its compound (e.g. Mg + 2HCl → MgCl2 + H2). In double displacement, ions are exchanged between two compounds (e.g. 2KBr + BaI2 → 2KI + BaBr2).

14.

Question: In refining silver, recovery of silver from silver nitrate solution involves displacement by copper metal. Write the reaction.

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

15.

Question: What is a precipitation reaction? Give examples.

Solution: When two soluble salt solutions react and an insoluble solid forms, that solid is the precipitate. Examples:
CdSO4(aq) + K2S(aq) → CdS(s) + K2SO4(aq)
2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)

16.

Question: Explain oxidation and reduction in terms of gain of oxygen with two examples each.

Solution:

(a) Oxidation — addition of oxygen to an element. Examples:
4Na + O2 → 2Na2O
H2S + O2 → H2O + SO2

(b) Reduction — removal of oxygen from a compound. Examples:
CuO + H2 → Cu + H2O
2HgO → 2Hg + O2

17.

Question: A shiny brown element ‘X’ on heating in air becomes black. Name the element and the black compound formed.

Solution: Element is copper (Cu). On heating it forms copper oxide (CuO): 2Cu + O2 → 2CuO

18.

Question: Why do we apply paint on iron articles?

Solution: Paint prevents contact of iron with air and moisture, thereby protecting it from rusting (formation of iron oxide).

19.

Question: Oil and fat containing food items are flushed with nitrogen. Why?

Solution: Nitrogen is inert and prevents oxidation of oils and fats (rancidity). Flushing with nitrogen therefore prolongs shelf life and prevents off-flavours.

20.

Question: Explain the following terms with one example each.

(a) Corrosion — gradual oxidation of a metal to form more stable compounds (e.g. rusting of iron to form iron oxide).
(b) Rancidity — aerial oxidation of fats/oils producing unpleasant smell and taste; refrigeration slows this process.