CBSE Class 10 Science Chemical reactions and equations Sure Shot Questions Set 01

Question. In the equation, NaOH + HNO₃ → NaNO₃ + H₂O nitric acid is acting as
(A) an oxidizing agent
(B) an acid
(C) a nitrating agent
(D) a dehydrating agent
Answer. B

Question. Which of the following metal is protected by a layer of its oxide ?
(A) Copper
(B) Silver
(C) Iron
(D) Aluminium
Answer. D

Question. When copper turnings are added to silver nitrate solution, a blue colored solution is formed after some time. It is because, copper
(A) oxidizes silver from the solution
(B) forms a blue colored complex with AgNO₃
(C) is oxidized to Cu²⁺
(D) is reduced to Cu²⁺
Answer. C

Question. In the reaction, 2H₂S + SO₂ → 3S + 2H₂O
(A) H₂S has been oxidized
(B) SO₂ has been oxidized
(C) H₂S is the oxidizing agent
(D) SO₂ is the reducing agent
Answer. A

Question. In the reaction xPb(NO₃)₂ Heat-yPbO(s) + zNO₂(g) + O₂(g) Values of x, y and z respectively are
(A) 1, 1, 2
(B) 2, 2, 4
(C) 1, 2, 4
(D) 4, 2, 2
Answer. B

Question. In the balanced equation Na₂CO₃ + xHCl → 2NaCl + CO₂ + H₂O. The value of x is
(A) 1
(B) 2
(C) 3
(D) 4
Answer. B

Question. Which of the following statement is incorrect ?
(A) In oxidation, oxygen is added to a substance
(B) In reduction, hydrogen is added to a substance.
(C) Oxidizing agent is oxidized
(D) Reducing agent is oxidized
Answer. C

Question. Which of the statement about the following reaction is correct ?
ZnO + CO → Zn + CO₂
(A) ZnO is being reduced
(B) CO is being reduced
(C) CO₂ is being oxidized
(D) ZnO is being oxidized
Answer. A

Question. Which of the following chemical equations is an unbalanced one ?
(A) 2NaHCO₃ - Na₂CO₃ + H₂O + CO₂
(B) 2C₄H₁₀ + 12O₂ - 8CO₂ + 10H₂O
(C) 2Al + 6H₂O - 2Al(OH)₃ + 3H₂
(D) 4NH₃ + 5O₂ - 4NO + 6H₂O
Answer. B

Question. Which one of the following reaction will not take place ?
(A) Zn + FeSO₄ → ZnSO₄ + Fe
(B) 2 Al + 3 MgSO₄ → Al₂(SO₄)₃ + 3 Mg
(C) Fe + NiSO₄ → FeSO₄ + Ni
(D) Cu + 2 AgNO₃ → Cu(NO₃)₂ + 2 Ag
Answer. B

Question. Fill in the blank in the chemical equation : ……. + 4HNO₃ → Cu(NO₃)₂ + 2NO₂ + 2H₂O
(A) Cu
(B) 2Cu
(C) CuO
(D) 2CuO
Answer. A

Question. When lead nitrate reacts with potassium iodide, yellow precipitate of
(A) PbI₂ is formed
(B) KNO₃ is formed
(C) Pb(NO₃)₂ is formed
(D) PbIO₃ is formed
Answer. A

Question. When ferrous hydroxide reacts with hydrochloric acid, ….. and H₂O are produced
(A) FeCl₃
(B) FeCl₂
(C) FeCl₄
(D) FeCl
Answer. B

Question. Which of the following reactions involves the combination of two elements ?
(A) CaO + CO₂ → CaCO₃
(B) 4Na + O₂ → 2Na₂O
(C) SO₂ + 1/2 O₂ → SO₃
(D) NH₃ + HCl → NH₄Cl
Answer. B

Question. Symbol Δ used while representing some chemical changes stands for -
(A) light
(B) heat
(C) catalyst
(D) pressure
Answer. B

Question. In the balanced equation : a Fe₂O₃ + bH₂ → - cFe + dH₂O
The value of a, b, c, d are respectively –
(A) 1, 1, 2, 3
(B) 1, 1, 1, 1
(C) 1, 3, 2, 3
(D) 1, 2, 2, 3
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 the above
Answer. D

ASSIGNMENT QUESTIONS SET – 1

CHAPTER – 1

CHEMICAL REACTIONS AND EQUATIONS

Very Short Answer Type Questions

How are chemical reactions expressed in the shortest way?

Answer: By chemical equations.

What is the type of reaction in which the reactant gives simpler products?

Answer: A decomposition reaction.

What is the type of reaction in which two or more reactants combine to give a single product?

Answer: A combination reaction.

In which type of reaction does an exchange of partners take place?

Answer: A double displacement reaction.

Why are chemical equations balanced?

Answer: To obey the law of conservation of mass: the number of atoms of each element must be the same on both sides of the equation.

What symbol is used to indicate a solution made in water?

Answer: (aq)

What type of reaction occurs during the digestion of food inside our body?

Answer: A decomposition reaction (the large food molecules are broken into simpler ones by enzymes).

What type of reaction is represented by the following equation? \(\mathrm{CaCO_{3}\,}\rightarrow \mathrm{\,CaO\,+\,CO_{2}}\)

Answer: A thermal decomposition reaction.

What type of reaction occurs when silver bromide is exposed to sunlight?

Answer: A photolytic decomposition reaction: \(\mathrm{2AgBr\,}\rightarrow \mathrm{\,2Ag\,+\,Br_{2}}\)

A solution of a substance is used for whitewashing. Name the substance with its formula.

Answer: Calcium hydroxide (slaked lime), \(\mathrm{Ca(OH)_{2}}\).

Name the type of reaction which is represented by the following equation: \(\mathrm{Fe_{2}O_{3}\,+\,2Al\,}\rightarrow \mathrm{\,Al_{2}O_{3}\,+\,2Fe}\)

Answer: A displacement reaction (also a redox reaction; it is called the thermite reaction).

Balance the following equation using state symbols: \(\mathrm{Fe\,+\,HCl\,}\rightarrow \mathrm{\,FeCl_{2}\,+\,H_{2}}\)

Answer: \(\mathrm{Fe(s)\,+\,2HCl(aq)\,}\rightarrow \mathrm{\,FeCl_{2}(aq)\,+\,H_{2}(g)}\)

Express the following statement in the form of a balanced chemical equation: "Sodium reacts with water to form sodium hydroxide and hydrogen."

Answer: \(\mathrm{2Na\,+\,2H_{2}O\,}\rightarrow \mathrm{\,2NaOH\,+\,H_{2}}\)

Write the balanced chemical equation for the reaction that occurs between aluminium hydroxide and sulphuric acid forming aluminium sulphate and water.

Answer: \(\mathrm{2Al(OH)_{3}\,+\,3H_{2}SO_{4}\,}\rightarrow \mathrm{\,Al_{2}(SO_{4})_{3}\,+\,6H_{2}O}\)

What type of reaction occurs when ammonia is allowed to react with hydrogen chloride?

Answer: A combination reaction: \(\mathrm{NH_{3}\,+\,HCl\,}\rightarrow \mathrm{\,NH_{4}Cl}\)

Name the type of reaction involved when a zinc plate is dipped in a solution of copper sulphate.

Answer: A displacement reaction: \(\mathrm{Zn\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,ZnSO_{4}\,+\,Cu}\)

In the reaction \(\mathrm{CuO(s)\,+\,H_{2}(g)\,}\rightarrow \mathrm{\,Cu(s)\,+\,H_{2}O(l)}\), pick out: (i) the substance which is oxidised, (ii) the substance which is reduced, (iii) the oxidising agent, (iv) the reducing agent.

Answer: (i) Hydrogen is oxidised.
(ii) Copper oxide is reduced.
(iii) The oxidising agent is CuO.
(iv) The reducing agent is H₂.

What chemical reaction is involved in the corrosion of iron?

Answer: Oxidation. Iron reacts with oxygen and moisture to form hydrated iron(III) oxide (rust), \(\mathrm{Fe_{2}O_{3}.xH_{2}O}\).

Aluminium metal when burnt in air forms aluminium oxide. Write the balanced chemical equation for the reaction.

Answer: \(\mathrm{4Al\,+\,3O_{2}\,}\rightarrow \mathrm{\,2Al_{2}O_{3}}\)

Is the reaction represented by the following equation a displacement reaction? \(\mathrm{2KI\,+\,Cl_{2}\,}\rightarrow \mathrm{\,2KCl\,+\,I_{2}}\)

Answer: Yes. Chlorine is more reactive than iodine, so it displaces iodine from potassium iodide.

Express the following reaction in the form of a balanced chemical equation: "When a strip of copper is dipped in a solution of silver nitrate, silver metal is precipitated and a solution of copper nitrate is produced."

Answer: \(\mathrm{Cu(s)\,+\,2AgNO_{3}(aq)\,}\rightarrow \mathrm{\,Cu(NO_{3})_{2}(aq)\,+\,2Ag(s)}\)

Write the following equation in a balanced form: \(\mathrm{NH_{3}\,+\,O_{2}\,}\rightarrow \mathrm{\,N_{2}\,+\,H_{2}O}\) (the source printed the product as 2H₂O, which is not a skeleton equation; it is balanced from the skeleton).

Answer: \(\mathrm{4NH_{3}\,+\,3O_{2}\,}\rightarrow \mathrm{\,2N_{2}\,+\,6H_{2}O}\)

What is the process called in which a substance is converted into a new substance?

Answer: A chemical reaction (chemical change).

Name the gas evolved when a magnesium ribbon is dropped into dilute sulphuric acid.

Answer: Hydrogen gas.

Give an example of a double displacement reaction.

Answer: \(\mathrm{Na_{2}SO_{4}(aq)\,+\,BaCl_{2}(aq)\,}\rightarrow \mathrm{\,BaSO_{4}(s)\,+\,2NaCl(aq)}\)

Is copper more reactive than iron? Give the equation of the reaction in support of your answer.

Answer: No, iron is more reactive than copper. Iron displaces copper from copper sulphate solution: \(\mathrm{Fe\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,FeSO_{4}\,+\,Cu}\)

Can a combustion reaction be a redox reaction?

Answer: Yes. In combustion the fuel is oxidised and oxygen is reduced, so it is a redox reaction.

Can a double displacement reaction be a redox reaction?

Answer: No. There is no change in the oxidation states of the elements, so it is not a redox reaction.

State one characteristic of the chemical reaction when lemon juice is added gradually to potassium permanganate solution.

Answer: A change in colour: the purple colour of the potassium permanganate solution disappears.

Which gas evolves when dilute HCl is added to sodium carbonate?

Answer: Carbon dioxide.

Why is a photochemical reaction considered an endothermic reaction?

Answer: Because it takes place by absorbing energy in the form of light.

Which term is applied for the process in which an unpleasant smell and taste develop in foods containing fats and oils?

Answer: Rancidity.

What are the substances called which are added to foods containing fats and oils to protect them from becoming rancid?

Answer: Antioxidants.

Why are potato chips packaged in nitrogen?

Answer: Nitrogen is an unreactive gas and prevents the oxidation of the oil and fat in the chips, so the chips do not become rancid.

In the refining of silver, silver is obtained from silver nitrate by using copper metal. Write down the reaction involved.

Answer: \(\mathrm{Cu\,+\,2AgNO_{3}\,}\rightarrow \mathrm{\,Cu(NO_{3})_{2}\,+\,2Ag}\)

A shiny brown coloured element when heated in air becomes black. Name the element and the black coloured substance so formed.

Answer: The element is copper and the black substance is copper(II) oxide, CuO: \(\mathrm{2Cu\,+\,O_{2}\,}\rightarrow \mathrm{\,2CuO}\)

Name the substance which is oxidised in the following reaction: \(\mathrm{H_{2}S\,+\,Cl_{2}\,}\rightarrow \mathrm{\,2HCl\,+\,S}\)

Answer: Hydrogen sulphide (H₂S) is oxidised, since it loses hydrogen.

Why are all decomposition reactions endothermic?

Answer: Energy (heat, light or electricity) must be supplied to break the bonds of the compound, so the reaction absorbs energy. (This holds for the usual decomposition reactions described in the chapter.)

Is the decomposition of vegetable matter into compost an exothermic reaction?

Answer: Yes. Heat is released during the decomposition of vegetable matter.

Why is photosynthesis an endothermic reaction?

Answer: It takes place by absorbing energy from sunlight.

Balance the following equations: (i) MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O; (ii) Na + O₂ → Na₂O; (iii) H₂O₂ → H₂O + O₂; (iv) Mg(OH)₂ + HCl → MgCl₂ + H₂O; (v) Fe + O₂ → Fe₂O₃; (vi) Al(OH)₃ → Al₂O₃ + H₂O; (vii) NH₃ + CuO → Cu + N₂ + H₂O; (viii) Al₂(SO₄)₃ + NaOH → Al(OH)₃ + Na₂SO₄; (ix) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O; (x) NaOH + H₂SO₄ → Na₂SO₄ + H₂O; (xi) BaCl₂ + H₂SO₄ → BaSO₄ + HCl; (xii) calcium hydroxide + carbon dioxide → calcium carbonate + water; (xiii) aluminium + copper chloride → aluminium chloride + copper; (xiv) calcium + water → calcium hydroxide + hydrogen; (xv) sulphur dioxide + oxygen → sulphur trioxide; (xvi) MnO₂ + HCl → MnCl₂ + Cl₂ + H₂O; (xvii) Zn + HCl → ZnCl₂ + H₂ (the source omitted H₂).

Answer:

1. \(\mathrm{MnO_{2}\,+\,4HCl\,}\rightarrow \mathrm{\,MnCl_{2}\,+\,Cl_{2}\,+\,2H_{2}O}\)
2. \(\mathrm{4Na\,+\,O_{2}\,}\rightarrow \mathrm{\,2Na_{2}O}\)
3. \(\mathrm{2H_{2}O_{2}\,}\rightarrow \mathrm{\,2H_{2}O\,+\,O_{2}}\)
4. \(\mathrm{Mg(OH)_{2}\,+\,2HCl\,}\rightarrow \mathrm{\,MgCl_{2}\,+\,2H_{2}O}\)
5. \(\mathrm{4Fe\,+\,3O_{2}\,}\rightarrow \mathrm{\,2Fe_{2}O_{3}}\)
6. \(\mathrm{2Al(OH)_{3}\,}\rightarrow \mathrm{\,Al_{2}O_{3}\,+\,3H_{2}O}\)
7. \(\mathrm{2NH_{3}\,+\,3CuO\,}\rightarrow \mathrm{\,3Cu\,+\,N_{2}\,+\,3H_{2}O}\)
8. \(\mathrm{Al_{2}(SO_{4})_{3}\,+\,6NaOH\,}\rightarrow \mathrm{\,2Al(OH)_{3}\,+\,3Na_{2}SO_{4}}\)
9. \(\mathrm{2HNO_{3}\,+\,Ca(OH)_{2}\,}\rightarrow \mathrm{\,Ca(NO_{3})_{2}\,+\,2H_{2}O}\)
10. \(\mathrm{2NaOH\,+\,H_{2}SO_{4}\,}\rightarrow \mathrm{\,Na_{2}SO_{4}\,+\,2H_{2}O}\)
11. \(\mathrm{BaCl_{2}\,+\,H_{2}SO_{4}\,}\rightarrow \mathrm{\,BaSO_{4}\,+\,2HCl}\)
12. \(\mathrm{Ca(OH)_{2}\,+\,CO_{2}\,}\rightarrow \mathrm{\,CaCO_{3}\,+\,H_{2}O}\)
13. \(\mathrm{2Al\,+\,3CuCl_{2}\,}\rightarrow \mathrm{\,2AlCl_{3}\,+\,3Cu}\)
14. \(\mathrm{Ca\,+\,2H_{2}O\,}\rightarrow \mathrm{\,Ca(OH)_{2}\,+\,H_{2}}\)
15. \(\mathrm{2SO_{2}\,+\,O_{2}\,}\rightarrow \mathrm{\,2SO_{3}}\)
16. \(\mathrm{MnO_{2}\,+\,4HCl\,}\rightarrow \mathrm{\,MnCl_{2}\,+\,Cl_{2}\,+\,2H_{2}O}\)
17. \(\mathrm{Zn\,+\,2HCl\,}\rightarrow \mathrm{\,ZnCl_{2}\,+\,H_{2}}\)

Short Answer Type Questions

What do you mean by a chemical reaction? Give an example.

Answer: A process in which one or more substances change into new substances with new properties is a chemical reaction. Example: \(\mathrm{Mg\,+\,O_{2}\,}\rightarrow \mathrm{\,MgO}\) (magnesium burns in air to form magnesium oxide).

What do you mean by a combination reaction? Give an example.

Answer: A reaction in which two or more reactants combine to form a single product. Example: \(\mathrm{CaO\,+\,H_{2}O\,}\rightarrow \mathrm{\,Ca(OH)_{2}}\)

What do you mean by a displacement reaction? Give an example.

Answer: A reaction in which a more reactive element displaces a less reactive element from its compound. Example: \(\mathrm{Fe\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,FeSO_{4}\,+\,Cu}\)

What do you mean by a decomposition reaction? Give an example.

Answer: A reaction in which a single compound breaks down into two or more simpler substances. Example: \(\mathrm{CaCO_{3}\,}\rightarrow \mathrm{\,CaO\,+\,CO_{2}}\)

What do you mean by a double displacement reaction? Give an example.

Answer: A reaction in which two compounds exchange their ions or groups to form two new compounds. Example: \(\mathrm{Na_{2}SO_{4}\,+\,BaCl_{2}\,}\rightarrow \mathrm{\,BaSO_{4}\,+\,2NaCl}\)

Explain the term "electrolytic decomposition", giving a suitable example.

Answer: The decomposition of a compound by passing electricity through it is called electrolytic decomposition. Example: electrolysis of water gives hydrogen and oxygen, \(\mathrm{2H_{2}O\,}\rightarrow \mathrm{\,2H_{2}\,+\,O_{2}}\).

Mention any two uses of decomposition reactions.

Answer: Decomposition by heat is used to get quicklime from limestone, and the electrolytic decomposition of molten compounds is used to extract metals (for example sodium from molten sodium chloride).

Give an example of a reaction in which a less reactive non-metal is displaced by a more reactive non-metal.

Answer: \(\mathrm{Cl_{2}\,+\,2KI\,}\rightarrow \mathrm{\,2KCl\,+\,I_{2}}\) (chlorine displaces iodine).

Why does the blue colour of copper sulphate change when a piece of iron is dropped into it?

Answer: Iron is more reactive than copper, so it displaces copper from the blue copper sulphate solution and forms light green iron sulphate. A brown deposit of copper forms on the iron. \(\mathrm{Fe\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,FeSO_{4}\,+\,Cu}\)

In the reactions given below, identify the substances that act as oxidising and reducing agents: (i) \(\mathrm{4Na(s)\,+\,O_{2}(g)\,}\rightarrow \mathrm{\,2Na_{2}O(s)}\); (ii) \(\mathrm{ZnO(s)\,+\,C(s)\,}\rightarrow \mathrm{\,Zn(s)\,+\,CO(g)}\)

Answer: (i) Sodium is the reducing agent (it is oxidised) and oxygen is the oxidising agent.
(ii) Carbon is the reducing agent and zinc oxide is the oxidising agent.

Write the balanced chemical equation for the following reactions and identify the type of reaction in each case: (i) potassium chloride (aq) + barium iodide (aq) → potassium iodide (aq) + barium chloride (s); (ii) zinc carbonate (s) → zinc oxide (s) + carbon dioxide (g); (iii) hydrogen (g) + chlorine (g) → hydrogen chloride (g); (iv) magnesium (s) + hydrochloric acid (aq) → magnesium chloride (aq) + hydrogen (g).

Answer: (i) \(\mathrm{2KCl(aq)\,+\,BaI_{2}(aq)\,}\rightarrow \mathrm{\,2KI(aq)\,+\,BaCl_{2}(s)}\) – double displacement (as printed in the source; barium chloride is actually soluble in water).
(ii) \(\mathrm{ZnCO_{3}(s)\,}\rightarrow \mathrm{\,ZnO(s)\,+\,CO_{2}(g)}\) – decomposition.
(iii) \(\mathrm{H_{2}(g)\,+\,Cl_{2}(g)\,}\rightarrow \mathrm{\,2HCl(g)}\) – combination.
(iv) \(\mathrm{Mg(s)\,+\,2HCl(aq)\,}\rightarrow \mathrm{\,MgCl_{2}(aq)\,+\,H_{2}(g)}\) – displacement.

Name the type of reaction involved in the reactions represented by the following equations: (i) \(\mathrm{CaO\,+\,H_{2}O\,}\rightarrow \mathrm{\,Ca(OH)_{2}}\); (ii) \(\mathrm{Zn\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,ZnSO_{4}\,+\,Cu}\); (iii) \(\mathrm{Al_{2}(SO_{4})_{3}\,+\,6NH_{4}OH\,}\rightarrow \mathrm{\,2Al(OH)_{3}\,+\,3(NH_{4})_{2}SO_{4}}\); (iv) \(\mathrm{CaCO_{3}\,}\rightarrow \mathrm{\,CaO\,+\,CO_{2}}\); (v) \(\mathrm{H_{2}\,+\,Cl_{2}\,}\rightarrow \mathrm{\,2HCl}\)

Answer: (i) Combination. (ii) Displacement. (iii) Double displacement. (iv) Decomposition. (v) Combination.

Why is a magnesium ribbon cleaned before burning it in air?

Answer: Magnesium is covered with a layer of magnesium oxide, which stops it from burning. The ribbon is cleaned with sandpaper to remove this layer.

State the characteristics of a chemical reaction.

Answer: A change in state, a change in colour, evolution of a gas, formation of a precipitate and a change in temperature.

What is a chemical equation?

Answer: The representation of a chemical reaction in terms of symbols and formulae of the reactants and products is a chemical equation.

Write the balanced chemical equations for the following chemical reactions: (i) hydrogen + chlorine → hydrogen chloride; (ii) barium chloride + aluminium sulphate → barium sulphate + aluminium chloride; (iii) sodium + water → sodium hydroxide + hydrogen (the source printed sodium chloride, which is incorrect).

Answer: (i) \(\mathrm{H_{2}\,+\,Cl_{2}\,}\rightarrow \mathrm{\,2HCl}\)
(ii) \(\mathrm{3BaCl_{2}\,+\,Al_{2}(SO_{4})_{3}\,}\rightarrow \mathrm{\,3BaSO_{4}\,+\,2AlCl_{3}}\)
(iii) \(\mathrm{2Na\,+\,2H_{2}O\,}\rightarrow \mathrm{\,2NaOH\,+\,H_{2}}\)

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 a precipitate of barium sulphate and a solution of sodium chloride; (ii) sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer: (i) \(\mathrm{BaCl_{2}(aq)\,+\,Na_{2}SO_{4}(aq)\,}\rightarrow \mathrm{\,BaSO_{4}(s)\,+\,2NaCl(aq)}\)
(ii) \(\mathrm{NaOH(aq)\,+\,HCl(aq)\,}\rightarrow \mathrm{\,NaCl(aq)\,+\,H_{2}O(l)}\)

How can you explain that respiration is an exothermic reaction?

Answer: During respiration, glucose combines with oxygen in our cells and gives carbon dioxide, water and energy, and the energy is released as heat: \(\mathrm{C_{6}H_{12}O_{6}\,+\,6O_{2}\,}\rightarrow \mathrm{\,6CO_{2}\,+\,6H_{2}O}\) + energy. Since heat is released, respiration is an exothermic reaction.

What do you mean by a precipitation reaction? Explain by giving an example.

Answer: A reaction in which an insoluble solid (precipitate) is formed when two solutions are mixed is a precipitation reaction. Example: \(\mathrm{Na_{2}SO_{4}(aq)\,+\,BaCl_{2}(aq)\,}\rightarrow \mathrm{\,BaSO_{4}(s)\,+\,2NaCl(aq)}\) – the white solid is barium sulphate.

What are antioxidants? Name two substances which are usually used as antioxidants. State any two ways to prevent the rancidity of food containing oils and fats.

Answer: Antioxidants are substances which prevent the oxidation of food (and so prevent rancidity). Examples: BHA (butylated hydroxyanisole) and BHT (butylated hydroxytoluene).
Rancidity can be prevented by adding antioxidants and by packing the food in nitrogen gas. Keeping the food in airtight containers or in the refrigerator also helps.

What observations do you expect to get when granulated zinc taken in a test tube is treated with dilute sulphuric acid?

Answer: Bubbles of hydrogen gas are produced, the zinc slowly dissolves, and the test tube becomes warm. \(\mathrm{Zn\,+\,H_{2}SO_{4}\,}\rightarrow \mathrm{\,ZnSO_{4}\,+\,H_{2}}\)

Give an example of a chemical reaction which takes place with a fall in temperature.

Answer: Dissolving ammonium chloride (or ammonium nitrate) in water, or mixing barium hydroxide with ammonium chloride. These endothermic processes absorb heat and the temperature falls.

State one characteristic of the chemical reaction taking place when: (i) dilute sulphuric acid is made to react with marble chips; (ii) lemon juice is added to a solution of potassium permanganate; (iii) dilute hydrochloric acid is added to a solution of lead nitrate in the cold; (iv) water is added to quicklime.

Answer: (i) A colourless gas (carbon dioxide) is evolved with bubbles.
(ii) The purple colour of the solution disappears (a change in colour).
(iii) A white precipitate of lead chloride is formed.
(iv) Heat is evolved (a change in temperature) and the quicklime is slaked with a hissing sound.

Which of the following reactions are exothermic and which are endothermic? (i) burning of natural gas; (ii) photosynthesis; (iii) electrolysis of water; (iv) respiration.

Answer: (i) Exothermic. (ii) Endothermic. (iii) Endothermic. (iv) Exothermic.

What would you observe when lead nitrate is heated in a test tube?

Answer: Brown fumes of nitrogen dioxide are evolved, and a yellow residue of lead oxide is left. The reaction is accompanied by a crackling sound. \(\mathrm{2Pb(NO_{3})_{2}\,}\rightarrow \mathrm{\,2PbO\,+\,4NO_{2}\,+\,O_{2}}\)

Why is respiration considered an exothermic reaction?

Answer: Because the oxidation of glucose in the cells releases energy (heat).

Give an example of a decomposition reaction. Describe an activity to illustrate such a reaction by heating.

Answer: Example: heating ferrous sulphate crystals, \(\mathrm{2FeSO_{4}\,}\rightarrow \mathrm{\,Fe_{2}O_{3}\,+\,SO_{2}\,+\,SO_{3}}\).
Activity: take about 2 g of ferrous sulphate crystals in a dry boiling tube and note their green colour. Heat the tube over a flame. The crystals first lose water, and then they change into a brown solid, ferric oxide, and the smell of burning sulphur is noticed from the gases SO₂ and SO₃.

When hydrogen is passed over copper oxide, copper and steam are formed. Write a balanced equation for this reaction and state which of the chemicals are (i) elements, (ii) compounds, (iii) reactants, (iv) products, (v) metals, (vi) non-metals.

Answer: \(\mathrm{CuO\,+\,H_{2}\,}\rightarrow \mathrm{\,Cu\,+\,H_{2}O}\)
(i) Elements: H₂ and Cu.
(ii) Compounds: CuO and H₂O.
(iii) Reactants: CuO and H₂.
(iv) Products: Cu and H₂O.
(v) Metal: Cu.
(vi) Non-metal: H₂.

(a) What is a balanced chemical equation? Why should chemical equations be balanced? (b) Aluminium burns in chlorine to form aluminium chloride. Write a balanced chemical equation for this reaction.

Answer: (a) An equation in which the number of atoms of each element is the same on both sides is a balanced chemical equation. Equations are balanced because of the law of conservation of mass: mass can neither be created nor destroyed in a chemical reaction.
(b) \(\mathrm{2Al\,+\,3Cl_{2}\,}\rightarrow \mathrm{\,2AlCl_{3}}\)

Long Answer Type Questions

Balance the chemical equations for the following reactions: (i) Cu + H₂SO₄ → CuSO₄ + SO₂ + H₂O; (ii) NaOH + H₂SO₄ → Na₂SO₄ + H₂O.

Answer: (i) \(\mathrm{Cu\,+\,2H_{2}SO_{4}\,}\rightarrow \mathrm{\,CuSO_{4}\,+\,SO_{2}\,+\,2H_{2}O}\)
(ii) \(\mathrm{2NaOH\,+\,H_{2}SO_{4}\,}\rightarrow \mathrm{\,Na_{2}SO_{4}\,+\,2H_{2}O}\)

Differentiate between a balanced and an unbalanced chemical equation.

Answer: In a balanced equation the number of atoms of each element is equal on both sides, so it follows the law of conservation of mass. In an unbalanced (skeleton) equation the number of atoms of one or more elements is different on the two sides, so it does not obey this law.

Write the following chemical equations with state symbols: (i) \(\mathrm{CaCO_{3}\,}\rightarrow \mathrm{\,CaO\,+\,CO_{2}}\); (ii) \(\mathrm{2Na\,+\,2H_{2}O\,}\rightarrow \mathrm{\,2NaOH\,+\,H_{2}}\); (iii) \(\mathrm{AgNO_{3}\,+\,NaCl\,}\rightarrow \mathrm{\,AgCl\,+\,NaNO_{3}}\); (iv) \(\mathrm{6CO_{2}\,+\,6H_{2}O\,}\rightarrow \mathrm{\,C_{6}H_{12}O_{6}\,+\,6O_{2}}\).

Answer: (i) \(\mathrm{CaCO_{3}(s)\,}\rightarrow \mathrm{\,CaO(s)\,+\,CO_{2}(g)}\)
(ii) \(\mathrm{2Na(s)\,+\,2H_{2}O(l)\,}\rightarrow \mathrm{\,2NaOH(aq)\,+\,H_{2}(g)}\)
(iii) \(\mathrm{AgNO_{3}(aq)\,+\,NaCl(aq)\,}\rightarrow \mathrm{\,AgCl(s)\,+\,NaNO_{3}(aq)}\)
(iv) \(\mathrm{6CO_{2}(g)\,+\,6H_{2}O(l)\,}\rightarrow \mathrm{\,C_{6}H_{12}O_{6}(aq)\,+\,6O_{2}(g)}\) (in the presence of sunlight and chlorophyll).

What do you mean by exothermic and endothermic reactions? Give examples.

Answer: Reactions in which heat is released along with the products are exothermic reactions. Examples: burning of natural gas, respiration, the reaction of quicklime with water, and decomposition of vegetable matter into compost.
Reactions in which heat is absorbed are endothermic reactions. Examples: photosynthesis, electrolysis of water and the decomposition of calcium carbonate on heating.

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

Answer: In a displacement reaction a more reactive element displaces a less reactive element from its compound: \(\mathrm{Zn\,+\,CuSO_{4}\,}\rightarrow \mathrm{\,ZnSO_{4}\,+\,Cu}\).
In a double displacement reaction two compounds exchange their ions to form two new compounds: \(\mathrm{BaCl_{2}\,+\,Na_{2}SO_{4}\,}\rightarrow \mathrm{\,BaSO_{4}\,+\,2NaCl}\).

Explain how oxidation and reduction processes occur simultaneously.

Answer: Oxidation is the gain of oxygen (or loss of hydrogen), and reduction is the loss of oxygen (or gain of hydrogen). When one substance gains oxygen, the oxygen has to come from another substance, which therefore loses oxygen. For example, in \(\mathrm{CuO\,+\,H_{2}\,}\rightarrow \mathrm{\,Cu\,+\,H_{2}O}\) hydrogen gains oxygen (it is oxidised) while copper oxide loses oxygen (it is reduced). So both happen together; such reactions are called redox reactions.

What is corrosion? Write the chemical reaction that takes place during the corrosion of iron.

Answer: Corrosion is the slow eating away of metals by the action of air, moisture or chemicals on their surface. When iron is exposed to moist air it forms a reddish-brown coating called rust: \(\mathrm{4Fe\,+\,3O_{2}\,+\,xH_{2}O\,}\rightarrow \mathrm{\,2Fe_{2}O_{3}.xH_{2}O}\)

What are the various ways to make a chemical equation more informative?

Answer: By writing the physical states of the reactants and products, using the symbols (s), (l), (g) and (aq); by writing the conditions such as temperature, pressure and catalyst above or below the arrow; and by indicating the heat change, and the gas or precipitate formed with an arrow up (↑) or down (↓).

Explain the following terms: (i) corrosion; (ii) rancidity.

Answer: (i) Corrosion is the gradual damage of a metal by air, moisture and other substances, as in the rusting of iron.
(ii) Rancidity is the oxidation of fats and oils in food that gives an unpleasant smell and taste.

When metal X is treated with a dilute acid Y, a gas Z is evolved which burns readily with a little explosion. (a) Name any two metals which can behave like metal X. (b) Name any two acids which can behave like acid Y. (c) Name the gas Z. (d) Is the gas Z lighter than or heavier than air? (e) Is the reaction between metal X and acid Y exothermic or endothermic? (f) By taking a specific example of metal X and dilute acid Y, write a balanced chemical equation for the reaction. Also indicate the physical state of all the reactants and products.

Answer: (a) Zinc and magnesium (also iron).
(b) Hydrochloric acid and sulphuric acid.
(c) Hydrogen gas.
(d) Hydrogen is lighter than air.
(e) Exothermic, because heat is released.
(f) \(\mathrm{Zn(s)\,+\,2HCl(aq)\,}\rightarrow \mathrm{\,ZnCl_{2}(aq)\,+\,H_{2}(g)}\)

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

Question. In the equation, a Al + bH₂SO₄ - c Al₂(SO₄)₃ + dH₂ a, b, c, d are respectively -
(A) 2, 3, 1, 1
(B) 2, 3, 1, 3
(C) 2, 3, 2, 3
(D) 2, 2, 3, 3
Answer. B

Question. aMg₃N₂ + bH₂O - cMg(OH)₂ + dNH₃. When the equation is balanced, the coefficients a, b, c, d respectively are -
(A) 1, 3, 3, 2
(B) 1, 6, 3, 2
(C) 1, 2, 3, 2
(D) 2, 3, 6, 2
Answer. B

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

Question. When dilute hydrochloric acid is added to zinc pieces taken in a test tube
(A) no change takes place
(B) the color of the solution becomes yellow
(C) a pungent smelling gas gets liberated
(D) small bubbles of hydrogen gas appear on the surface of zinc pieces
Answer. D

Question. Plastic bags containing potato chips are filled with an antioxidant, known as
(A) oxygen
(B) nitrogen
(C) hydrogen
(D) carbon monoxide
Answer. B

Question. AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq). Above reaction is
(A) Precipitation reaction
(B) Double Displacement reaction
(C) Combination reaction
(D) A and B both
Answer. D

Question. Which of the following is a decomposition reaction ?
(A) NaOH + HCl → NaCl + H₂O
(B) NH₄CNO → H₂NCOONH₂
(C) 2KClO₃ → 2KCl + 3O₂
(D) H₂ + I₂ → 2HI
Answer. C

Question. Which of the following is a displacement reaction ?
(A) CaCO₃ → CaO + CO₂
(B) CaO + 2HCl → CaCl₂ + H₂O
(C) Fe + CuSO₄ → FeSO₄ + Cu
(D) NaOH + HCl → NaCl + H₂O
Answer. C

Question. Which of the following is not a decomposition reaction ?
(A) CaCO₃ → CaO + CO₂
(B) 2KClO₃ → 2KCl + 3O₂
(C) Digestion of food in body
(D) H₂ + Cl₂ → 2HCl
Answer. D

Question. Reaction of phosphorus and oxygen is an example of
(A) oxidation
(B) reduction
(C) rancidity
(D) none of these
Answer. A