Chapter 1 Chemical Reactions and Equations

Question 1.
Why should magnesium ribbon be cleaned before burning in air?
Magnesium is a reactive metal. When it’s exposed to atmosphere for extended time, the oxygen present within the atmosphere combines and MgO is created on the metal surface. This oxide layer doesn’t burn in the flame so magnesium ribbon has got to be cleaned with sandpaper before burning in air.

Question 2.
Write the balanced equations for the following chemical reactions:
(a) Hydrogen + Chlorine ➝ hydrogen chloride
(b) Barium chloride + Aluminium sulphate ➝ Barium sulphate + aluminum chloride
(c) Sodium + Water ➝ sodium hydroxide + Hydrogen
The balanced equations are written as:
(a) H2 + Cl2 ➝ 2HCl
(b) 3BaCl2 + Al2(SO4)3 ➝ 3BaSO4 + 2AlCl3
(c) 2Na + 2H2O ➝ 2NaOH + H2

Question 3.
Write the balanced equations with state symbols for the following reactions:
(a) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(b) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride and water.
The symbol equations in balanced form for the reactions are:
(a) BaCl2(aq) + Na2SO4(aq) ➝ BaS04(s)+ 2NaCl(aq)
(b) NaOH(aq) + HCl(aq) ➝ NaCl(aq) + H2O(l)

Question 4

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.


i) The substance ‘X’ used in whitewashing is quick lime or Calcium Oxide with chemical formula of CaO.

ii) CaO + H2O → Ca(OH)2

Question 5

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


In activity 1.7, It is so because in water, the hydrogen is two times more than oxygen by mass and volume. After electrolysis two molecules of Hydrogen and one molecule of oxygen gas is released, hence the amount of Hydrogen collected would be double than that of oxygen.


When you mix solutions of lead(II) nitrate and potassium iodide.
(a) What is the colour of the precipitate formed? Name the compound involved.
(b) Write a balanced chemical equation for the reaction.
(c) What type of a reaction is this?
(a) The color of precipitate is yellow. The compound is lead(II) iodide with chemical formula PbI2.
(b) Pb(NO3)2(aq) + 2KI(aq) ➝ Pbl2(s) + 2KNO3(aq)
(c) It’s a double displacement reaction.

Question 7.
Why does the colour of copper sulphate change when an iron nail is dipped in it?


Iron nail acquires a brown coating of copper as a result of displacement reaction because iron is more reactive than copper. The solution becomes light green.

Question 8.
Give one example of the double displacement reaction.


Ag+ + NO3 + Na+ + Cl– → AgCl + Na+ + NO3

Question 9.
Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(a) 4Na(s) + O2(g) ➝ 2Na2O(s)
(b) CuO(s) + H2(g) ➝ Cu(s) + H2O(l)


In first equation,Na is getting oxidised. In second equation, hydrogen (H2) is oxidised to form water (H2O) while copper(II) oxide (CuO) is reduced to copper (Cu).

Question 10

Magnesium ribbon burns with a dazzling flame in air (or oxygen) and changes to a white substance magnesium oxide. Is magnesium being oxidised or reduced in this reaction?
The balanced chemical equation is:
2Mg(s) + O2(g) ➝ 2MgO(s)
Yes, magnesium (Mg) gets oxidised to form magnesium oxide (MgO) in this reaction. It has the same as the total mass of products. It implies that mass can neither be created nor be destroyed in a chemical reaction.

Chapter Ends

Question 1

Which of the following statements about the reaction given below are incorrect?
2PbO(s) + C(s) ➝ 2Pb(s) + CO2(g)
(i) Lead is getting reduced.
(ii) Carbon dioxide is getting oxidized.
(iii) Carbon is getting oxidized.
(iv) Lead oxide is getting reduced.
(a) (i) and (ii)
(b) (i) and (iii)
(c) (i), (ii) and (iii)
(d) all are incorrect


(a) (i) and (ii)

Explanation: (i) because oxygen is removed (ii) because the oxygen is added to carbon.

Question 2
Fe2O3 + 2Al ➝ Al2O3 + 2Fe
The above reaction is an example of:
(a) combination reaction
(b) double displacement,reaction
(c) decomposition reaction
(d) displacement reaction


The Oxygen with Ferrous oxide is getting displaced to Aluminium metal to form Aluminium Oxide.

Question 3
What happens when dilute hydrochloric acid is added to iron filings? Tick 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.


(a) Hydrogen gas and iron chloride are produced.

2HCl + Fe → FeCl+ H2

Question 4.
What is a balanced chemical equation? Why should chemical equations be balanced?


An equation is said to be equal if, atoms on both sides reactant and product sides are equal. A chemical equation should be balanced to obey the law of conservation of energy.

Question 5.
Transfer the following into chemical equations and balance them:
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.


(a) Unbalanced equation: H2 + N2 ➝ NH3

Balanced Equation: 3H2(g) + N2(g) ➝ 2NH3(g)

(b) Unbalanced Equation: H2S + O2 ➝ H2O + SO2

Balanced Equation: 2H2S(g) + 3O2(g) ➝ 2H2O(g) + 2SO2(g)

(c) Unbalanced Equation: BaCl2 + Al2(SO4)3 ➝ AlCl3 + BaSO4

Balanced Equation: 3BaCl2(aq) + Al2(SO4)3(aq) ➝ 2AlCl3(aq) + 3BaSO4(s)

(d) Unbalanced Equation: K + H2O ➝ KOH + H2

Balanced Equation: 2K(s) + 2H2O(l) ➝ 2KOH(aq) + H2(g)

Question 6
Balance the following chemical equations:
(a) HNO3 + Ca(OH)2 ➝ Ca(NO3)2 + H2O
(b) NaOH + H2SO4 ➝ Na2SO4 + H2O
(c) NaCl + AgNO3 ➝ AgCl + NaNO3
(d) BaCl2 + H2SO4 ➝ BaSO4 + HCl


(a) 2HNO3 + Ca(OH)2 ➝ Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 ➝ Na2SO4 + 2H2O

(c) NaCl + AgNO→ AgCl + NaNO3

(d) BaCl2 + H2SO4 ➝ BaSO4 + 2HCl

Question 7
Write the balanced equations for the following reactions:
(a) Calcium hydroxide + Carbon dioxide ➝ Calcium carbonate + Water
(b) Aluminium + Copper chloride ➝ Aluminium chloride + Copper
(c) Barium chloride + Potassium sulphate ➝ Barium sulphate + Potassium chloride
(d) Zinc + Silver nitrate ➝ Zinc nitrate + Silver


(a) Ca(OH)2 + CO2 ➝ CaCO3 + H2O

(b) 2Al + 3CuCl→ 2AlCl+ 3Cu

(c)  BaCl2 + K2SO4 ➝ BaSO4 + 2KCl

(d) Zn + 2AgNO→ Zn(NO3)+ 2Ag

Question 8.
Write the balanced chemical equations for the following reactions and identify the type of reaction:
(a) Potassium bromide(aq) + Barium iodide(aq) ➝ Potassium iodide(aq) + Barium bromide(aq)
(b) Zinc carbonate(s) ➝ Zinc oxide(s) + Carbon dioxide (g)
(c) Hydrogen(g) + Chlorine (g) ➝ Hydrogen chloride (g)
(d) Magnesium(s) + Hydrochloric acid(aq) ➝ Magnesium chloride(aq) + Hydrogen(g)


(a) 2KBr(aq) + BaI2(aq) ➝ 2KI(aq) + BaBr2(aq) (Double displacement Reaction)

(b) ZnCO3(s) ➝ ZnO(s) + CO2(g) (Decomposition Reaction)

(c) H2(g) + Cl2(g) ➝ 2HCl (g) (Combination Reaction)

(d) Mg(s) + 2HCl(aq) ➝ MgCl2(aq) + H2(g) (Displacement Reaction)

Question 9.
What does one mean by exothermic and endothermic reactions? Give examples.


An endothermic reaction is when the energy gets absorbed from the surroundings in the form of heat.

Example: C(s) + H2O(g) + heat ➝ CO(g) + H2(g)

An exothermic reaction is one in which energy is released from the system and the resulting content gets heated up.

For example: 2H2(g) + O2(g) ➝ 2H2O(l) + heat

Question 10
Why is respiration considered an exothermic reaction?


For survival, we need energy. We get the energy from the food we eat. When the food comes in contact with oxygen present in body, carbon dioxide and water  and energy are formed and the process is known as respiration process. This process of energy in the form of energy, the respiration process is considered as  exothermic reaction.

The Reaction: C6H12O+ 6O→ 6CO+ 6H2O + Energy

Question 11
Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.


Decomposition is one in which larger molecules breaks to give simple molecules while in combination reactions, gives two or more smaller molecules. This subsequentaly explains that the decomposition reaction is the opposite of the combination reaction.

To understand more, let us give you an example of decomposition reaction:

ZnCO3 → ZnO + CO2

CaCO3 + Energy → CaO + CO2

Question 12
Write one equation each for decomposition reaction where energy is supplied in the form of heat, light and electricity.


(a)Thermal decomposition reaction (Thermolysis)



(b) Electrolytic decomposition reaction (Electrolysis)



(c) Photo decomposition reaction (Photolysis)



Question 13
What is the difference between displacement and double displacement reactions? Write equations for these reactions.


In a displacement reaction, only a single displacement occurs whereas in the double displacement reaction, two displacements takes place between the molecules.

Example for displacement reaction: Mg + 2HCl → MgCl+ H2

Example for double displacement reaction: 2KBr + BaI2 → 2KI + BaBr2

Question 14.
In the refining of silver, the recovery of silver from silver nitrafe solution involved displacement by copper metal. Write down the reactions involved.


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

Question 15.
What do you mean by precipitation reaction? Explain giving examples.


When two solutions are combined, double displacement reaction occurs. And when one of the products formed dont get dissolved in the solution and gets settled on the surface of container. This is known as precipitate.

For Example: 2NaOH(aq) + MgCl2(aq) → 2NaCl(aq) + Mg(OH)2(s)

Question 16.
Explain the following in terms of gain or loss of oxygen with two examples of each:
(a) oxidation
(b) reduction


(a) Oxidation is the gain of oxygen in a chemical reaction.

4Na(s) + O2(g) → 2Na2O(s)

(b) Reduction is the loss of oxygen in a chemical reaction.

CuO(s) + H2(g) → Cu(s) + H2O(l)

Question 17
A shining brown-coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black-coloured compound formed.


The black coloured element ‘X’  is copper oxide

2Cu(s) + O2(g) → 2CuO(s)

Question 18.
Why do you apply paint on iron articles?


Iron is a very reactive metal, In the presence of air and exposed to moisture it rusts to form Fe2O3.xH2O. To prevent direct contact, paint is applied so there is no direct contact with air and moisture.

Question 19
Oil and fat containing food items are flushed with nitrogen. Why?


Food items containing oil and fat have a property of releasing unpleasant smell and taste as it reacts with oxygen which is known as randicity. To avoid this, nitrogen is flushed and thus prevents randicity.

Question 20.
Explain the following terms with one example of each.
(a) Corrosion
(b) Rusting


(a) Corrosion is a chemical process in which a metal decays when exposed to moisture.

(b) Corrosion of iron is called rusting. It is an oxidation process that takes place with the presence of moisture.

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