Exemplar Questions and Solutions

Important CBSE Questions – 2 Mark Questions

13) What is the difference between chemical and physical change?

Chemical change is any change that results in the formation of new chemical substances whereas in a chemical change, no new substances are formed. E.g., Rusting of iron (iron oxide is formed) is a chemical change whereas dissolving sugar in water is a physical change (sugar molecules are dispersed within the water, but the individual sugar molecules are unchanged.

14) What is a chemical reaction? Give one example:

Ans) The process in which one or more substances interact to form a new substances having entirely new properties is called a chemical reaction. For example, when iron and oxygen react, they change to a new substance, iron oxide (rust). Iron oxide (Fe2O3) has different chemical properties to iron and oxygen.

4Fe + 3O₂ → 2Fe₂O₃.

15) Define combination reaction. Give two examples:

Ans) A chemical reaction in which two or more substances combine to form a new substance is called a combination reaction.

We can represent these reactions using the general equation

A + B → AB

E.g.,

C + O₂ → CO₂

H₂ + O₂ → 2H₂O

16) What is galvanised iron? How does galvanizing protect iron from rusting?

Ans) Galvanization is the process of applying a protective zinc coating on iron or steel, to prevent rusting. Galvanized iron is a type of iron that has been coated in a layer of zinc to protect it from corrosion. Zinc is used because it is more electrochemically active than iron (or steel). Thus, Zinc will get oxidized and will protect the iron sacrificially. The Zinc oxide formed is sticky in nature and will protect the inner layer of iron from oxidation.

17) State one characteristic each of a chemical reaction which take place when:

a) Dilute hydrochloric acid is added to sodium carbonate.

b) Lemon juice is added gradually to potassium permanganate solution.

c) Dilute sulphuric acid is added to barium chloride solution.

d) Quicklime is treated with water.

e) Wax is burned in the form of candle

Ans) a) Dilute hydrochloric acid is added to sodium carbonate- The chemical reaction between Sodium carbonate and dilute hydrochloric acid is characterized by the evolution of carbon dioxide gas.

Na₂CO₃ + 2 HCl → 2NaCl + CO₂ + H₂O

b) Lemon juice is added gradually to potassium permanganate solution- The chemical reaction between lemon juice(citric acid) and purple coloured potassium permanganate solution is characterized by a change in colour from purple to a colourless solution.

c) Dilute sulphuric acid is added to barium chloride solution- The chemical reaction between dilute sulphuric acid and purple barium chloride solution is characterized by the formation of a white precipitate of barium sulphate.

BaCl₂ + H₂SO₄ → BaSO₄ + 2 HCl

d) Quicklime is treated with water- The chemical reaction between quicklime and water to form slaked lime(Ca(OH)₂) is characterized by a change in temperature. There is a rise in temperature as it is an exothermic reaction.

CaO(s) + H₂O(l) → Ca(OH)₂(s)

e) Wax is burned in the form of candle- When wax is burned in the form of candle, water and carbondioxide are formed. The combustion reaction of candle wax, is characterised by a change in state from solid (wax) to liquid(water) and gas (Carbondioxide).

18) What is the difference between a displacement reaction and a double displacement reaction? Support your answers using examples.

Ans) In a (single)displacement reaction, only one element in a compound is displaced by a more reactive element. In other words, a free element replaces one of the ions in a compound.

Eg., CuSO₄(aq) + Fe(s) → Cu(s) + FeSO₄(aq)

Here, as Fe is more reactive than Cu, it displaces Cu from CuSO4 and forms FeSO₄

In a double displacement reaction, exchange of ions takes place or rather parts of two compounds exchange their places to form two new compounds.

E.g., AgNO₃ (aq) + NaCl (aq) → AgCl (s) + NaNO₃ (aq).

When silver nitrate (AgNO₃) combines with sodium chloride(NaCl), two new compounds namely silver chloride (AgCl )and sodium nitrate (NaNO₃)are formed as sodium and silver exchanged their places.

19)Give two characteristics of chemical reaction which take place when dilute sulphuric acid is poured over Zinc granules:

When dil.sulphuric acid is poured over zinc granules, a displacement reaction takes place and results in the formation of a new substance (zinc sulphate) and hydrogen gas.

H₂SO₄(aq)+Zn(s) → ZnSO₄(aq)+H₂(g)

The reaction vessel becomes hot indicating that the reaction is exothermic.

20)When hydrogen burns in oxygen, water is formed and when water is electrolyzed, then hydrogen and oxygen are produced. What type of reaction takes place: In the first case? In the second case?

Ans) When hydrogen burns in oxygen, water is formed. The given reaction is a clear example of combination reaction as two or more substances (hydrogen and oxygen) combine to form a single substance(water)

2H₂ + O₂ → 2H₂O

When water is electrolyzed, then hydrogen and oxygen are produced. The given reaction is a clear example of decomposition reaction, as a single substance (water) decomposes into two or more substance (hydrogen and oxygen).

2H₂O → 2 H₂ + O₂

21) List five observations which help us to determine whether a chemical reaction has taken place or not

Ans) The five observations which help us to determine whether a chemical reaction has taken place or not are-

1. Change in state.

2. Change in colour.

3. Evolution of a gas.

4. Change in temperature

5. Formation of new substance

22) Why do we add dilute sulphuric acid during the electrolysis of water?

Ans) Dil. Sulphuric acid is added during the electrolysis of water because water is a poor conductor of electricity and dil. Sulphuric acid is used to increase the conductivity of water. The acid dissociates to produce ions which in turn increase conductivity.

23) What is a skeletal equation? Give an example:

Ans) An unbalanced chemical equation is called the skeletal chemical equation.

When a chemical equation is written using symbols and formulae, such that the number of atoms of different elements towards the side of the reactants is not equal to number of atoms of the same elements towards the side of the products, is called a skeletal equation or an unbalanced equation.

Eg., H₂ + O₂ → H₂O

CO₂ + H₂O(l) → C₆H1₂O₆ + O₂(photosynthesis)

24) Why should we balance a skeletal equation?

Ans) According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants. In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation in order to satisfy the law of conservation of mass.