Question

33. (a) What is meant by a chemical reaction ? Explain with the help of an example.
(b) Give one example each of a chemical reaction characterised by :
(i) evolution of a gas
(ii) change in colour
(iii) formation of a precipitate
(iv) change in temperature
(v) change in state.

Answer 1

Answer:

a. A chemical reaction is the phenomena in which atoms of two or more elements re arrange themselves or a chemical change happens.

Ex - C + O2 -> CO2

B.

1. Na + H20 - > NAOH + H2

2. PB(NO3)2 - > PBO + NO2 +O2

3. KI + PB(NO3)2 - > KNO3 + PBI2

4.CAO + H20 - > CA(OH) 2

5. FE2O3 + AL - > FE + AL2O3

Explanation:

1. Hydrogen gas is evolved.

2.lead oxide is yellow in color.

3. Lead iodide is a yellow precipitate.

4. It is an exothermic reaction meaning the test tube will become hot.

5. Iron and aluminium oxide will be formed in molten states.

Answer 2

Answer:

1. Change in colour :

The reaction between lead nitrate solution and poassium iodide solution.

Chemical equation as follow :

Pb ( NO₃ )₂ + 2 KI  ⇒  PbI₂ + 2 KNO₃

In above reaction colour change from colourless to yellow.

2. Change in temperature :

The reaction between dil. sulphuric acid on zinc.

Chemically :

Zn + H₂SO₄  ⇒  ZnSO₄ + H₂

Heat evolved in the above reaction.

3. Formation of precipitation​ :

The reaction between barium chloride on sodium sulphate.

Chemically :

BaCl₂ + Na₂SO₄  ⇒  BaSO₄ + 2 NaCl

In the above reaction white ppt. of BaSO₄ will form.