Question

. Give one example of each : (a) Chemical reaction showing evolution of gas. (b) Change in substance’s colour during a chemical reaction.

Answer 1

1. Reaction of dil. Sulphur acid with zinc granules.
H2SO4 + Zn - ZnSO4 + H2
2. White silver chloride turns grey in sunlight. This is due to decomposition of silver chloride into silver and chlorine by light.
2AgCl(s) -----(sunlight) 2Ag(s) + Cl2(g)

Answer 2

A chemical reaction is one in which a product that has chemical properties that differ from the original reactants is produced.  

(a) Chemical reaction showing evolution of gas.

In a chemical reaction, the gas is evolved as product. Most of the gas evolution reactions can be carried out in  fume chamber.

${ H }_{ 2 }{ CO }_{ 3 }(aq)\quad \rightarrow \quad { H }_{ 2 }O(l)\quad +\quad { CO }_{ 2 }(g)$

(b) $\quad Change\quad in\quad color$

some chemical reactions are characterized by change in color.

Citric acid reacts with potassium permanganate, then the purple colour disappears.