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Answer» Characteristics of Chemical Reactions: When a chemical reaction takes place, it is accompained by one or more of the following characteristics: Evolution of a gas:
A gas may evolve during a chemical reaction, if one of the reactants is a liquid or in solution form. Evolution of gas is confirmed by appearance of effervescence (bubbling) in the liquid. The gas can be identified by testing it. In a chemical equation, the evolution of a gas is shown by putting an arrow pointing upwards
(↑).
Example:
When zinc reacts with dilute sulphuric acid, hydrogen gas is given off. The gas can be tested with the help of burning matchstick when the burning matchstick is taken near the test tube containing hydrogen gas, hydrogen burns with a pop sound.
Zn + H2SO4 → ZnSO4 + H2↑
Change of colour:
One of the product formed during a chemical reaction may have a different colour than that of the reactants. In such case, change of colour accompanies the chemical reaction.
Example:
When white crystal of lead nitrate are heated strongly, yellow-coloured lead monoxide is formed. A reddish – brown gas, nitrogen dioxide is also given off.
2Pb (NO3)2 → 2PbO + 4NO2 ↑+ O2 ↑
Lead nitrate (white) Lead monoxide (yellow) Nitrogen dioxide (reddish brown) oxygen.
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