Page 122 - ICSE Chemistry 8

Example: Sodium carbonate reacts with hydrochloric acid to form sodium chloride, water and carbon
dioxide.
The chemical equa on can be represented as shown.

Na CO (s) + 2HCl(aq) 2NaCl(aq) + H O(l) + CO (g) 
2
3
2
2
Sodium Hydrochloric Sodium Water Carbon
carbonate acid chloride dioxide
Sodium carbonate and hydrochloric acid are reactants, and sodium chloride, water and carbon dioxide
are products in the above chemical reac on.

Characteristics of a Chemical Reaction

In addi on to the forma on of new substances, a chemical change may be accompanied by one or more
of the following changes.
1. Evolu on of gas: Some chemical reac ons are characterised by evolu on of gas. The forma on gas
is indicated as bubbles if the reac on takes place in a liquid medium. The evolu on of gas in the
form of bubbles is known as eff ervescence. The gas that is released is indicated by an arrow in the
upward direc on ().

Examples:
Eff ervescence
• When zinc granules are added to dilute sulphuric acid,
hydrogen gas is evolved as a brisk eff ervescence. If a burning The bubbling of a solu on
due to the escape of gas is
matchs ck is brought near the mouth of a test tube, it burns
called eff ervescence.
with a pop sound.

dil. H SO (aq) + Zn(s) ZnSO (aq) + H (g) 
2
2
4
4
Dilute sulphuric acid Zinc Zinc sulphate Hydrogen
• When solid sodium nitrate is heated, oxygen gas is evolved. If a glowing splinter is brought
near the mouth of the test tube, it bursts into fl ame.
Heat
2NaNO (s) 2NaNO (s) + O (g) 
3
2
2
Sodium nitrate Sodium nitrite Oxygen
• When solid calcium carbonate is heated, carbon dioxide gas is evolved. If this gas is passed
through lime water, lime water turns milky.

Heat
CaCO (s) CaO(s) + CO (g) 
2
3
Calcium carbonate Calcium oxide Carbon dioxide
Let’s perform Ac vity 6.1 to observe the evolu on of gas.

Activity 6.1

Aim: To observe the evolu on of gas when zinc granules are added to dilute sulphuric acid
Materials required: Zinc granules, a conical fl ask, dilute sulphuric acid

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