Page 126 - ICSE Chemistry 8

1. Close contact: The reactants must be in direct contact with each other for a chemical reac on to
occur and there should be a rac on between the physical states of the reactants.
Example: Sodium reacts vigorously with cold water.
2Na(s) + 2H O(l) 2NaOH(aq) + H (g) 
2
2
Sodium Water Sodium hydroxide Hydrogen
2. Presence of solu on: Some chemical reac ons take place only when the reactants are mixed in a
solu on form. This results in bringing the reactants into contact at a faster rate.
Example: When sodium sulphate and barium chloride are mixed in an aqueous solu on, a white
precipitate of barium chloride is formed.

Na SO (aq) + BaCl (s) BaSO (aq) + 2NaCl(aq)
4
4
2
2
Sodium sulphate Barium chloride Barium sulphate Sodium chloride
(white ppt)
3. Heat energy: Some chemical reac ons take place only in the presence of heat.
Example: Calcium carbonate on hea ng gives carbon dioxide and calcium oxide.
Heat
CaCO (s) CaO(s) + CO (g)
2
3
Calcium carbonate Calcium oxide Carbon dioxide
4. Light energy: Some chemical reac ons can take place in the presence of light. Such reac ons are
called photochemical reac ons.
Examples:
• Photosynthesis, a process by which the leaves of green plants prepare glucose and oxygen
from carbon dioxide and water in the presence of chlorophyll and sunlight, is a photochemical
reac on.
Sunlight
6CO (g) + 6H O(l) chlorophyll C H O (s) + 6O (g)
2
2
6
6 12
2
Carbon dioxide Water Glucose Oxygen
• The decomposi on of silver chloride in the presence of light is also a photochemical reac on.
Light
2AgCl(s) 2Ag(s) + Cl (g)
2
Silver chloride Silver Chlorine
(greenish-yellow gas)
5. Electricity: Some chemical reac ons occur when electricity is passed through the reactants. Such
reac ons are called electrochemical reac ons.
Example: Water dissociates into hydrogen and oxygen when electric current is passed through it.
Electric
2H O(s) current 2H (g) + O (g)
2
2
2
Water Hydrogen Oxygen
6. Pressure: Some chemical reac ons occur when the reactants are subjected to high pressure.
Example: Nitrogen reacts with hydrogen under high pressure to form ammonia.
450°C
N (g) + 3H (g) 2NH (g)
2
2
3
Nitrogen Hydrogen 200 atm Ammonia
7. Catalyst: Some reac ons take place in the presence of a catalyst.
Let’s now study about catalysts and enzymes in detail.

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