Condition required for a chemical reaction to take palce :

Chemical reactions can be done through:

1. Physical contact 

2. Solution of reactants 

3. Electricity 

4. Heat 

5. Light 

6. Catalyst

1. Chemical Reactions Based On Solution Of Reactants: 

When we mix two substances (Reactants) in solution form, the chemical reaction takes place to form new substances (Products).

Example: 

• Take small amount of solid silver nitrate and sodium chloride in a test tube. The reactants in solid state have no reactions.
• Dissolve the same reactants in water in separate test tubes. 
• Mix both the solutions. Silver nitrate solution reacts with sodium chloride solution to form a white precipitate of silver chloride and sodium nitrate solution. 
• From the above reaction, we infer that some chemical reactions proceed only in solution form not in solid form.

2. Chemical Reaction Based On Electricity :

• When electricity is passed through water containing small amounts of sulphuric acid, hydrogen and oxygen gases are liberated. 
• Similarly, a concentrated solution of sodium chloride called Brine is electrolysed to produce chlorine and hydrogen gases along with sodium hydroxide.
• This is a very important reaction to produce chlorine industrially. 
• From the above two reactions, we infer that some chemical reactions proceed only by the passage of electricity. 
• Hence, such reactions are called as electrochemical reaction or electrolysis.

3. Chemical Reactions Based On Light: 

Sunlight is important not only for us but also for plants as welll. As you know photosynthesis is a process in which light energy from the sun is used by the plants to prepare starch from carbon dioxide and water.

Example: 

The sunlight uses the chemical reaction between carbon dioxide and water, which finally ends up in the production of starch (photo means light and synthesis means production). These chemical reactions is used by light are called as photochemical reactions.