Answer 1

1. Close contact— For a chemical reaction to take place the reactants should be brought in close contact i.e., they should be mixed,
2. Solution form— Some substances react with each other only when they are mixed in the solution form,
   e.g.,
    
3. Heat— Some reactants need to be heated to undergo a chemical change,
   e.g.,
    
4. Light— Some reactions take place in the present of light.