(a) What is a chemical reaction?

(b) State the conditions necessary for a chemical change or reaction.

(a) A chemical reaction is a process of breaking chemical bonds of the reacting substances (reactants) and making new bonds to form new substances (products).

(b) Conditions necessary for a chemical change or reaction are :

1. Mixing (close contact) — Some chemical reactions take place when two substances are mixed in their solid state.
   Pb(NO₃)₂ (s) + 2KI (s) ⟶ 2KNO₃ + PbI₂ (s)

2. Solution — In some cases, a chemical reaction occurs when substances are mixed either in molten or aqueous state.
   AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

3. Heat — Some chemical reactions occur only on heating.
   CuCO₃ (s) $\xrightarrow{\Delta}$ CuO (s) + CO₂ ↑ (g)

4. Light — Some chemical reactions take place by the action of light. They are called photochemical reaction.
   2AgNO₃ $\xrightarrow{\text{sunlight}}$ 2Ag + 2NO₂ + O₂ ↑

5. Electricity — Some reactions are caused or accompanied by the passage of an electric current. They are called electrochemical reaction.
   2NaCl $\xrightarrow{\text{electricity}}$ 2Na + Cl₂ ↑

6. Pressure — Some chemical reaction take place only when the involved substances are subjected to high pressure.
   N₂ + 3H₂ $\xrightleftharpoons{\text{above 200 atm}}$ 2NH₃ ↑

7. Catalyst — Some chemical reactions need a catalyst to accelerate or decelerate the rate at which they occur.
   2H₂O₂ $\xrightarrow{\text{MnO}_2}$ 2H₂O + O₂ ↑