When a chemical reaction occurs, one or more of the following changes take place :

(i)    Change in state
(ii)    Change in colour
(iii)    Evolution of a gas    
(iv)    Change in temperature.

(i) Burning of magnesium ribbon in air gives a powder (of MgO). So there is a change of state.

Magnesium + Oxygen --> Magnesium oxide(MgO)

(ii) Addition of colourless lead nitrate solution to potassium iodide solution gives yellow
coloured precipitate (lead iodide). So there is a change of colour.

(iii) Addition of dilute hydrochloric acid to zinc granules gives a gas (H₂) with effervescence.

The mode of communicating facts in chemical language about a chemical reaction is called chemical equation. Thus, a chemical equation is an expression for given chemical change in terms of symbols or formula of the reactants and products. For example, the reaction of zinc with dilute sulphuric acid to produce zinc sulphate and hydrogen is given by the following chemical equation:
Zn + H₂SO→ ZnSO₄ + H₂

In aobve reaction those are left-hand side is reactant and other part that is left hand side is product.