Following substance is used in fire extinguishers :

Sodium hydrogen carbonate (NaHCO₃) which is also known as baking soda or eatable soda.

i. Soda acid fire extinguishers contain a mixture of sulphuric acid and baking soda inside it.

ii. When knob of the fire extinguisher is pressed, sulphuric acid get mixed with baking soda which produces a lot of CO₂ gas.

iii. CO₂ gas covers the surroundings of fire.

iv. It stops the supply of air to the burning substance.

v. Hence, fire gets extinguished.