Compound A is sodium hydrogencarbonate-NaHCO3
(i) It is used in fire extinguishers because it produces CO2 gas on reaction with acid.
H2SO4 + 2NaHCO3 ----> Na2SO4 + 2H2O + 2CO2
(ii) It is used as an antacid because it neutralises excess acid (HCl) present in stomach.
HCI + NaHCO3 ----> NaCl + H2O + CO2
(iii) It is used in making bakery items because on reaction with acid it produces CO2 gas which makes bread, cakes, etc., soft and spongy.
NaHCO3 + H+ ------> H2O + CO2 + Na-salt of acid