Carbon dioxide is a chemical compound and contains two oxygen atoms and one carbon atom in one molecule. Acid reacts with a metallic carbonate to form carbon dioxide. For example, CaCO3 + 2HCl ------> CaCl2 + H2O + CO2. One mole of calcium carbonate reacts with two moles of hydrochloric acid to yield one mole of calcium chloride, one mole of water and one mole of carbon dioxide. When carbonates of metals except alkali metals are heated, carbon dioxide is formed. For example, MgCO3 ------> MgO + CO2. One mole of magnesium carbonate decomposes in one mole of magnesium oxide and one mole carbon dioxide.
