Question

When carbon dioxide is bubbled through a clear calcium hydroxide solution, the solution appears milky. Write an equation for this reaction, which illustrates that CO2 is an acidic oxide.

Answer 1

1. When carbon dioxide is bubble through a clear solution of calcium hydroxide, a milky solution of calcium carbonate will be formed. The balanced chemical equation for the reaction is as follows: 
CO2 + Ca[OH]2 = CaCO3 + H2O.
That is, one mole of carbon dioxide react with one mole of calcium hydroxide to produce one mole each of water and calcium carbonate.

2. An acidic oxide is defined as those compounds of oxygen which react with water to form acids or react with bases to form salts. They are usually oxides of metals or of metals in high oxidation states.
 In the chemical equation given in answer 1, one can easily see that carbon dioxide is an acidic oxide because it reacts with calcium hydroxide [which is a base] to produce calcium carbonate, which is a salt.