 Metal hydrogen carbonates decompose to give the metal oxide, water, and carbon dioxide. We're going to heat some sodium hydrogen carbonate, and as the sodium hydrogen carbonate decomposes, the carbon dioxide will be bubbled through some lime water. The lime water turning cloudy will be an indication of the formation of carbon dioxide. When carbon dioxide bubbles through lime water, which is a solution of calcium hydroxide, carbon dioxide forms carbonic acid, which reacts with the calcium hydroxide to form calcium carbonate. The calcium carbonate is not very water soluble and settles to the bottom of the test tube. The water vapor that's produced will condense at the top of the test tube, and we will test for the presence of the water using some cobalt blue paper.