Testing for carbon dioxide. Apparatus used to test for the production of carbon dioxide by fermenting yeast. The yeast is placed in a test tube (right) with sugar so that it begins to ferment. The gas produced by the fermentation passes out of the tube in the bung (red) and passes into a test tube containing lime water (calcium hydroxide solution, centre). Carbon dioxide (CO2) passed into limewater produces a milky suspension of calcium carbonate. When yeast ferments it converts sugar to carbon dioxide and alcohol. Therefore the lime water turns milky.