The reaction
As the seeds respire they take in oxygen and release carbon dioxide at roughly the same rate. If left alone in a sealed test tube, the carbon dioxide would replace any oxygen utilized by the seeds and the air pressure would remain relatively constant. In this experiment, any carbon dioxide released in the test tube reacts with the calcium hydroxide to form solid calcium carbonate, also known as calcite or limestone. This process essentially removes all gaseous carbon dioxide from the air space in the test tube and converts it to a solid. As more carbon dioxide is produced, more carbon dioxide is removed from the air and the air pressure in the test tube declines, essentially sucking water up into the test tube. If atmospheric pressure is higher outside the test tube than inside the test tube, water will rise in the test tube. Theoretically, the difference in air pressure should equal the weight of the water that rose in the test tube (P1- P2 = weight of water). Thus, the height of the water in the test tube is an indicator of the amount of respiration that occurred. It is theoretically possible to calculate a respiration rate from the change in the volume of air in the test tube, but changes in humidity and barometric pressure could complicate the calculations.