In a new bottle of soda, the concentration of carbon dioxide in the liquid phase has a particular value. If half of the liquid is poured out and the bottle is sealed, carbon dioxide will leave the liquid phase at an ever-decreasing rate, and the partial pressure of carbon dioxide in the gas phase will increase until equilibrium is reached. At that point, due to thermal motion, a molecule of CO 2 may leave the liquid phase, but within a very short time another molecule of CO 2 will pass from the gas to the liquid, and vice versa. At equilibrium, the rate of transfer of CO 2 from the gas to the liquid phase is equal to the rate from liquid to gas. In this case, the equilibrium concentration of CO 2 in the liquid is given by Henry's law, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. This relationship is written as