<P> In chemistry, a dynamic equilibrium exists once a reversible reaction ceases to change its ratio of reactants / products, but substances move between the chemicals at an equal rate, meaning there is no net change . It is a particular example of a system in a steady state . In thermodynamics, a closed system is in thermodynamic equilibrium when reactions occur at such rates that the composition of the mixture does not change with time . Reactions do in fact occur, sometimes vigorously, but to such an extent that changes in composition cannot be observed . Equilibrium constants can be expressed in terms of the rate constants for elementary reactions . </P> <P> 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 may leave the liquid phase, but within a very short time another molecule of CO will pass from the gas to the liquid, and vice versa . At equilibrium the rate of transfer of CO from the gas to the liquid phase is equal to the rate from liquid to gas . In this case, the equilibrium concentration of CO 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 </P>

A system is said to be in state of dynamic equilibrium when the
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