Consider a gas inside a container. The pressure exerted by the gas is the sum of the forces exerted due to the molecular collisions between its molecules and the surrounding surface. A gas with many molecules exerts a higher pressure than a gas with a smaller number of molecules in the same container. So, the pressure from the individual component is called its partial pressure. Consider a mixture of ideal gases in a container at thermal equilibrium. The total pressure inside the container equals the sum of the partial pressures of its components. This observation is known as Dalton's law of partial pressures. Recalling the ideal gas equation and substituting the partial pressures of the individual gases, the total pressure can be determined. The partial pressure of a vapor which is in equilibrium with the liquid phase of the same substance is called vapor pressure. The ratio of the partial pressure of water vapor to the vapor pressure of water at a temperature is known as relative humidity.