Water vapor pressure, at various temperatures

A mixture of gases always occurs when a gas is collected by displacement of water. For example, Figure 13.8 shows the collection of the oxygen gas that was produced by the decomposition of solid potassium chlorate. The gas is collected by bubbling it into a bottle initially filled with water. Thus the gas in the bottle is really a mixture of water vapor and oxygen. (Water vapor is present because molecules of water escape from the surface of the liquid and collect as a gas in the space above the liquid.) Therefore, the total pressure exerted by this mixture is the sum of the partial pressure of the gas being collected and the partial pressure of the water vapor. The partial pressure of the water vapor is called the vapor pressure of water. Because water molecules are more likely to escape from hot water than from cold water, the vapor pressure of water increases with temperature. This is shown by the values of water vapor pressure at various temperatures shown in Table 13.2. 

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