Q:

What causes water's low vapor pressure?

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Quick Answer

The vapor pressure of water is relatively low because it is held together through hydrogen bonds. This means that the molecules in a glass of water do not evaporate from the water very easily, as the molecules are attracted to each other. However, as with most liquids, the vapor pressure of water rises as temperature increases.

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Full Answer

Hydrogen bonds are much weaker than covalent or ionic bonds, which explains why water molecules do not break apart as easily as they evaporate. Nevertheless, the hydrogen bonds between water molecules cause the molecules to tend to stay in the liquid state. Additionally, the hydrogen bonds cause the surface tension of water to be relatively high.

The amount of air pressure over a sample of water influences its vapor pressure and boiling point. For example, if the amount of air pressure over the water is doubled, much fewer water molecules break their bonds and evaporate, resulting in lower vapor pressure. This also causes water to have a lower boiling point when the atmospheric pressure is lower. Temperature also affects the vapor pressure of a liquid. When water is heated, the individual molecules have more energy, which allows them to break the hydrogen bonds more easily.

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