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Vapor pressure is the amount of gas in equilibrium with the liquid and solid phases

Chemistry

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Vapor pressure is the amount of gas in equilibrium with the liquid and solid phases. The higher the vapor pressure, the more gas in equilibrium, and thus the easier it is for the substance to vaporize (turn to gas), and vice versa.

Intermolecular forces are the attractive interactions between molecules. For something to vaporize, you have to get enough energy together to break these interactions. As a result, you'll need more energy (due to higher temperature) to break apart stronger intermolecular forces.

The common intermolecular forces ordered by strength are:

London Dispersion ##<## Dipole-Dipole ##<## H-"bonding" ##<## Ion-Dipole ##<## Ion Pair

Since higher vapor pressures corresponds to more gas in equilibrium, the intermolecular forces are therefore primarily weaker for higher vapor pressures.

We can draw from this that molecules that are primarily attracted by London dispersion interactions will have higher vapor pressures than those attracted by dipole-dipole interactions, which will in turn have higher vapor pressures than those with H-bonding interactions.

In the end, we can make the overall relationship that:

The stronger the intermolecular forces, the stronger the interactions that hold the substance together, the lower the vapor pressure of a liquid at the given temperature, and the harder it is to vaporize a substance.