The vapour pressure of a liquid primarily depends on the nature of the liquid and its temperature.
Let's break this down:
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Nature of the Liquid (Intermolecular Forces): Different liquids have different intermolecular forces (IMFs). IMFs are the attractive or repulsive forces between molecules. Liquids with stronger IMFs require more energy to overcome these attractions for molecules to escape into the gaseous phase. Therefore:
- Stronger Intermolecular Forces → Lower Vapour Pressure
- Weaker Intermolecular Forces → Higher Vapour Pressure
For example, ethanol has weaker hydrogen bonding than water. Therefore, at the same temperature, ethanol has a higher vapour pressure than water.
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Temperature: As temperature increases, the average kinetic energy of the liquid molecules increases. This means more molecules have enough energy to overcome the intermolecular forces and escape into the gas phase, leading to a higher vapour pressure.
- Higher Temperature → Higher Vapour Pressure
- Lower Temperature → Lower Vapour Pressure
This is why water evaporates faster on a hot day than on a cold day. The increased temperature provides more molecules with the energy needed to transition into the vapour phase.
In summary, vapour pressure reflects the tendency of a liquid to evaporate. This tendency is determined by how strongly the molecules are held together (intermolecular forces) and how much kinetic energy they possess (temperature).
