What increases vapour pressure?
As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.
As for vapor pressure being higher at higher temperatures, when the temperature of a liquid is raised, the added energy in the liquid gives the molecules more energy and they have greater ability to escape the liquid phase and go into the gaseous phase.
This vapour pressure is controlled or affected by certain factors. These factors include the nature of liquid, temperature, boiling point of the liquid, and solute concentration.
The vapor pressure is highly dependent not only on the nature of the liquid, but also on its temperature: It increases with increasing temperature. In such instances, it increases until it matches the ambient pressure above the liquid (usually 1 atm or 760 mmHg).
A liquid's vapor pressure is directly related to the intermolecular forces present between its molecules. The stronger these forces, the lower the rate of evaporation and the lower the vapor pressure.
As temperature increases the molecular activity at the surface of the water would increase. This means that more molecules of water would be transitioning to gas. With more gas molecules there would be an increase in the vapor pressure assuming the volume of the container is remaining constant.
Relative humidity is related to the partial pressure of water vapor in the air. At 100% humidity, the partial pressure is equal to the vapor pressure, and no more water can enter the vapor phase. If the partial pressure is less than the vapor pressure, then evaporation will take place, as humidity is less than 100%.
On adding a non-volatile solute to a solvent, the vapor pressure of the solution becomes less compared to the vapor pressure of the pure solute. This lowers the vapor pressure of the solvent.
- Temperature: To raise VPD: increase temperature. To lower VPD: decrease temperature.
- Humidity. To raise VPD: decrease humidity (turn dehumidifier on) To lower VPD: increase humidity (turn dehumidifier off)
Pentane, hexane and heptane
The London forces between the smaller pentane molecules are much weaker than the forces between the larger heptane molecules. Therefore pentane has the largest vapor pressure and heptane has the smallest vapor pressure.
Does vapor pressure increase with boiling point?
In general, liquids with a lower boiling point will exert higher vapor pressures.
When the temperature of a liquid increases the kinetic energy of its molecules also increases. Due to this the molecules which undergo the process of vapour also increase which ultimately increases the vapour pressure.
The vapor pressure of a liquid lowers the amount of pressure exerted on the liquid by the atmosphere . As a result, liquids with high vapor pressures have lower boiling points.
In the atmosphere, the dry air exerts more pressure than most air. As dry air consists mostly of nitrogen and oxygen and other gases which exerts high pressure on the other hand in moist air-water vapour contain increases, the weight of which is less than nitrogen and oxygen. So moist air is lighter than dry air.
Substances with high vapor pressures are said to be volatile - that is, they easily evaporate. Many small organic compounds are volatile, including most scent compounds. If you can smell a liquid, it is because molecules of the liquid are evaporating into the gas phase and traveling through the air to your nose!
Relationship between vapor pressure, relative humidity and temperature. The relative humidity is the ratio of vapor pressure and saturation vapor pressure times 100. As the saturation vapor pressure increases with rising temperature, the relative humidity decreases when the vapor pressure is constant.
Vapor pressure depends on the actual water vapor in the air, and it may vary from near zero in cold, dry air to about 2 inches of mercury in warm, moist air. High values can occur only in the warm, lower layers of the troposphere.
Instead, the amount of water vapor is controlled by the temperature. This is because the temperature of the surrounding atmosphere limits the maximum amount of water vapor the atmosphere can contain.
