What is Partial Pressure?

Partial Pressure is the pressure given by a particular gas into a mixture of gases.

Imagine, you have some amount of Nitrogen (N2), Oxygen (‎O2) and Carbon Dioxide (CO2) gas and you mixed them in a jar. According to the property of the gas, the gases will spread and take the full space of the jar. Now measure the pressure given by the gases individually. Take out (Hypothetically) any two of the three gases and measure the pressure for that remaining gas. Do that for all the three gases. Then it’ll be found that the pressure given by the mixture will be the summation of the pressures given by the gases individually. The pressure given by the individual gas to the whole jar is called the partial pressure of the gas. Here,

Pressure of the  mixture(P)= Partial Pressure of Oxygen + Partial Pressure of Nitrogen + Partial Pressure of Carbon dioxide (Dalton’s law of Partial Pressure)

Why does water evaporate at room temperature?

There can be a question in our mind that why water evaporates from sea, river and even from a glass of water? The boiling point temperature of water at 1 atmospheric pressure (1 atm) is 100° Celsius. So, to change its state from water to vapor, water must be in the required temperature. But the temperature of any place isn’t that much high. So, how is it possible? Moreover, how do wet clothes become dry without any heat?

partial pressure visualization

Now, let’s come to the point,

Air is a mixture of many gases. The total pressure in atmosphere given by air (1 atm) is the combination of the partial pressures of all the gases. Again, the percentage of vapor in the air is small (up to 4 % at 30° Celsius temperature). The partial pressure of vapor in the air is 2.3 KPa (1 atm=101.325 KPa) at 20°C. So, the partial pressure of vapor is too low. We know that 100°C is the boiling point of water. At this temperature, water gets the energy to break its bonds and become vapor. So, we must need 100°C to make vapor at 1 atm.

Partial pressure increases with the increase of temperature. At 100°C, its partial pressure becomes equal to the vapor pressure (the pressure of the vapor when it is in equilibrium with water or ice) and the surrounding atmospheric pressure. But, when the partial pressure of vapor is less than atmospheric pressure, water can go into its gaseous state below the boiling point temperature. Because water does not need high temperature to become vapor at a partial pressure lower than 1 atm. So, water can evaporate at room temperature. Similarly, wet clothes become dry in air(water particles from the cloth can go to the air by evaporation).


  • Evaporation and boiling are not the same, but by both processes, water turns to vapor. Evaporation can happen at the temperature below boiling point. But for boiling, there must need temperature 100°C or more.
  • Boiling means the excitation of water particles to increase the kinetic energy to become vapor. But in evaporation, there is no excitation. So, the rate of changing its state is not large.

For any information, please comment. Thank you !!!


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