Vapor Pressure and Water

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The vapor pressure of a liquid is the point at which equilibrium pressure is reached, in a closed container, between molecules leaving the liquid and going into the gaseous phase and molecules leaving the gaseous phase and entering the liquid phase. To learn more about the details, keep reading!

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Vapor Pressure and Water

With any body of water, water molecules are always both evaporating and condensing. The vapor pressure of water is the pressure at which the gas phase is in equilibrium with the liquid phase. The high surface tension of water (water "sticks" to itself, so it doesn't "want to" evaporate) means water has a low vapor pressure.

An explanation of vapor pressure

Drawing of pressure cooker with water molecules in equilibrium-same amount evaporating as condensing

Vapor pressure is constant when there is an equilibrium of water molecules moving between the liquid phase and the gaseous phase, in a closed container.

Vapor pressure is constant when there is an equilibrium of water molecules moving between the liquid phase and the gaseous phase, in a closed container.

The vapor pressure of a liquid is the point at which equilibrium pressure is reached, in a closed container, between molecules leaving the liquid and going into the gaseous phase and molecules leaving the gaseous phase and entering the liquid phase. Note the mention of a "closed container". In an open container the molecules in the gaseous phase will just fly off and an equilibrium would not be reached, as many fewer gaseous molecules would be re-entering the liquid phase. Also note that at equilibrium the movement of molecules between liquid and gas does not stop, but the number of molecules in the gaseous phase stays the same—there is always movement between phases. So, at equilibrium there is a certain concentration of molecules in the gaseous phase; the pressure the gas is exerting is the vapor pressure. 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.

 

Your nose knows about vapor pressure

Let's say you liked to eat turnip greens but didn't like the smell of them cooking. What you would want to do is cook them quicker, in that case. To cook your greens you put them in a pot of boiling water...water that, at sea level, boils at 212°F. In an open container water won't get hotter than that. Rather you'll notice a lot of steam coming out of the pot due to evaporation. Adding more heat won't raise the water temperature, and thus, won't cook your greens faster.

If you wanted to cook your turnip greens quicker, you would want the water temperature to be higher. But, there is a solution that will lessen the time you have to smell the greens. You can use vapor pressure to "trick" your turnip greens by using a closed container to cook in—known as a pressure cooker. Pressure cookers have lids that can be secured to the pot which prevents steam from escaping the pot, which raises the pressure of the vapor inside the container. There is a pressure-release valve on the top of the pot to prevent pressures from getting so high that the pot explodes (although there are many instances of the valve malfunctioning with the disastrous effect being a pot that literally explodes). We mentioned that with a higher vapor pressure higher water temperatures can be reached, meaning that in a pressure cooker the vapor pressure is much higher and thus, the water doesn't boil until it reaches a higher temperature, which cooks the food faster.