The USGS Water Science School
Vapor Pressure and Water
Your nose knows about vapor pressure
The equilibrium pressure, in a closed container, between molecules moving between the liquid and gaseous phases.
So, what do turnip greens, your nose, and vapor pressure have in common? To try to explain vapor pressure to you, maybe the best place to start is your nose. Assuming that the gentleman in this picture does not care for the smell of turnip greens, he is adjusting the vapor pressure to help keep the smell down (sorry, no thermostat-like device really exists that would actually allow you to adjust the vapor pressure for your whole house, but it would be handy, right?). The vapor pressure of liquids helps to determine the extent to which molecules in the liquid stay as liquid or escape into the air as gas or vapor.
As far as your nose is concerned, when you boil water to cook a delicious pot of turnip greens, the added heat in the pot energizes the water molecules so that some of them escape into the kitchen air as gas. Indirectly, the smell of turnips will escape into the air, too, which is why you smell that turnip aroma when you boil them in water. The added heat raises the normal vapor pressure of water, with the result being water vapor wafting through the kitchen, and with it, the smell of turnips.
The example above is a little deceptive as when water boils water vapor is released, but not necessarily "turnip vapor". But, the higher heat and agitation of boiling water helps to break down molecules in the turnips and those are released as particles into the air with the water vapor. So, you're not really smelling the water vapor, but you are smelling tiny bits of turnips. The same concept does hold true for various liquids, and here vapor pressure does explain why you smell some liquids more readily than others.
An explanation of vapor pressure
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.
Turnip greens in a hurry
If you wanted to cook your turnip greens quicker you would want the water temperature to be higher. But, in an open container, water boils at 212°F (at sea level), and if you continue to heat the water you will release more molecules as water vapor but the temperature of the water won't go higher than 212°. Now, this assumes you live at sea level, since as the higher your kitchen is above sea level, the lower the air pressure pushing down on the water in the pot of turnip greens, and the lower the vapor pressure, and thus, the water will boil at a lower temperature. That is why it takes longer to cook food at higher altitudes.
The point is, in an open container once water reaches the boiling point it will not get hotter. You can use vapor pressure to "trick" your turnip greens, though, 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.
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