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| Pressure cooker. Image courtesy www.yourbestdigs.com/reviews/the-best-pressure-cooker, CC BY 2.0 via Wikimedia Commons. |
Water boils at 100 °C. But that's only at atmospheric pressure at sea level. All liquids boil when their vapour pressure
exceeds the ambient atmospheric pressure. Vapour pressure varies with
temperature (see graph below) and also different liquids have different
vapour pressures at the same temperature. The ambient atmospheric
pressure effectively "holds" the molecules of a liquid in, preventing
them from escaping. Vapour pressure is defined as the pressure exhibited
by vapour present above a liquid's surface. When water reaches 100 °C,
its vapour pressure is 1 atmosphere. That's equivalent to 1 bar, 15
pounds per square inch (PSI) or 101325 pascals (Pa) in metric. Since
that equals the ambient atmospheric pressure (which is 1 atm absolute
above vacuum), any increase in temperature will increase the water's
vapour pressure, allowing it to boil.
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| Vapour pressure versus temperature for water. No machine-readable author provided. Adam Rędzikowski assumed (based on copyright claims), CC BY-SA 3.0 |
Volatile Liquids
Some
liquids have higher vapour pressure for the same temperature. Alcohol
for instance which has a value greater than that of water and ethanol, a
type of alcohol, boils at 78 °C.
Pressure cookers
Pressure
cookers work by increasing pressure within the cooking vessel as the
temperature of the contents increases. Gay-Lussac's law states that at
constant volume, pressure of a gas increases proportionately with
absolute temperature. If you've ever put an empty juice or milk carton
in the fire, with the cap tightly shut, you'll know that it bulges up
and eventually bursts. A pressure cooker typically operates at a
pressure of 2 atm absolute (or 1 atm gauge above atmospheric pressure).
Water temperature has to increase so its vapour pressure equals this
before it boils. This occurs at approximately 120 °C and it's this
higher temperature that increases the cooking speed of the contents.
Also steam at 120 degrees has more energy than water at the same
temperature. This is because when water undergoes a phase change to
steam, it gains energy, known as latent heat of vaporisation.
When steam comes in contact with food, it has to lose that energy first
before it turns back into water, transferring that energy to the food.
Cups of Tea on Mount Everest
The
reverse is true for temperature and pressure. At high altitudes,
ambient atmospheric pressure is reduced. So water boils at a lower
temperature of 68 °C on Mount Everest, not so good if you like milky
tea.