How can you boil a liquid without heating it? Why is this possible?
Asked by: Betty Arnold
The boiling point of a liquid depends on both temperature and pressure. As pressure
increases, so does the boiling temperature. Pressure cookers are used in cooking to raise
the temperature at which liquids within will boil. Conversely, the lower atmospheric
pressure on a mountain top makes it harder to get boiling water hot enough for good tea or
Boiling occurs when a liquid's molecules have enough energy to break free from surrounding
molecules. Think of higher pressure as making that escape more difficult by offering a
Water boils under normal atmospheric pressure at 212°F (100°C). Imagine it at 221°F (105°C)
but NOT boiling in a pressurized container. If the pressure is quickly reduced, the
221°F (105°C) water at normal pressure will now boil.
For another example, put water at room temperature into a vacuum chamber and begin removing
the air. Eventually, the boiling temperature will fall below the water temperature and
boiling will begin without heating.
Answered by: Paul Walorski, B.A. Physics, Part-time Physics Instructor
Boiling just means allowing the particles of a liquid to separate and begin flying about
independently as a gas. There are two competing forces at work: the kinetic energy of the
individual particles, which all together is the heat in the substance, and the attractive
forces holding the particles together. In addition, the pressure of other substances on
the surface of the liquid can squeeze the liquid, pushing on the liquid's molecules and
holding them together. Thus, in order to let the liquid's molecules zoom off as a gas, you
1. Add heat to the substance, so that a larger percentage of the total molecules in the
liquid will have enough kinetic energy to break free from the intermolecular attractions,
2. Remove some of the outside pressure which is holding the surface molecules of the
liquid in place.
Thus if you want to make cold liquids boil, put them in an area of low pressure! If you
have access to a vacuum pump, this is easy to demonstrate. Also note that this is why
liquids boil more easily in the mountains than on coastal plains, and therefore why there
are special 'high altitude cooking instructions' on many foods. Boiling water in Colorado
just isn't as hot as boiling water in Florida.
Answered by: Rob Landolfi, Science Teacher, Rockville, MD
'Physics is mathematical not because we know so much about the physical world, but because we know so little; it is only its mathematical properties that we can discover.'