How can you boil a liquid without heating it? Why is this possible?

Asked by: Betty Arnold

Answer

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 coffee.

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 counteracting force.

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 can:

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, or

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