Boiling is the change of state from a liquid into a vapour, occurring at a constant temperature called the boiling point. The boiling point of a substance is the temperature of a substance at which it boils. During the process of boiling, the heat supplied to the liquid does work in separating the molecules and pushing back the surrounding atmosphere (increase in volume). The heat that is absorbed without a change in temperature is called latent heat of vaporisation of the substance.
Condensation is the reverse of boiling, that is when the vapour of the same substance is cooled at the same temperature as its boiling point. Heat is given out during condensation.
Effect of impurities on the boiling point of water
The addition of impurities such as salt or sugar to water raises its boiling point. Sea water has a slightly higher boiling point than water.
Effects of pressure on the boiling point of water
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.
Therefore, reduced pressure decreases the boiling point of water, and increased pressure raises the boiling point of water. The boiling point of water (and water-based liquids) is determined by the ambient pressure. Pressure cookers always require liquid in order to cook food under pressure. At atmospheric pressure and at sea level, the boiling point of water is 100 °C (212 °F) and excess heat only increases the rate at which water evaporates into steam vapour; more heat does not increase the temperature of the water. At higher altitudes above sea level, the atmospheric pressure is lower and thus the boiling point of water is lower, because the lower pressure pushing on the water makes it easier for the water molecules to escape to the surface compared to higher pressure.