Boiling - liquid to gas Boiling point Boiling point of pure and impure substances Evaporation Condensation - gas to liquid

Boiling, evaporation and condensation are terms that refer to the changes in state between liquids and gases:

• Boiling occurs when a liquid is heated and turns to a gas.
  
• Evaporation occurs when a liquid changes to a gas at a temperature below its normal boiling point.
  
• Condensation occurs when a gas is cooled and turns to a liquid.

Boiling - liquid to gas
As a liquid is heated, more and more particles will evaporate off the liquid surface. It is when the pressure of these escaping particles is equal to the external air pressure on the surface of the liquid, that bubbles of gas start to form in the liquid.

Boiling occurs when enough particles in a liquid escape en masse from the liquid to form bubbles of gas in the body of the liquid. These gas bubbles then rise to the surface of the liquid and the liquid is said to boil.

Boiling

Boiling point
The boiling point (or condensation point) of a substance is defined as the temperature at which the vapour pressure of the liquid is exactly equal to the external pressure. Above the boiling point, the substance exists as a gas and below, it exists predominately as a liquid.

The temperature at which a liquid boils is affected by atmospheric pressure. The higher the altitude, the lower the temperature at which boiling occurs (and the lower the liquid's boiling point).

When quoting a boiling point, scientists also quote the air pressure. All standard boiling points are measured at sea level where the air pressure is an average of 1 atmosphere or 101.3 kPa pressure.

Different liquids have different boiling points depending on the strength of bonding between the particles and the mass of the particles. The heavier the particles in the liquid, and the stronger the bonding, the higher the boiling point will be.

Helium, the lightest and weakest bonded substance known, has a boiling point very near absolute zero at -269°C. At the other end of the scale, Tungsten, made of much heavier particles with very strong bonding, boils at 5660°C.

Boiling point of pure and impure substances
For pure substances, the temperature at which boiling and condensing occurs is quite sharp. For impure substances, boiling and condensing occurs more gradually over a range of temperatures.

This is one way that chemists can measure the purity of a substance. A pure substance will boil at a specific temperature, the more impure a substance the more its boiling point will vary over a range of temperature.

Evaporation
When a liquid is heated, its particles eventually gain enough energy to overcome the forces holding them loosely in place in the liquid and become free, fast moving individual particles in the gas state.

Evaporation occurs when particles in a liquid pass directly into the gas phase, at a temperature below the boiling point of the liquid. Evaporation is dependent on individual particles gaining enough energy to escape the surface of the liquid and becoming gas particles.

Evaporation at high temperature

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