Change of state
All matter exists normally in three states as solids, liquids or gases. Matter can be changed from one state to another either by heating or cooling.
1. Melting (solid to liquid)
2. Evaporation (liquid to gas)
3. Sublimation (solid to gas)
4. Freezing / Solidification (liquid to solid)
5. Condensation (gas to liquid)
While boiling a pot of water, the temperature of the water increases until it reaches 100 ˚C which is the boiling point of water, and then the temperature remains constant until all the water changes from liquid to gas. During this process heat is continuously added to the water. But the temperature of water does not increase above its boiling point. This is the concept of latent heat capacity.
Latent heat capacity of a substance is defined as the amount of heat energy required to change the state of a unit mass of the material.
Where L = Latent heat capacity of the substance
Q = Amount of heat
m = mass of the substance
The SI unit for Latent heat capacity is J kg-1 .
• The latent heat for a solid - liquid state change is called the latent heat of fusion (Lf)
• The latent heat for a liquid - gas state change is called the latent heat of vaporization (Lv)
• The latent heat for a solid - gas state change is called the latent heat of sublimation (Ls)
the triple point of a substance is the temperature and pressure at which the three phases (gas, liquid and solid) of that substance coexist in thermodynamic equilibrium.
The triple point of water is at 273.1 K and a partial vapour pressure of 611.657 Pascal.