Latent Heat & Change of State Adding or removing heat does not always result in a change of temperature. During a change of state, the heat added is called latent heat because there is no change in temperature. Latent means "hidden".
Melting When a solid is melting the heat energy added is building up the energy of the molecules so they can break free of the forces holding them together.
Evaporation or Boiling Similarly, when liquids are turning to gases the heat energy increases the energy of the molecules so they get further apart and become gas molecules.
Latent Heat of Fusion ( H f ) Latent heat of fusion is the amount of heat required to melt 1 g of a substance at its melting/freezing point without changing its temperature. It is also the amount of heat that must be removed to solidify 1 g of a substance at its melting/freezing point without changing its temperature.
Latent Heat of Vaporization ( H v ) Latent heat of vaporization is the amount of heat required to vaporize 1 g of a substance at its boiling point without changing its temperature. It is also the amount of heat that must be removed to condense 1 g of a gas at its boiling point without changing its temperature.
Water The latent heat of fusion for water is 330 J/g, which means that 330J of energy are needed to change 1 g of ice at 0°C into water at 0°C. The latent heat of vaporization for water is 2260 J/g, which means that 2260 J of energy are needed to change 1 g of liquid water at 100 o C into 1 g of steam at 100 o C.
Latent Heat Formula Latent heat can be calculated using the following formula: Q = m H where Q = the quantity of heat in Joules (J) m = the mass of the material in g H = the latent heat of the substance in J/g
Latent Heat Values SubstanceMelting Point ( o C) Latent Heat of Fusion H f (J/g) Boiling Point ( o C) Latent Heat of Vaporization H f (J/g) Water03331002260 Aluminum659399232710530 Copper108320725664730 Gold106362.828081720 Mercury-38.911.4357296 Nitrogen-21025.7-196200 Oxygen-21913.9-183213
Latent Heat Applications Water has one of the highest latent heat of fusion values of all substances and therefore has several applications. 1. Picnic Coolers The heat required to melt the ice comes from the food or drinks in the cooler. Since heat leaves the food, it gets cold. 2. Preventing Frost Damage When a frost is predicted, farmers will turn on the water sprinklers. As the water falls on the plants and starts to freeze, heat is released to the surroundings and plants. The heat helps the plants stay warm enough to prevent damage. This only works when the temperature does not drop much below freezing.
Latent Heat Applications 3. Cooling Off When Wet You can cool off your body by just coming out of a shower or swimming pool if you do not dry off with a towel. When you are wet, heat from your body is used to vaporize the water. Because heat leaves your body, you feel cool. If you don't want to cool off, just dry yourself well with a towel. 4. Avoid Steam Burns When steam condenses it releases heat to the surroundings. Then the condensed water cools from 100°C to 37°C (body temperature), so even more heat is released and absorbed by the skin. That's why a steam burn is much worse than a burn from boiling water.