Change of State
The 3 Basic States of Matter
What about Plasma?
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 (DHf)


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
(DHv)


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.
Heating Curves
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
100oC into 1 g of steam at 100oC.
Heating Curves
Latent Heat Formula

Latent heat can be calculated using the
following formula:
Q = mDH
where
Q = the quantity of heat in Joules (J)
m = the mass of the material in g
DH = the latent heat of the substance in J/g
Latent Heat Values
Substance
Melting Point
(oC)
Latent Heat
of Fusion
DHf (J/g)
Boiling Point
(oC)
Latent Heat
of
Vaporization
DHf (J/g)
Water
0
333
100
2260
Aluminum
659
399
2327
10530
Copper
1083
207
2566
4730
Gold
1063
62.8
2808
1720
Mercury
-38.9
11.4
357
296
Nitrogen
-210
25.7
-196
200
Oxygen
-219
13.9
-183
213
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.