SPECIFIC HEAT
Imagine you are visiting the beach on a hot summer’s day. You might notice that
the ocean remains much cooler than the hot sand and air. This can seem puzzling
since energy from the same hot sun is falling on the sand, the air, and the ocean
water at the same rate, but the temperature of the water changes much less than
the temperature of the sand and air.
This is because as a substance absorbs heat, its temperature change depends on
the nature of the substance. Not all substances have the same capacity to change
temperature as they absorb heat. This phenomenon explains the temperature
differences between the hot sand beach and the cool ocean water.
For example, imagine you have 1 kg of sand and 1 kg of ocean water. The
temperature of each is 20 ° C. What would happen to the temperature of both
substances if both were placed in the hot sun for 15 minutes? The temperature of
the ocean water would increase 2 Celsius degrees, while the sand would increase
about 12 degrees. This is because, compared to 1 kg of sand, the amount of heat
that is needed to raise the temperature of the 1 kg of water by 1 degree Celsius is
about six times greater. In other words, the ocean water would have to absorb six
times more heat energy as the sand to be at the same temperature. The water has
a higher specific heat capacity than the sand. Specific heat capacity is the amount
of heat that is needed to raise the temperature of 1 g of an object by 1 Celsius
degree. Specific heat is measured in Joules / gram / Celsius degree (J / g / C °) or
calories / gram / Celsius degree (cal / g / C°). Specific heat capacities of some
common materials are listed in table 1.
Table 1: Specific Heat Capacity of Common Materials
Substance
Water
Wood
Carbon (graphite)
Glass
Iron
Table 2:
Specific heat (J / g / C °)
4.18
1.76
0.71
0.66
0.45
Specific heats and molar heat
capacities for various substances at
20 C
c in cal/gm K or Molar C
Btu/lb F
J/mol K
Substance
c in J/gm K
Aluminum
0.900
0.215
24.3
Bismuth
0.123
0.0294
25.7
Copper
0.386
0.0923
24.5
Brass
0.380
0.092
...
Gold
0.126
0.0301
25.6
Lead
0.128
0.0305
26.4
Silver
0.233
0.0558
24.9
Tungsten
0.134
0.0321
24.8
Zinc
0.387
0.0925
25.2
Mercury
0.140
0.033
28.3
2.4
0.58
111
Water
4.186
1.00
75.2
Ice (-10 C)
2.05
0.49
36.9
Granite
.790
0.19
...
Glass
.84
0.20
...
Alcohol(ethyl)
Compared with other common materials, water has a very high specific heat
capacity. This means that water can absorb heat without a large change in
temperature. This makes water a useful substance to cool or warm other
substances.
CALCULATING HEAT ENERGY
The temperature, the amount of a substance, and the nature of the substance are
variables that are needed to measure energy. By measuring those variables, we can
calculate the amount of heat energy that is absorbed by an object. It is related to
the mass of the object, its specific heat, and its change in
temperature. Therefore, to calculate the amount of energy, you will apply the
following equation:
Heat energy = mass of object x change in temperature x specific heat capacity
To make life easier, scientists measure the energy of substances working with one
substance – water. They calculate heat energy by heating a specific mass of water
and measuring temperature changes. From the table above, note that water has a
specific heat of 4.18 J / g / C °. That means that to increase the temperature of 1
gram of water by 1 C °, 4.18 joules are needed. Water that has a mass of 1 g has a
volume of 1 mL.
So to find the amount of energy needed to raise the temperature of 15 g (15 mL)
of water by 20 C °, plug the numbers into the equation:
Heat energy = 15 g x 20 C ° x 4.18 J / g / C °
Heat energy = 1,254 J