What happens to the temperature
of a block of ice when you put a Bunsen burner underneath it? You
might think that the temperature goes up smoothly, but that's not what
happens. The graph of temperature against time is called a heating
curve. Let's look at the heating curve for water.
Notice that, in
general, the temperature goes up the longer the heating continues.
However, there are two horizontal flat parts to the graph. These
happen when there is a change of state. The
plateaus are also
called phase changes.
The first change of state is melting (changing from a solid to a
liquid). The temperature stays the same while a substance melts. For
water, this temperature is 0°C because the
melting point for water is
The second change of state is boiling (changing from a liquid to a
gas). The temperature stays the same while a substance boils. For
water, this temperature is 100°C because the boiling point for water
Different substances have different melting points and boiling points,
but the shapes of their heating curves are very similar. For example,
this is the heating curve for iron, a metal that melts at 1538°C and
boils at 2861°C.
Heating curves show how the
temperature changes as a substance is heated up. Cooling curves are
the opposite. They show how the temperature changes as a substance is
cooled down. Just like heating curves, cooling curves have horizontal
flat parts where the state changes from gas to liquid, or from liquid
You are likely to have used salol or stearic acid in a school
practical lesson to make your own cooling curve. Salol has a melting
point of about 45°C and stearic acid has a melting point of about 69°C.
They are easily melted in a boiling tube placed in a beaker of hot
water. The temperature can be followed using a thermometer or
temperature probe connected to a data logger. The liquid may be cooled
by putting the boiling tube in a beaker of cold water or just leaving
it in the air.
***Note- The melting and
freezing occur at the same temperature.
During freezing, energy is removed
and during melting, energy is absorbed.