(The temperature at
which a liquid boils
to form a gas or a
gas condenses to
form a liquid)
HEATING CURVE
BOILING POINT
MELTING POINT
(The temperature
at which a solid
melts to form a
liquid or a liquid
freezes to form a
solid)
Time (mins)
As heat is added at a constant rate to a solid, the substance changes from a solid to a liquid and
finally to a gas. A graph of the temperature change versus the heat added is called a heating
curve.
As heat is added to the ice, the temperature increases.
When the temperature reaches 0°C, the melting point of ice (the solid state of water), the ice will
begin to melt. Any heat added at this point goes to breaking the bonds in the ice, NOT
increasing the temperature of the water.
Once all the water is in the liquid form, any additional heat will now raise the temperature of the
water, until we reach 100°C, the boiling point of water.
At 100°C, the water begins to boil. The temperature remains constant until all of the water is in
the gaseous form. Once the water is completely converted into steam (the gaseous state of
water), adding heat directly, raises the temperature of the steam.
COOLING CURVE
When a solid substance is heated until it is completely melted, and then heated further and
allowed to cool slowly, the curve obtained by plotting temperature against time is known as the
cooling curve.
(The temperature
at which a solid
melts to form a
liquid or a liquid
freezes to form a
solid)
Explanations for the cooling curve above:
Between A and B, the temperature falls with time - there is no change of state, hence the
substance is still in the liquid state.
Between B and C, there is a constant temperature over time. This suggests that there is a change
of state going on from liquid to solid. The heat lost between these points is that gained in
liquefying the solid substance. Hence, there is a balance of heat, which explains why there is no
reading of temperature change- this steady temperature is the melting point of the substance and
the section, B and C represents liquid/solid state.
Between C and D, the temperature falls again with time. Here, the substance is totally in the solid
state.