10 · GASES, LIQUIDS AND SOLIDSHeating and cooling curves
The phase changes shown in Fig. 10.3 may be represented using a heating curvefor
the substance under study (Fig. 10.4).156
Amount of heat energy added/kJTemperature/°CAB
CDSolid
meltingSolidLiquidLiquid
boilingGasEFFig. 10.4Heating curve for a pure substance.A heating curve is a plot of the temperature of a substance as it is being heated (i.e. as
heat energy is added to the substance). The general shape of heating curves is the same
for most pure substances.Melting
As we start to heat the solid substance slowly, its temperature rises (from A to B). The
slope of the line AB depends upon the specific heat capacityof the solid. As soon as
the first drop of liquid is produced (at B), the temperature of the liquid–solid mixture
remains constant (from B to C). This constant temperature is the meltingorfreezing
point(ormelting temperature) of the substance. The constant temperature arises
because all the added heat is used to melt the solid: no energy is used to raise the tem-
perature of the substance. At point C, all the solid has melted. The melting point of
water at a surrounding air pressure of 101 kPa (1 atm) is 0C.Boiling
As we continue heating, the liquid becomes hotter (C to D). The slope of the line CD
depends upon the specific heat capacity of the liquid. At D, the liquid begins to boil.Kinetic theory
Use the kinetic theory to explain the following:
(i) Two beakers are heated with identical bunsen flames for the same length of time. Beaker
A contains less water than beaker B. Why does the temperature of the water in beaker A
rise faster than that of beaker B?
(ii)The drying of clothes on a line is an example of evaporation. Explain why clothes dry faster
on hot, windy days.
(iii)A smell of a stink bomb released in a corner of a room soon spreads to the rest of the
room.Exercise 10B
For more about
specific heat
capacity and heat
changes during
changes of state
see Appendix 10
on the website.