Evaporation and condensation take place over a range of temperatures; boiling takes place at a specific temperature.
melting
freezing or solidification
liquid Melting: a pure substance melts suddenly at a particular temperature. Freezing: the reverse takes place sharply at the same temperature.
increasing temperature
sublimation
Sublimation: a few solids change directly from solid to gas on heating; the term sublimation is used for the change in either direction.
condensation or liquefaction
evaporation or vaporisation
gas
solid Figure 2.2
Changes of physical state and the effect of increasing temperature at atmospheric pressure.
increasing temperature solid
liquid
melting point (m.p.)
gas
boiling point (b.p.)
decreasing temperature Figure 2.3 a substance.
The relationship between the melting point and boiling point of
a purple vapour, but then condenses again on a cold surface (Figure 2.5, overleaf). Evaporation, boiling and condensation If a liquid is left with its surface exposed to the air, it evaporates. Splashes of water evaporate at room temperature. After rain, puddles dry up! When liquids change into gases in this way, the process is called evaporation. Evaporation takes place from the surface of the liquid. The larger the surface area, the faster the liquid evaporates.
Figure 2.4 The metal gallium has a melting point just above room temperature. It will literally melt in the hand.
The warmer the liquid is, the faster it evaporates. Eventually, at a certain temperature, it becomes hot enough for gas to form within the liquid and not just at the surface. Bubbles of gas appear inside the liquid. This process is
Original material © Cambridge University Press 2014
Chapter 2: The nature of matter
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