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Changes in Physical State
Changes in Physical State
Daphny Bian
The interconversion of physical
states of matter
Melting and freezing
The temperature at which a pure substance turns to a
liquid is called the melting point (m.p.). This always
happens at one particular temperature for each pure
substance.
? The process is reversed at precisely the same
temperature if a liquid is cooled down. It is then
called the freezing point (f.p.).
? The melting point and freezing point of any given
substance are both the same temperature.
? For example, the melting and freezing point of pure
water take place at 0℃.
Evaporation
? If a liquid is left with its surface exposed to the air, it
evaporates. When liquids change into gases over a range of
temperature, the process is called evaporation.
? Evaporation takes place from the surface of the liquid.
? The rate of evaporation depends upon:
Temperature: the warmer the liquid, the faster it evaporates.
Wind speed: the higher the wind speed, the more evaporation.
Surface area: the larger the surface area, the faster the liquid
evaporates.
Boiling
? Eventually, at a certain temperature, it becomes hot
enough to form gas within the liquid and not just at
the surface. Bubbles of gas appear inside the liquid.
This process is known as boiling.
? It takes place at a specific temperature, at the boiling
point (b.p.) for each pure liquid.
Condensation (gas-liquid)
? The reverse of evaporation is condensation. Also there is a
condensation point (c.p.) for each pure liquid at a specific
temperature,
? This is usually brought about by cooling. However, it is
possible at normal temperatures to condense a gas into a liquid
by increasing the pressure, without cooling.
? The word vapour is used for a gas that can be compressed into
liquid without cooling.
? Normally, though, we use the words gas or vapour when
talking generally about the gas state.
Sublimation
? A few solids – carbon dioxide (dry ice) and iodine,
for example – do not melt whe
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