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Notes on the differences between evaporation and boiling
The bubbles usually seen in water which is being heated but
is still below the boiling point are bubbles of dissolved
air, rather than of water in the gas state.
Notes on particle explanation of the differences between evaporation
and boiling
Books often say that water is evaporating when it is boiling.
In the sense that water particles end up mixed with air particles
this is fine. However, it is important to understand the difference
between boiling and evaporation below boiling point. We could
think of boiling as a special case of evaporation.
Notes on the Think for Yourself questions
'The propanone evaporates faster than water because
propanone particles are more energetic than water particles.'
No. Both samples are at the same
temperature, so the proportion of high energy particles should
be about the same.
'Propanone particles have a weaker hold on each other.'
Yes. This is a better explanation.
The propanone particles have a weaker hold on each other,
so they do not need
to have as much energy to leave on their own and are easier
for air particles to knock out. Note
Some students may also suggest that the difference could be
due to propanone particles being bigger than water particles.
Although this is a sensible idea –more of the smaller
water particles would have to escape from the same volume
of liquid – it does not account for most of the observed
difference. (Propanone particles are four times larger but
the sample of propanone evaporates roughly ten times faster).
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Science Enhancement Programme CD-ROM 2005 |
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