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TGS1:
What speeds up dissolving?
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The
topic of dissolving was introduced in I1. If you want to review
that section with your class before going on, return to the
main content and use the drop -down menu at the top right
to go straight to the start of I1. You can use the same method
to get back to S1 quickly.
Notes on ‘Think for Yourself’ questions
Why does 1g of sugar as small crystals dissolve faster than
1g as big crystals?
The sugar particles are knocked out
from the surface of the crystals. The smaller the sugar crystals
the bigger the total surface area, so more particles can be
hit at the same time.
Why do sugar crystals dissolve faster at higher temperatures?
Only high energy water particles
can knock sugar particles away from a crystal. The warmer
the water, the more
high energy water particles there are, so more sugar particles
can be knocked out at the same time. (There are also
more high energy sugar particles in the crystal).
Look at the animation again. Notice that some sugar particles
are returning. If 50 sugar particles leave and 20
sugar particles return every second, what is the overall rate
of dissolving?
30 particles per second. (These
numbers aren’t ‘real’: they just illustrate
the point).
How would stirring affect the rate at which sugar particles
leave the surface of the crystals?
The rate would not change because
stirring just creates bulk movement in the water: it doesn’t
increase the number of high energy water particles so sugar
particles would not be knocked out at a higher rate.
So how can stirring speed up the rate of dissolving?
It reduces the number of sugar particles
which can return. The bigger the difference in numbers leaving
and returning, the faster the rate of dissolving.
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Science Enhancement Programme CD-ROM 2005 |
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user
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