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TGS1: What speeds up dissolving?
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.

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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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