Aspirin SAC

[C dogga]

I have a few qns:

Why must you need to swirl frequently when heating up the mixture of aspirin under a fume hood??
Why do crystals form when you scratch the side of the test tube?
Why do we only add minimum amounts of water when dissolving the  aspirin upon recrystalisation.
Why is it important we remove as much water as possible from the crystals.

[lzxnl]

I have a few qns:

Why must you need to swirl frequently when heating up the mixture of aspirin under a fume hood??
Why do crystals form when you scratch the side of the test tube?
Why do we only add minimum amounts of water when dissolving the  aspirin upon recrystalisation.
Why is it important we remove as much water as possible from the crystals.

I'm not sure about the second one to be honest, but I can answer the others.
1. Swirling increases the rate of collisions and thus the reaction rate. Think about it; if you do a titration and you're close to the endpoint, if you just add more titrant, the colour may persist for quite some time. Swirl, and the colour disappears.

3. If you add too much water, you may not be able to recrystallise much of the aspirin at the end; some may have dissolved.

4. If you don't remove all of the water, your product is impure and not as effective.