Do crystals grow faster when the solution was in an ice bath or if it was at room temp?

Question

The chemicals used in my lab were:
- Aluminum metal
- Potasssium Hydroxide
- Sulfuric acid


A guy I know guessed and said: The slow cool lets the molecules move faster, which increases the likelihood of bonds but the ice bath would slow them down much more, meaning there is less of a chance of bonds.


But people on Yahoo Answers said that...

"Usually things dissolve much easier in warm water. Crystals should form more rapidly in an ice bath because they are more supersaturated. Rapid moving around is not necessarily conducive to crystal formation. "

" Ice bath. Crystallization is faster lower the temperature. "

"They'll form faster under an ice bath if you think about it...crystals are essentially solids...when water freeze into ice and ice is crystal lattices this is what is occurring with other crystal formations."

Answer 1

The people on YA are essentially right; for most compounds, the crystalization is faster if you cool the solution, as you are forcing it to crystallize by reducing solubility. However, if you let the crystals grow more slowly you will generally get more pure, larger and better shaped crystals.

Answer 2

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RE:
Do crystals grow faster when the solution was in an ice bath or if it was at room temp?
The chemicals used in my lab were:
- Aluminum metal
- Potasssium Hydroxide
- Sulfuric acid


A guy I know guessed and said: The slow cool lets the molecules move faster, which increases the likelihood of bonds but the ice bath would slow them down much more, meaning there is less of a chance...

Answer 3

Again - YA has it. Lowering the temperature lowers the solubility so the solid is precipitated more quickly. Sometimes in the lab the products of a reaction are put into an icebath specifically to make the crystals form faster (I'm thinking especially of organic chemistry preps).
If you want a fewer number of larger, regular crystals then slow formation is the way to go - but it does take longer.

Answer 4

The people on Yahoo! Answers are correct; lowering the temperature reduces the ability of the water to contain the dissolved substances.

When that happens, they have to go somewhere, so they will either precipitate out of solution, or crystallize, depending on the nature of the dissolved solids.

Answer 5

I want to ask the same question as the op.