Why add specifically hot solvent during recrystallization procedures instead of heating it up after?

Question

Recrystallization methods that I have seen online and in manuals involve adding a minimum amount hot solvent to the solid mixture. Is there a reason I should do this instead of adding an equivalent amount of solvent at room temperature and then heating?

I've tried dissolving table salt and sugar both ways, but I didn't notice a substantial difference other than adding hot solvent directly requires less time to dissolve because it took time to heat the cooler solvent.

Answer 1

The goal of recrystallisation is to obtain purified crystals from impure solid, as much as possible. The solute is therefore dissolved in a solvent where the solubility is high at the boiling point, but low (or insoluble) at the room temperature. Additionally the impurities should be soluble in the solvent at room temperature.

The standard recrystallisation workflow would be something like this:

1. Heat solvent to its boiling point
2. Pour the boiling solvent slowly onto the impure solid with heating and swirling
3. When all of the impure solid has dissolved stop adding solvent
4. Allow the container to return to room temperature, then cool with ice if you wish

You want to extract as much pure crystals as you can, so the impure solid is dissolved in the least amount of hot solvent possible. This is the crucial point—you don't want to have extra solvent, which would mean that when the solution returns to room temperature, there is some of your target compound that remains in the solution (which could have been extracted if you had added less solvent). You also don't want to add less solvent because then the impure solid won't dissolve completely and your obtained crystal would be contaminated with that.

The only way to do this is to keep everything (i.e. pure solvent and the impure solid with hot solvent) at the boiling point of the solvent and add the hot solvent drop by drop until everything has dissolved. (Don't dry heat the impure solid btw, that causes a whole range of other problems!)

If you do this while the solvent is cold, you won't be able to estimate how much of the solvent is required to completely dissolve the solid beforehand (because the solubilities are different). You will either overestimate or underestimate, which would cause problems as mentioned above.

Note that sometimes there are insoluble impurities in the solid, then you would have to filter everything at high temperature. The same logic applies.