Solubility in recrystallisation process help needed

For this qusetion, I don't understand the explanation for B. What does it mean by there must be a difference between the solubility?

B is stating that the impure product must be dissolved in the hot solvent but that must occur after the impurity has been filtered right? I am uncertain as to why B is also right and am also unsure about the process....

Thanks

When you recrystallise in a lab, you dissolve all the reaction products in hot solvent, then cool the solvent down so the product you want crystallises.

The idea is, the impurities should be soluble at high and low temperatures (hopefully) so they stay in solution as you cool down, but your product should be soluble at high temperature but a lot less soluble at low temperature. So if you have a saturated solution (ie a solution that has the maximum amount of product dissolved in a certain volume of solvent) at high T then when you cool down lots of product crashes out (which just means it precipitates to form a solid).


So the impure product should be dissolved in hot solvent, because thats when it ismost soluble, just using cold solvent would result in a solution that, when cooled with ice, would lead to very little product being precipitated out.

This is why a big difference in solubilities at different T is important, if it dissolved equally well at high T and low T, then once you'd dissolved it, you wouldn't be able to get the solid to precipitate!

As an example (using the number you're given) lets say I dissolved 56.3grams of benzoic acid in 1litre or 100^oC water, that would be saturated, then when i cooled it to room T (25^oC) only 3.4grams would stay in solution, so id have 52.9grams of purified product, that's pretty good yield.

If instead I used a solvent where i could dissolve 56.3 grams per litre at 100^oC, and 50.0 grams per litre at 25^oC, then cooling would only give 6.3 grams of purified product which is rubbish yield!

so the difference in solubility with T is crucial, as is dissolving in hot solvent (the hotter, the more soluble which means the more crashes out when I cool).

The third point, washign with warm solvent, defeats the point of recrystallising because your product is more soluble in warm solvent so you'll just wash away product you worked hard to keep. Instead using ice cold solvent means the the product isn't washed away so much, again giving better yield.

Sorry for the long post, hope it clears things up

Am I understanding this right?
-Impurity should be more soluble at high temperature. Here is what I don't get, why do they also need to be soluble at low temperature since you want to remove them? Can you explain this bit more?

-the pure product is more soluble at lower temperature because at higher temperature they will be lost. At lower temperature the pure product can be recrytallised?

Thanks a lot

So, you dissolve the solid, in hot solvent, then cool the solvent down, your product crashes out, you filter off the solid product and the impurities stay in solution so they aren't in your solid after you recrystallise.

If the impurities weren't soluble at low T, they would precipitate out with your desired product, and when you filtered, you'd be left with impurity in your solid, which defeats the point of a recryst.

So impurities need to be soluble at both high and low T otherwise you couldn't seperate them by recrystallisation...

The pure product is more soluble at high temperature, that's the reason recrystallisation works!!!!