In extraction experiements, why is it important to know the densities of the two solvents being used?

Question

Also, why must they not be too close in value?

Answer 1

Because you have to know which is the top and which is the bottom layer.

For example, if you're using diethyl ether to extract something from water, the density of water is 1.00g/mL and of ether 0.71g/mL. So if you shake up a mixture of water and ether, the water will settle to the bottom and the ether will collect on top. You open a stopcock ("faucet") on the bottom of your vessel and let the water drain out and stop before the ether goes.

If you're using chloroform to extract from waster, the density of chloroform, is 1.48. So after shaking, it's the chloroform that winds up on the bottom.

The other solvent should not be too close to 1.00g/mL, because you'll get a liquid mush that doesn't separate into layers.