Ethanol is the more volatile component.

乙醇是更容易挥发的组分。

Because a is the more volatile component.

因为A的挥发性更好。

Because water's the less volatile component.

因为水的挥发性不高。

So I'm going to make B the more volatile component.

那么我让B是较易挥发的成分。

So now I want to stick with a as the more volatile component.

假设A是，挥发性更强的组分。