Solubility vs. Temperature

A fact of chemistry that people sometimes find counterintutive is that the temperature dependence of solubility is the inverse for solids and for gases.

Chemistry textbooks that I have seen, though, usually try to motivate this by calculations involving stuff like partial vapor pressures in the solvent, or enthalpy of crystallization, none of which is especially memorable or intuitive.

This pattern can be reduced to a more general chemistry rule of thumb, however: like dissolves like. Hotter solids are closer to melting, which means they will mix better with other liquids; similarly colder gases are closer to liquifying, which again means they will mix better with liquids.

It’s still possible to get confused and remember this the wrong way around, though. If, say, colder CO₂ is closer to liquid and therefore dissolves better in water, why does colder water — i.e. closer to ice but further away from steam — not become a poorer solvent at the same time?

The thing to do here is to think about what kind of a mixture are we making in the first place. If you are trying to make a gaseous mixture, colder water indeed "dissolves” worse (— in other words, colder air, or the like, is able to retain less moisture). Buf if we’re trying to make a liquid solution, it does matter if the water is 1°C or 99°C, it’s already liquid in any case. There is no way to stabilize water as a solid or a gas in a homogeneous mixture, when conditions are within its liquid range (gels exist of course, but they’re not truly molecular-level homogeneous, as much as little networks to trap water in pouches and such). The “work” is in making the dissolved material also into a liquid; to coax it to not solidify or evaporate. And the closer we are to liquid range, the less work this will be.

There is also a pre-existing concept for this line of thinking that is already in use, which may also help in better understanding solutions, but it requires some termological care.

Consider e.g. a solution of sugar in water. We may not usually think of this as a mixture of liquid water and liquid sugar. But this is still the same kind of a thing as e.g. a solution of antifreeze and water. A cooled antifreeze solution can be thought of as water whose “freezing point has been lowered” — but also, as “a solution of ice in antifreeze”: with the antifreeze being the solvent, ice being merely a dissolved solid. Equally well, a thick sugar syrup that contains more sugar than water could be considered simply liquid sugar, with water serving as an “antisolidifier” than as a “solvent”. There is no fundamental difference: they’re both the same thing, seem from two different angles.

In other words, the colloquial use of “to melt” for “to dissolve” is indeed motivated. For a solid, to be dissolved IS to be molten, no matter if aided by solvent molecules rather than by heat.

Note though that I use above the adjectives dissolved and molten, specifically. If we now consider the verbs, dissolve and melt still cannot be considered synonyms (this much we must grant to all the high school chemistry teachers out there to insist on the distinction). But this has more to do with the way a mixture is prepared. Antifreeze solution is made from liquid water with added antifreeze, and then cooled below freezing point; sugar syrup is made from solid sugar with added water, and heated above the mixture’s melting point. In principle we could do the opposite as well — dissolve ice in liquid antifreeze, or mix liquid sugar with water and then cool the mixture. This would not be very practical, given the usual temperatures we do our work at… but the resulting solution would be the same in any case.