Why do salts dissolve better in warm water, but not gases?

29 April 2012


Chris, here's a question of the week for you. I use lots of soda water with a home charging unit, and the first thing I learned is to chill the water before injecting the CO2. Otherwise the gas is poorly absorbed, and the water doesn't have much fizz. On the other hand, in chemistry I learned that most compounds (usually salts of various types) dissolve much better in warm rather than cold water. Why does it work one way with salts and the opposite way with gasses?
John Gamel


Chris - It does sound a bit counterintuitive doesn't it? The simple reason, chemically, is if you were trying to dissolve some salts, let's take table salt - sodium chloride - as an example. Sodium chloride consists of sodium ions Na+ and chloride ions Cl- and they're in an ionic lattice where the sodium has given an electron to the chlorine so you've got the sodium plus and the chloride minus.

Water has a dipole. What that means is that water molecules, H2O, have an oxygen in the centre which loves electrons and it's a bit minus, and the hydrogen which holds on to its electron slightly less well ends up being a little bit plus.

This means that when salt mixes with water, the hydrogen ions can grab hold of the chloride because they're minus and the hydrogen is a bit plus. The oxygen can grab onto the sodium and form an interaction and as a result, the ions want to move in to contact with the water molecules.

If you warm the solution up, the water molecules have more kinetic energy because the temperature is higher, therefore they will interact with more energy with the salt and therefore, they'll find it easier to bring those ions into a solution and hang onto them.

On the other hand with gas, it's slightly different and the best explanation I came across to explain this was in terms of what we call partial pressure. In other words, the relative proportion of a mixture that a gas makes up. If you increase the temperature of a gas, it wants to expand so it takes up more space effectively, the pressure is higher. So if the pressure is higher, the gas wants to take up more room or push harder on other things around it, electrically speaking, and as a result it's going to find it harder to sit in a solution of water molecules which were all trying to interact with each other. So therefore, when the temperature is higher, gases find it harder to dissolve but when the temperature is lower, gases dissolve much better. And that's why you find there's actually much more oxygen in the water around the Antarctic, so you get creatures that are very, very big there compared with warmer water which can sustain less oxygen tension and also, if you want to keep your fizzy drinks fizzier, putting them in the fridge works exactly the same way; the CO2 will dissolve better when they're actually cold in the fridge.

Dave - Another way to think about it is that for salt to dissolve, you've got to break bonds and that takes energy. So the hotter it is, the more energy there is about, so the easier it is to break those bonds. So the hotter it is, the more likely the salt is to be dissolved. But similarly, for the gas to escape from the water, it's going to take energy because there will be some bonds in there, so the hotter it is, the more likely it is to come out as a gas.

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