ELI5, Why do solids dissolve better in hot liquids, but gasses dissolve better in cold liquids?
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The speed of dissolution when far away from the maximal amount usually increases with temperature. That is because **temperature corresponds to moving molecules**, and __more movement means more collisions__, hence more dissolving. The closer to the maximum it gets, the slower it becomes.
**Stuff bumping into each other** is also very _very_ roughly why solids tend to dissolve better the hotter a liquid is: more bumping into each other to keep it “afloat”. This is however far from the entire truth, see further below.
**Gases are usually more in a kind of equilibrium with a liquid they dissolve into.** The gas hits the liquid and gets sometimes dissolved, while gas in the liquid can often escape from the surface when it gets close. **The hotter the liquid, the faster the gas gets to a surface**, or another dissolved gas molecule it can start a new bubble with.
At the same time, **hotter gas also means it will dissolve quicker into a liquid**, but this does rarely enough to counteract the former effect because gas hitting the liquid mostly just bounces off. Similarly increasing pressure means more gas hitting the almost uncompressed liquid, thus higher solubility. That’s why soda bottles are in a state of equilibrium, with carbon dioxide escaping when you open it.
That I think answers your intended question, but there is a bit more to it yet, mostly with solids:
The maximal amount that can be dissolved can depend on temperature differently than portrayed by the naive explanations above. The **solubility of table salt for example does not depend much on the temperature** of water. Some substances such as **baking soda even have an inverse dependence**, they dissolve less if the water is hot (one can try this at home).
This happens **because the various parts of the solid substance (sodium and chloride/carbonate in the given examples) attract each other** while repelling their own kind, and both are furthermore in attraction with the water. Those many mutual forces can furthermore depend on temperature as well, and in some cases the chemicals even change their structure with temperature. Altogether that causes a quite complex landscape on how solubility depends on temperature. The solubility sometimes even has a peak or low in water far away from the freezing and boiling point.
Lastly those things are also why dissolving stuff changes the freezing point, and in higher amounts even the boiling point.
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