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Megalomania192 t1_iycwpsm wrote

ELI5: Almost everything is more soluble in hot water than cold water. Including Soaps. The soap is what cleans the germs off. More soap in the water = better germ cleaning.

Soaps help water remove things that don't usually mix with water - like oils, fats and germs. The soap has one end that like water and one end that likes fat/oil/"not water". The soap surrounds the germs and the water surrounds the soap

Beyond ELI5:

A few interesting examples occur of things are NOT more soluble in hot water than cold water: triethylamine is soluble in water below 19C only. Nicotine is soluble in water below 61C and insoluble above that (weirdly, it becomes soluble again above 210C in pressurised containers). Some polymers show similar behaviour. It's called a lower critical solution temperature

The explanation for this comes down to a Gibbs Free Energy change which is too advanced for this sub.

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Chromotron t1_iyd6vvv wrote

Maybe some examples for lesser solubility at higher temperatures that are salts:

  • many calcium salts such as its carbonate (gypsum), sulfate (chalk), phosphate, etc.;
  • sodium sulfate has its highest solubility at 33°C, it falls off in both directions;
  • unlike what many believe, table salt's solubility increases almost not with temperature; only the speed of dissolving does.

It is also interesting to note that some salts produce a lot of heat (e.g. NaOH a.k.a. caustic soda) or cold (e.g. ammonium nitrate a.k.a. the stuff that blew up Beirut, or urea). This and solubility are related, but all combinations can happen.

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