[John]

A question that has long plagued me:

Hot water can hold more of a dissolved substance than cold water. Thus, for example, we make rock candy by dissolving sugar in hot water, then cooling the water and letting the sugar precipitate out.

So what is the mechanism by which hot water holds less of the dissolved minerals found in hard well water than cold water does?

Thanks!
JL

[Jd1828]

Im not exactly sure why but here is my best guess:

Some reactions will proceed spontaneously at lower temperatures.  If the reaction has a negative but small enthalpy (dH)  it will only be able to spontaneously react at low temps.  This relationship is shown in the gibbs free energy equation.  

dG=dH-T(dS)

in order for the reaction to be spontaneous dG has to be negative.

[savoy7]

Hard water

cold water comes in and goes to the tap

hot water comes in and is heated - calcium carbonate is less soluble at higher temperature and will precipitate out.  This tends to happen around the heating coils.  If you have ever replaced a hot water heater, you will see (and feel - weight) all of the calcium (and potentially magnesium) at the bottom of it.  Therefore, the water coming out of the tap has lower hardness compared to the cold water.

savoy

[John]

So calcium carbonate really does have reduced solubility at higher temps? Interesting, and (as mentioned) very counter-intuitive.

Thanks!

[DrCMS]

No calcium carbonate is more soluble in hot water than cold but only by a tiny degree, say 19ppm and 14ppm.

In water from the tap most hardness is calcium bicarbonate as that is heated it becomes calcium carbonte which is less soluble and you get scale in your kettle.  It happens most at the heating element, so that is where the scale is worst.

[xiankai]

my guess as to why substances are usuaully more soluble in hot water is because hot water has a greater volume (expansion), and thus can contain more solute.

[Donaldson Tan]

doesn't higher temperature suggest greater proportion of water molecules with sufficient energy to overcome the activation energy to hydrate a salt?

[xiankai]

o_O i didnt know hydrating a salt was a chemical reaction (thought it was physical all along)

if there's activation energy involved in that reaction, would it be possible to have an exothermic reaction resulting from hydrations of certain salts?

[arnyk]

o_O i didnt know hydrating a salt was a chemical reaction (thought it was physical all along)

if there's activation energy involved in that reaction, would it be possible to have an exothermic reaction resulting from hydrations of certain salts?

Mixing water with an anhydrous salt is generally exothermic while making an anhydrous salt from a hydrate is generally endothermic.