Solubility

[chiddler]

The amount of salt that dissolves in water ALWAYS increases with which of the following:

A. Increasing temp
B. Decreasing temp
C. Increasing water
D. Decreasing water

Answer: C

Tricky question. The explanation for not picking A is that solvation can either be endo or exothermic. My question: when is solvation ever endothermic? I thought solvation was exothermic. The reason it dissolves in the first place is because it forms stronger bonds with water than with its ionic pair, and therefore releases energy. Right?

Can I please have an explanation for this?

thank you.

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[MedPR]

Cant really explain it, but here are a couple of examples (NH4NO3, NaCl): http://en.wikipedia.org/wiki/Enthalpy_change_of_solution#Steps_in_dissolution

Edit: I got this question wrong too.

 

[ozzi22]

I thought that increasing temp always increases solubility but other factors like pressure, increasing solvent etc , will increase the rate of solubility. Any thoughts?

 

[chiddler]

I thought that increasing temp always increases solubility but other factors like pressure, increasing solvent etc , will increase the rate of solubility. Any thoughts?

rate of solubility? i've never heard of that phrase. do you mean rate of reaction?

remember that dissolving salts with low solubility is an equilibrium process. the maximum solubility is when forward reaction = backward reaction.

 

[chiddler]

Cant really explain it, but here are a couple of examples (NH4NO3, NaCl): http://en.wikipedia.org/wiki/Enthalpy_change_of_solution#Steps_in_dissolution

Edit: I got this question wrong too.

thanks very much. good enough for me.

 

[ozzi22]

rate of solubility? i've never heard of that phrase. do you mean rate of reaction?

remember that dissolving salts with low solubility is an equilibrium process. the maximum solubility is when forward reaction = backward reaction.

Yeah you're right. What i meant was the rate at which it takes a particular compound to dissolve. For example, a compund might already be soluble but increasing the solvent(WATER) might lower the time it takes to dissolve and hence, increase the solubility rate. I'm not sure but your explanations makes more sense.

 

[DrRichand1]

Your overthinking this one way to much. what happens if i keep raising the temp of water, will salt be indefinetly able to dissolve forever? no the will be a point where temp doesn't matter and no more salt can disolve.
The key to getting this question right in realizing that if we add more water we can disolve more salt, and then add more water disolve more salt. and that is true forever.

 

[Dasypus]

One example that's always worth remembering is calcium carbonate, otherwise known as lime scale. It scales up inside a home boiler because it's actually less soluble in hot water.