Question

Why is calcium iodate dissolved in warm water first and then cooled down when preparing a saturated solution?

Answer 1

Well, what is a `"saturated solution"`?

Explanation:

I find that the definition is a problem area in AS/A2 level chemistry that is rarely addressed even at undergraduate level. And so an attempt...

See this old spray. And to repeat myself, a saturated solution contains an amount of dissolved solute that is equal to that amount of solute that would be in equilibrium with undissolved solute. A temperature is normally specified because reasonably a HOT solution can hold more solute than a cold one (and again I sound like a beer commercial!). This is because dissolution of a salt is in fact a bond-breaking chemical reaction in which solute-solute bonds are broken.

`"Solid solute"+Deltarightleftharpoons"Dissolved solute"`

Now calcium iodate is fairly insoluble stuff. The literature gives
`K_"sp"=7.1xx10^-7` ...we assume that this value refers to a solubility at `298*K`. At higher temperatures, reasonably, more salt would be in solution. But as long as we can see solid material, we know that the solute is saturated. And so heating and then cooling would establish the equilibrium such that....

`Ca(IO_3)_2(s) rightleftharpoons Ca^(2+) + 2IO_3^(-)`

And so....`K_"sp"=4S^3`, where `"S=molar solubility of the salt"`

`S=""^3sqrt((7.1xx10^-7)/4)=5.62xx10^-3*mol*L^-1`..

..you can work out the mass solubility.

And so for your problem....if we cool down the solution of the salt, MORE salt should precipitate in order to satisfy the equilibrium value. The supernatant solution is regarded `"TO BE SATURATED AT THAT TEMPERATURE"`.