Trends in solubility of group 2 nitrates

Question

In my lab report, we are required to explain the trends in solubility of group 2 salts, going down the group. I had explained all of the trends except one, group 2 nitrates. The following is the data provided.

$\ce{Mg(NO3)2}$ – $\pu{0.49 mol}$ per $\pu{100 g}$ of water

$\ce{Ca(NO3)2}$ – $\pu{0.62 mol}$ per $\pu{100 g}$ of water

$\ce{Sr(NO3)2}$ – $\pu{0.16 mol}$ per $\pu{100 g}$ of water

$\ce{Ba(NO3)2}$ – $\pu{ 0.04 mol}$ per $\pu{100 g}$ of water

So as the data shown, the solubility first increases and then decreases. My speculation is that the general trend should decrease but $\ce{Ca(NO3)2}$ has some special properties which makes it more soluble or $\ce{Mg(NO3)2}$ has some special which makes it less soluble. I searched the internet and all I got is "All group 2 nitrates are soluble" with no explanation regarding the trend.

Please correct my speculations if they're wrong and also provide an explanation regarding this trend.

Answer 1

Solubility is one of the present scientific problems that nobody is able to explain thoroughly. As you are speaking of Calcium compounds, look at the series of Calcium compounds made with halogens. $\ce{CaCl2, CaBr2, CaI2}$ are so soluble in water that they can be dissolved in less than their weight of water. $\ce{CaF2}$ should be similar. It is not. On the contrary, $\ce{CaF2}$ is one of the most insoluble compounds, because it forms the mineral called fluorite, which is one of the main sources of Fluorine atoms on Earth. If it would have been at least partly soluble, it would have been washed away by the rains a long time ago. Nobody is able to explain such a difference of solubility.

There are plenty of theories explaining the solubilities of some groups of substances using electronegativities, ionic or covalent radius, and other parameters, which work pretty well with a lot of substances. But there are always exceptions. The final explanation remains to be discovered.