Question

Why solubility of grp 2 metal carbonates and sulphites in water decreases down the grp

Answer 1

Solubility of the sulphates

The sulphates become less soluble as you go down the Group.

The simple trend is true provided you include hydrated beryllium sulphate in it, but not if the beryllium sulphate is anhydrous.

The Nuffield Data Book quotes anyhydrous beryllium sulphate, BeSO4, as insoluble (I haven't been able to confirm this from any other source), whereas the hydrated form, BeSO4.4H2O is soluble. (The Data Books agree on this - giving a figure of about 39 g dissolving in 100 g of water at room temperature.)

Figures for magnesium sulphate and calcium sulphate also vary depending on whether the salt is hydrated or not, but nothing like so dramatically.

Two common examples may help you to remember the trend:

You are probably familiar with the reaction between magnesium and dilute sulphuric acid to give lots of hydrogen and a colourless solution of magnesium sulphate. Notice that you get a solution, not a precipitate. The magnesium sulphate is obviously soluble.

You may also remember that barium sulphate is formed as a white precipitate during the test for sulphate ions in solution. The ready formation of a precipitate shows that the barium sulphate must be pretty insoluble. In fact, 1 litre of water will only dissolve about 2 mg of barium sulphate at room temperature.

Solubility of the carbonates

The carbonates tend to become less soluble as you go down the Group.

None of the carbonates is anything more than very sparingly soluble. Magnesium carbonate (the most soluble one I have data for) is soluble to the extent of about 0.02 g per 100 g of water at room temperature.

I can't find any data for beryllium carbonate, but it tends to react with water and so that might confuse the trend.

The trend to lower solubility is, however, broken at the bottom of the Group. Barium carbonate is slightly more soluble than strontium carbonate.

There are no simple examples which might help you to remember the carbonate trend.

if my answer is helpful for you then please mark as brain list