Heat magnesium carbonate and it breaks down easily; barium carbonate barely notices. Dissolve the hydroxides and sulfates and the trends run in opposite directions. All of it traces back to the size of the M2+ ion.
Group 2 carbonates decompose on heating to the oxide and carbon dioxide; nitrates decompose to the oxide, nitrogen dioxide and oxygen. Thermal stability increases down the group: the M2+ ion gets larger with the same charge, so its charge density falls and it polarises (distorts) the large carbonate or nitrate anion less. The hydroxides become more soluble down the group, so the pH of their saturated solutions rises; the sulfates become less soluble, so barium sulfate is insoluble, which is the basis of the test for sulfate ions.
Heat each carbonate and nitrate in the simulator and compare the temperature at which it starts to decompose.
Compare how much of each hydroxide and sulfate dissolves in the solubility simulator.
| Mg | Ca | Sr | Ba | |
|---|---|---|---|---|
| hydroxide | sparingly soluble, pH about 10 | slightly soluble (limewater), pH about 12 | more soluble | quite soluble, pH about 13 to 14 |
| sulfate | soluble | sparingly soluble | very slightly soluble | insoluble |
The two solubility trends go in opposite directions: hydroxides become more soluble down the group, sulfates less. When explaining thermal stability, talk about the cation polarising the anion; the anion is the same throughout, so the change must come from the size of M2+.
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What are the products when calcium carbonate is heated strongly?
Which is the correct equation for heating magnesium nitrate?
How does the solubility of the sulfates change down Group 2?
Which Group 2 carbonate is the most thermally stable?
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Why does the thermal stability of the Group 2 carbonates increase down the group?
A colourless solution gives a white precipitate with acidified barium chloride. The solution contains:
Which saturated solution has the highest pH?
What mass of magnesium oxide is left when 7.42 g of magnesium nitrate (Mr 148.3) is heated to constant mass? (Mr MgO 40.3)
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