Group 2 Solubility & Uses (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching Chemistry, and this video is about the solubility of the Group 2 hydroxides and sulfates, and what those compounds are used for.

How soluble a Group 2 compound is decides what it gets used for: the insoluble ones are safe to swallow and useful as tests, and the more soluble ones act faster.

The Group 2 hydroxides get more soluble going down the group, and the sulfates get less soluble going down the group. Those two trends are the reason the compounds are used the way they are. Barium sulfate is safe to swallow for an X-ray because it is insoluble and so is not absorbed into the blood, and that same insolubility is what makes it the test for sulfate ions. Magnesium hydroxide is safe to drink as an antacid because it is only partially soluble, so the solution is only slightly alkaline. And calcium hydroxide acts faster on acidic soil than calcium carbonate does, because calcium carbonate is largely insoluble.

We start with the two solubility trends themselves. Then the uses of calcium, in agriculture and in flue gases. And last magnesium and barium, in medicine and in extracting titanium.

Going down Group 2, the hydroxides get more soluble. Magnesium hydroxide is sparingly soluble, and barium hydroxide is soluble. Each of these compounds dissolves by splitting into the metal 2 plus ion and either hydroxide ions or the sulfate ion. That trend is also why the solutions formed when the Group 2 oxides react with water get more alkaline down the group: the oxide ion itself reacts with water to give hydroxide ions, so a more soluble hydroxide puts more hydroxide ions into solution, and the higher the concentration of hydroxide ions, the more alkaline the solution. The sulfates go the other way. Going down the group they get less soluble: magnesium sulfate is soluble, and barium sulfate is insoluble. Those two opposite trends are what decide how each compound can be used.

Three calcium compounds turn up in industry and agriculture. Calcium carbonate, which as an impure rock is limestone. Calcium oxide, which is quicklime, formed by the thermal decomposition of calcium carbonate. And calcium hydroxide, which is slaked lime, formed when water is added to quicklime.

All three are bases, so all three are used to raise the pH of soil, calcium hydroxide neutralising the acid directly to give calcium ions and water. Soil becomes more acidic over time, and the optimum pH for many crops is around 6 to 6.5. Calcium carbonate is the one most commonly used as agricultural lime, because it is cheaper and safer to handle, but because it is largely insoluble it acts more slowly than calcium hydroxide does. Calcium compounds are also used to remove sulfur dioxide from flue gases, in a process known as sulfur scrubbing.

Magnesium hydroxide is partially soluble in water, and in suspension it is milk of magnesia, used to neutralise excess acid in the stomach and to treat constipation. It is safe precisely because it is only partially soluble: the low hydroxide concentration makes the solution only slightly alkaline, at about pH 10. Magnesium also has an industrial job. Titanium dioxide is heated in a stream of chlorine, in the presence of coke, to produce titanium chloride, and the titanium is then extracted from that chloride by reduction with magnesium, although sodium can be used instead.

Barium's use is medical. A barium meal or barium swallow containing barium sulfate is given to a patient who needs an X-ray on their intestines, because barium absorbs X-rays, so the gut shows up white on the image. Barium is toxic, but it is safe in this form because barium sulfate is insoluble and so is not absorbed into the blood, and because the amount swallowed is small.

The insolubility of barium sulfate is what the test for sulfate ions relies on. Add hydrochloric acid followed by barium chloride solution, or nitric acid followed by barium nitrate solution, and a white precipitate of barium sulfate means sulfate ions are present. The acid is there to remove other anions, such as carbonate, that could also form white precipitates, which is what makes the test specific to sulfate.

Going down Group 2, the hydroxides get more soluble and the sulfates get less soluble. Magnesium hydroxide is sparingly soluble, and barium sulfate is insoluble. That insolubility is what makes barium sulfate safe to swallow for an X-ray, and what makes it the test for sulfate ions. The partial solubility of magnesium hydroxide is what makes milk of magnesia safe to drink, and the insolubility of calcium carbonate is why it acts more slowly on soil than calcium hydroxide. In every case, how soluble the compound is decides what it can be used for.