Topic 8 · page 12 of 29

Solubility of Group 2 hydroxides and sulfates

Down Group 2 the hydroxides become more soluble and the sulfates become less soluble. The two trends run in opposite directions, and the insolubility of barium sulfate is the basis of the sulfate test.

10 min read8.15

Read first: Oxides and hydroxides of Groups 1 and 2

Some Group 2 compounds dissolve in water and some do not. The pattern down the group is one of the facts to learn.

Two rows of Group 2 compounds from magnesium to barium. Top row, hydroxides: magnesium hydroxide almost insoluble, calcium hydroxide slightly soluble, strontium hydroxide soluble, barium hydroxide soluble, with an arrow pointing down the group labelled solubility increases. Bottom row, sulfates: magnesium sulfate soluble, calcium sulfate slightly soluble, strontium sulfate almost insoluble, barium sulfate insoluble, with an arrow labelled solubility decreases. The shaded bars show how much dissolves, schematically.
Hydroxides get more soluble down the group, sulfates less soluble. The bars show the pattern, not measured values.
Compound Mg Ca Sr Ba
Hydroxide, M(OH)₂ almost insoluble slightly soluble soluble soluble
Sulfate, MSO₄ soluble slightly soluble almost insoluble insoluble

A saturated solution of calcium hydroxide is called limewater. It is slightly soluble, so limewater is only weakly alkaline and contains very little dissolved Ca(OH)₂. A suspension of undissolved calcium hydroxide in water is called milk of lime.

PracticalComparing the hydroxides and sulfates

  1. Put 1 cm³ of 0.1 mol dm⁻³ magnesium nitrate, calcium nitrate and barium nitrate solution into three separate test tubes.
  2. To each, add sodium hydroxide solution drop by drop. Compare the amount of precipitate.
  3. Repeat with fresh samples, adding dilute sulfuric acid (or sodium sulfate solution) instead. Compare the precipitates.
  4. Keep the volumes and concentrations equal, so that the test is fair.
Reagent added Magnesium Calcium Barium
NaOH(aq) thick white precipitate some white precipitate no precipitate (or very little)
H₂SO₄(aq) no precipitate some white precipitate, slower to appear thick white precipitate

The precipitates are the insoluble hydroxide or sulfate:

MgX2+(aq)+2 OHX−(aq)→Mg(OH)X2(s)BaX2+(aq)+SOX4X2−(aq)→BaSOX4(s) \ce{Mg^2+(aq) + 2OH-(aq) -> Mg(OH)2(s)} \qquad \ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}

The observations depend on the concentrations used. With very dilute solutions the calcium compounds may give little or no precipitate. The order Mg > Ca > Ba for hydroxide precipitation and Ba > Ca > Mg for sulfate precipitation is what matters.

Why this matters 8.15

The insolubility of barium sulfate is the basis of the test for sulfate ions, which is on page 16: acidified barium chloride gives a white precipitate of BaSO₄ only if sulfate ions are present. The solubility of magnesium sulfate is why Epsom salts (MgSO₄·7H₂O) dissolve readily in a bath.

Check yourself

  1. State the trend in solubility of the Group 2 hydroxides and of the Group 2 sulfates, going down the group.
  2. Which is the least soluble of magnesium sulfate, calcium sulfate and barium sulfate?
  3. Write an ionic equation with state symbols for the formation of the white precipitate when barium chloride solution is added to sodium sulfate solution.
  4. Magnesium nitrate solution and barium nitrate solution are each tested with sodium hydroxide solution. Predict the observation for each.
  5. Write the name and formula of the solution used to test for carbon dioxide, and explain why it is only weakly alkaline.
  6. A student adds dilute sulfuric acid to a very dilute solution of calcium nitrate and sees only a faint cloudiness. Explain this using the solubility trend.

Answers cover these until you have tried the questions

  1. Hydroxides: solubility increases down the group. Sulfates: solubility decreases down the group.
  2. Barium sulfate.
  3. BaX2+(aq)+SOX4X2−(aq)→BaSOX4(s)\ce{Ba^2+(aq) + SO4^2-(aq) -> BaSO4(s)}
  4. Magnesium nitrate: a white precipitate of Mg(OH)₂ (almost insoluble). Barium nitrate: no precipitate (or very little), because Ba(OH)₂ is soluble.
  5. Limewater, a solution of calcium hydroxide, Ca(OH)₂. It is only weakly alkaline because calcium hydroxide is only slightly soluble, so the OH⁻ concentration is low.
  6. Calcium sulfate is only slightly soluble, between magnesium sulfate (soluble) and barium sulfate (insoluble). A small amount precipitates, giving faint cloudiness rather than a thick white precipitate.