Group 2 Sulfates: Solubility, Testing, and Applications Revision Notes | A Level AQA Chemistry

1. The Solubility Trend in Group 2 Sulphates

Picture a row of white ionic solids — magnesium sulphate, calcium sulphate, strontium sulphate, barium sulphate — each dropped into water. The first dissolves readily; the last barely dissolves at all. That single observation captures one of the clearest trends in Group 2 chemistry.

As you move down Group 2 from magnesium (Mg) to barium (Ba), the sulphates become progressively less soluble in water. Magnesium sulphate ($\text{MgSO}_4$) is freely soluble — you may know it as Epsom salts. Calcium sulphate ($\text{CaSO}_4$) is sparingly soluble, which is why gypsum sets hard in plaster. Strontium sulphate ($\text{SrSO}_4$) is only slightly soluble, and barium sulphate ($\text{BaSO}_4$) is essentially insoluble.

The chart makes the trend unmistakable: solubility drops by several orders of magnitude across the group. At A-level, you are expected to know the direction of this trend and to recall that $\text{BaSO}_4$ is insoluble. The underlying reason involves the balance between lattice enthalpy and hydration enthalpy, but AQA requires you to know the trend rather than derive it from energetics.

This trend is the foundation for everything else in this lesson. Because $\text{BaSO}_4$ is insoluble, it forms a precipitate when barium ions meet sulphate ions in solution — and that precipitation reaction is the basis of the sulphate ion test.

2. Testing for Sulphate Ions Using Acidified Barium Chloride

The test for sulphate ions is one of the most commonly examined qualitative analysis procedures at A-level. The reagent is acidified barium chloride solution — barium chloride dissolved in water and then acidified with dilute hydrochloric acid. When this is added to a solution containing sulphate ions, a dense white precipitate of barium sulphate forms immediately.

The full ionic equation for the reaction is straightforward. Barium ions from the barium chloride solution combine with sulphate ions from the test solution to form the insoluble precipitate:

Notice the state symbols: both reactants are aqueous (aq) and the product is a solid (s). Examiners award marks specifically for correct state symbols, so including them is non-negotiable in any equation you write.

2a. Why Must the Solution Be Acidified?

Here is where the test becomes more sophisticated — and where many students lose marks by giving an incomplete answer. The barium chloride solution must be acidified with dilute hydrochloric acid before use. The reason is not to make the barium ions more reactive; it is to remove interfering ions that would otherwise give a false positive result.

Two ions in particular cause problems: carbonate ions ($\text{CO}_3^{2-}$) and sulphite ions ($\text{SO}_3^{2-}$). Both of these react with barium ions to form white precipitates — barium carbonate and barium sulphite respectively. If you added barium chloride to a solution containing carbonates, you would see a white precipitate even in the complete absence of sulphate ions. That would be a false positive.

Adding dilute hydrochloric acid before the barium chloride destroys these interfering ions. Carbonate ions react with the acid to produce carbon dioxide gas and water; sulphite ions are similarly removed. Crucially, sulphate ions do not react with dilute hydrochloric acid — they remain in solution, ready to react with the barium ions. This selectivity is what makes the acidification step essential.

3. The Procedure: Carrying Out the Sulphate Ion Test

Knowing the chemistry is one thing; being able to describe the procedure clearly is another. AQA questions often ask you to 'describe' the test, which means giving a sequential account of what you do and what you observe — without explaining the reasons unless the question specifically asks for them.

Worked example: Procedure: Testing for Sulphate Ions

Describe how you would test an aqueous solution for the presence of sulphate ions.

1. Add an excess of dilute hydrochloric acid to the test solution. — This removes carbonate ions ($\text{CO}_3^{2-}$) and sulphite ions ($\text{SO}_3^{2-}$), which would otherwise react with barium ions to give a false positive white precipitate.
2. Add a few drops of barium chloride solution ($\text{BaCl}_2$(aq)) to the acidified test solution. — Barium ions ($\text{Ba}^{2+}$) are now introduced. If sulphate ions are present, they will react immediately with the barium ions.
3. A dense white precipitate forms if sulphate ions are present. No precipitate (or the solution remains clear) if sulphate ions are absent. — The white precipitate is barium sulphate, $\text{BaSO}_4$(s). It is insoluble in dilute hydrochloric acid, which confirms it is not barium carbonate or barium sulphite.
4. $\text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{BaSO}_4(\text{s})$ — This is the net ionic equation. Chloride ions from the barium chloride and any other spectator ions are omitted. State symbols are required for full marks.

Answer: A dense white precipitate of $\text{BaSO}_4$(s) confirms the presence of sulphate ions. The precipitate is insoluble in dilute hydrochloric acid.

4. The Medical Use of Barium Sulphate

The same property that makes $\text{BaSO}_4$ useful in the laboratory — its insolubility — also makes it invaluable in medicine. This is a striking example of how a chemical property directly determines a real-world application, and it is explicitly required by the AQA specification.

Barium sulphate is used as a contrast agent in a procedure known as a barium meal or barium swallow. The patient drinks a suspension of $\text{BaSO}_4$ in water, and the barium sulphate coats the lining of the oesophagus, stomach, and intestines. Because barium is a heavy element, it absorbs X-rays strongly. This means the organs coated in barium sulphate appear white on an X-ray image, allowing doctors to identify abnormalities such as ulcers, tumours, or narrowing of the digestive tract.

The critical question is: why is it safe to swallow barium sulphate when barium compounds are generally toxic? The answer lies directly in the solubility trend you studied in Section 1. Barium sulphate is so insoluble that it passes through the digestive system without dissolving into the bloodstream. Because it cannot dissolve, barium ions are never released in significant quantities, and the compound is therefore non-toxic in this context. Soluble barium salts — such as barium chloride — are genuinely toxic and must never be ingested.

This application neatly ties together the solubility trend and the properties of $\text{BaSO}_4$: the same insolubility that produces a reliable precipitate in the laboratory is the property that prevents toxicity in the body. Recognising this link between chemical property and application is exactly the kind of reasoning AQA rewards in 'explain' and 'evaluate' questions.

5. Exam Technique and Mark-Winning Responses

The topics in this lesson appear frequently in AQA papers, and the marks are very achievable — but only if you use precise language. Vague answers about 'white precipitates' or 'barium being safe' will not score without the chemical reasoning behind them. The following guidance shows you how to convert your understanding into mark-winning responses.

When a question asks you to 'explain why barium chloride solution is acidified', a one-mark answer names the interfering ions; a two-mark answer also states what would happen without acidification (false positive white precipitate). Always push to the second mark by completing the reasoning chain.