Among the following sulphate salts, which one is practically insoluble in water under ordinary conditions?

Introduction / Context:
Solubility rules for ionic compounds in water are an important part of school chemistry because they help predict whether precipitates will form in double displacement reactions. Many sulphate salts are soluble, but certain heavy metal sulphates are sparingly soluble or insoluble. This question asks you to identify which sulphate among the options is not soluble in water, that is, forms a precipitate rather than dissolving appreciably.

Given Data / Assumptions:

• The options list sulphates of lead, zinc, potassium, sodium, and ammonium.
• General solubility rules for sulphate salts apply.
• We interpret not soluble as practically insoluble for exam purposes.

Concept / Approach:
General solubility rules state that most sulphate salts are soluble in water, with important exceptions such as sulphates of lead, barium, and calcium (which is moderately soluble). Alkali metal sulphates (like sodium and potassium sulphates) and ammonium sulphate are soluble. Zinc sulphate is also soluble in water. Lead sulphate, however, is considered insoluble or only very sparingly soluble, which means it forms a solid precipitate in water and in reactions producing PbSO4. Therefore, among the given choices, lead sulphate is the one that is not soluble in water.

Step-by-Step Solution:
Step 1: Recall the solubility rule that most sulphates are soluble except those of lead, barium, and partly calcium. Step 2: Identify the sulphates of alkali metals (sodium and potassium) and ammonium as definitely soluble in water. Step 3: Recognise that zinc sulphate is a typical metal sulphate that dissolves in water to give Zn2+ and SO4^2- ions. Step 4: Note that lead sulphate, PbSO4, is specifically mentioned in solubility tables as insoluble or sparingly soluble. Step 5: Conclude that lead sulphate is the sulphate that is not soluble in water among the options.

Verification / Alternative check:
Laboratory observations confirm that when solutions containing lead ions and sulphate ions are mixed, a white precipitate of lead sulphate forms and does not dissolve readily. In contrast, compounds like sodium sulphate and potassium sulphate dissolve easily to make clear solutions. Commercial fertilisers such as ammonium sulphate are also water soluble and used to supply nutrients to plants. Solubility tables summarise that PbSO4 is one of the important insoluble sulphates, while ZnSO4, Na2SO4, K2SO4, and (NH4)2SO4 are all soluble. These independent checks validate the conclusion.

Why Other Options Are Wrong:
- Zinc sulphate: ZnSO4 is soluble in water and is often used in solutions for agricultural and laboratory purposes.
- Potassium sulphate: As an alkali metal sulphate, K2SO4 is soluble in water and used as a soluble potash fertiliser.
- Sodium sulphate: Na2SO4 is also an alkali metal sulphate and is soluble in water, used in detergents and industry.
- Ammonium sulphate: (NH4)2SO4 is a soluble fertiliser salt and dissolves easily in water.

Common Pitfalls:
Students sometimes remember that some sulphates are insoluble but mix up which metal ions are involved, confusing lead sulphate with zinc sulphate or others. Another mistake is to assume that all sulphates behave in the same way. To avoid confusion, remember the pattern that most sulphates are soluble, but lead sulphate and barium sulphate are classic insoluble examples, with calcium sulphate having limited solubility. Linking these exceptions clearly with the corresponding metals helps when quickly analysing precipitation reactions in exam questions.

Final Answer:
The sulphate that is not soluble in water is Lead sulphate.