Which of the following ionic compounds is not soluble in water at room temperature?

Introduction / Context:
Solubility rules for ionic compounds are widely taught in introductory chemistry to help predict whether a precipitate will form in a given reaction. Many salts of sodium, potassium and nitrate are soluble in water, while some metal hydroxides are not. This question tests whether the learner can apply general solubility rules to identify which compound from a short list is insoluble or only sparingly soluble in water at room temperature.

Given Data / Assumptions:
- Option A is copper(II) hydroxide, Cu(OH)2.
- Option B is sodium chloride, NaCl.
- Option C is silver nitrate, AgNO3.
- Option D is potassium sulfate, K2SO4.
- Standard aqueous solubility rules at room temperature are assumed.

Concept / Approach:
Typical solubility rules state that all sodium and potassium salts are soluble in water, as are all nitrates. Many hydroxides, especially of transition metals such as copper, are insoluble or only very slightly soluble. Sulfates are often soluble, with notable exceptions like barium sulfate and lead sulfate. Using these rules, we can quickly classify the given compounds. Copper(II) hydroxide falls into the group of insoluble hydroxides, while sodium chloride, silver nitrate and potassium sulfate are well known as water soluble salts.

Step-by-Step Solution:
Step 1: Apply solubility rules to sodium chloride. Sodium salts are soluble, so NaCl dissolves readily in water. Step 2: Apply solubility rules to silver nitrate. All nitrate salts, including AgNO3, are soluble in water. Step 3: Apply solubility rules to potassium sulfate. Potassium salts are soluble, so K2SO4 also dissolves in water. Step 4: Consider copper(II) hydroxide, Cu(OH)2, which is a transition metal hydroxide. Most such hydroxides are insoluble or only very slightly soluble in water. Step 5: Conclude that copper(II) hydroxide is the compound that is not soluble in water at room temperature.

Verification / Alternative check:
A practical verification is to remember laboratory observations. Sodium chloride and potassium sulfate readily dissolve to give clear solutions, and silver nitrate is routinely used in solution form in analytical experiments. In contrast, when copper(II) ions react with hydroxide ions in aqueous solution, a blue precipitate of Cu(OH)2 forms, demonstrating its low solubility. This behaviour is consistent with solubility rules and confirms the classification of copper(II) hydroxide as insoluble in water.

Why Other Options Are Wrong:
Option B, sodium chloride, is a classic example of a soluble ionic compound and dissolves in water to form salt solutions. Option C, silver nitrate, is highly soluble and often used in titration and precipitation reactions. Option D, potassium sulfate, is also soluble because potassium salts are generally soluble. None of these compounds form precipitates simply by being placed in water, so they are not correct answers to the question about insolubility.

Common Pitfalls:
Students sometimes misremember solubility rules or overgeneralise, thinking all nitrates or all sulfates are insoluble, which is the opposite of the usual rule. Another pitfall is to assume that any compound containing a heavy metal must be insoluble, ignoring specific group trends. Confusion can also arise because solubility depends on temperature, but at room temperature the standard rules are very reliable. Repeated practice with these rules builds confidence in identifying soluble and insoluble salts.

Final Answer:
The ionic compound that is not soluble in water at room temperature is copper(II) hydroxide.