In aqueous chemistry, which of the following salts is considered insoluble in water under normal conditions?

Introduction / Context:
Solubility rules are an important tool in chemistry for predicting whether an ionic compound will dissolve in water to form a solution or remain largely undissolved as a precipitate. Many exam questions test your ability to apply basic solubility rules to common salts. This question asks you to identify which of the listed salts is considered insoluble in water under normal laboratory conditions, meaning it has very low solubility and tends to form a solid rather than a clear solution.

Given Data / Assumptions:

• The substances listed are sodium chloride, potassium sulfate, copper(II) hydroxide, and silver nitrate.
• We assume room temperature and pure water as the solvent.
• Solubility is considered in the general qualitative sense (soluble vs insoluble) used in introductory chemistry.
• We use standard solubility rules rather than detailed numerical solubility product values.

Concept / Approach:
General solubility rules state that all common salts of sodium, potassium, and ammonium are soluble in water. Most nitrates are also soluble. Many hydroxides, however, are insoluble, except for those of alkali metals and some alkaline earth metals such as calcium, strontium, and barium to limited extents. Copper(II) hydroxide is one of the classic examples of an insoluble hydroxide that forms a blue precipitate when copper(II) salts are treated with hydroxide ions. Therefore, among the given salts, copper(II) hydroxide is considered insoluble in water.

Step-by-Step Solution:
Step 1: Apply the rule that all sodium salts, including sodium chloride (NaCl), are soluble in water. Step 2: Apply the rule that all potassium salts, including potassium sulfate (K2SO4), are soluble in water. Step 3: Recall that almost all nitrates, including silver nitrate (AgNO3), are soluble in water. Step 4: Consider hydroxides and note that many transition metal hydroxides, including copper(II) hydroxide (Cu(OH)2), are insoluble or only very sparingly soluble in water. Step 5: Conclude that copper(II) hydroxide is the insoluble salt among the options and select it as the correct answer.

Verification / Alternative check:
In qualitative analysis experiments, when sodium hydroxide solution is added to a copper(II) salt solution, a blue precipitate of copper(II) hydroxide forms, demonstrating its low solubility in water. By contrast, sodium chloride dissolves readily and is used as common table salt. Potassium sulfate is also soluble and does not precipitate under normal conditions. Silver nitrate dissolves readily as well, forming a clear solution used for precipitation tests, such as forming a white precipitate of silver chloride with chloride ions. These laboratory observations confirm the solubility of sodium chloride, potassium sulfate, and silver nitrate and the insolubility of copper(II) hydroxide.

Why Other Options Are Wrong:
Sodium chloride is highly soluble in water and is a classic example used to illustrate ionic dissolution. Potassium sulfate is another alkali metal salt and follows the rule of alkali metal salts being soluble. Silver nitrate is soluble and is commonly used in solution form in analytical chemistry. None of these salts form a persistent precipitate under normal conditions when added to water, so they cannot be considered insoluble in the usual qualitative sense. Only copper(II) hydroxide fits the description of an insoluble salt among the options.

Common Pitfalls:
Students sometimes misinterpret solubility rules and think that because silver chloride is insoluble, all silver salts must be insoluble, which is not true. Silver nitrate is an important exception and is soluble. Another pitfall is failing to remember that most hydroxides, especially of transition metals, are insoluble, leading to confusion when identifying precipitates in experiments. To avoid these errors, memorise a few key solubility rules: nitrates are soluble, alkali metal salts are soluble, and many transition metal hydroxides are insoluble. With these rules in mind, questions like this become straightforward.

Final Answer:
The salt that is considered insoluble in water under normal conditions is copper(II) hydroxide.