In aqueous solution chemistry, which combination of solutions will produce a precipitate when mixed?

Introduction / Context:
Many important reactions in aqueous chemistry involve the formation of an insoluble solid called a precipitate. Recognising which combinations of soluble salts and acids lead to precipitation is a key skill in qualitative analysis and practical laboratory work. This question asks you to apply solubility rules to decide which pair of aqueous solutions will produce a solid precipitate when mixed.

Given Data / Assumptions:

• All reactants listed are aqueous solutions, indicated by the term solution or by (aq).
• The pairs involve common ions such as sodium, potassium, magnesium, nitrate, acetate, chloride, and silver.
• We assume standard solubility rules used in school chemistry.
• We neglect very slight solubility and focus on clear formation of an insoluble precipitate.

Concept / Approach:
To determine whether a precipitate forms, we write possible products of double displacement and then apply solubility rules. Nitrates and salts of sodium and potassium are almost always soluble. Many hydroxides are insoluble, especially those of magnesium and other alkaline earth metals. Simple salts like sodium chloride and sodium nitrate are soluble, and combinations producing only soluble products will not precipitate. Therefore, we look for a pair that yields an insoluble metal hydroxide or similar salt.

Step-by-Step Solution:
Step 1: Consider sodium hydroxide solution with hydrochloric acid solution. The reaction NaOH + HCl gives NaCl and water. Sodium chloride is soluble, so no solid forms. Step 2: Consider potassium hydroxide solution with magnesium nitrate solution. Possible products are potassium nitrate and magnesium hydroxide. Step 3: Potassium nitrate is soluble because all nitrates and all potassium salts are soluble in water. Step 4: Magnesium hydroxide is poorly soluble in water and is treated as an insoluble hydroxide, so it will appear as a solid precipitate. Step 5: Sodium acetate solution with hydrochloric acid solution reacts to form sodium chloride and acetic acid in solution, neither of which precipitates. Step 6: Silver nitrate solution with sodium nitrate solution produces no new insoluble salt because all involved ions produce soluble nitrates. Mixing hydrochloric acid with sodium chloride also does not create an insoluble product. Thus, only the second pair clearly produces a precipitate.

Verification / Alternative check:
Solubility charts confirm that most nitrate salts are soluble and that alkali metal salts are typically soluble. They also show that many hydroxides, particularly of magnesium, calcium, and other alkaline earth metals, are insoluble or only slightly soluble. In laboratory tests, when a solution of a soluble magnesium salt is treated with a strong base like potassium hydroxide, a white precipitate of magnesium hydroxide is observed. This practical evidence supports the conclusion that the combination of potassium hydroxide and magnesium nitrate produces a precipitate.

Why Other Options Are Wrong:
Sodium hydroxide solution and hydrochloric acid solution: This neutralisation forms soluble sodium chloride and water, so no precipitate appears.
Sodium acetate solution and hydrochloric acid solution: Forms sodium chloride and acetic acid, both soluble in water under normal conditions.
Silver nitrate solution and sodium nitrate solution: Only soluble nitrate salts are present; there is no new insoluble compound produced.
Hydrochloric acid solution and sodium chloride solution: All species (H plus, Cl minus, Na plus) remain in solution; no insoluble compound is formed.

Common Pitfalls:
A common mistake is to focus only on whether a reaction occurs, rather than whether a solid precipitate forms. Neutralisation reactions produce water but not necessarily a precipitate. Another error is to treat all double displacement reactions as precipitation reactions, ignoring solubility rules. To avoid this, always check whether any of the possible products is listed as insoluble or sparingly soluble. If all products are soluble, then no visible precipitate appears.

Final Answer:
The combination of potassium hydroxide solution and magnesium nitrate solution produces an insoluble precipitate of magnesium hydroxide.