The substance that reacts with sodium to form alkali and changes white anhydrous copper(II) tetraoxosulphate (VI) to blue is

1. Analyze the question

We are looking for a substance that meets two specific criteria:

• Reaction with Sodium: It must react with sodium metal to produce an alkali (a basic hydroxide).
• Hydration Test: It must turn white anhydrous copper(II) tetraoxosulphate(VI) into blue.

2. Explain the concept

• Reaction with Sodium: When sodium metal reacts with water ($H_2O$), it produces sodium hydroxide ($NaOH$), which is a strong alkali, and releases hydrogen gas. • Equation: $2Na(s) + 2H_2O(l) \rightarrow 2NaOH(aq) + H_2(g)$
• Hydration Test: Anhydrous copper(II) tetraoxosulphate(VI) ($CuSO_4$) is a white powder. It is a well-known chemical test for the presence of water. When water is added to it, it becomes hydrated, forming $CuSO_4 \cdot 5H_2O$, which is blue in color.

3. Show the solution

• Step 1: Evaluate the reaction with sodium. Acids react with sodium to produce salts and hydrogen gas, not necessarily an alkali. Water reacts with sodium to produce an alkali ($NaOH$).
• Step 2: Evaluate the color change. The change from white anhydrous $CuSO_4$ to blue hydrated $CuSO_4$ is the standard laboratory test for water.
• Step 3: Combine the findings. Since water satisfies both the production of an alkali when reacting with sodium and the hydration of copper(II) tetraoxosulphate(VI), it is the correct substance.

4. Identify the answer

Comparing our findings to the options provided:

• A. Base: Does not react with sodium to form an alkali.
• B. Acid: Reacts with sodium to form a salt, not an alkali.
• C. Water: Reacts with sodium to form $NaOH$ (alkali) and turns anhydrous $CuSO_4$ blue.
• D. Solvent: This is a general term; while water is a solvent, "water" is the specific chemical identity required here.

5. Exam Tip

Whenever you see "anhydrous copper(II) tetraoxosulphate(VI)" in a question, immediately associate it with the test for water. It is a classic JAMB/WASSCE indicator. Also, remember that Group 1 metals (like Sodium) react vigorously with water to form alkaline solutions—this is why they are called "alkali metals."