A student used the following step in testing for a non-reducing sugar. I. added Benedict's solution to the…

Analyze the question: The test is for a non-reducing sugar (e.g., sucrose). We need the colour seen in the final step, when Benedict's solution is added after hydrolysis and neutralisation.

Concept:

• Benedict's solution only reacts with reducing sugars (like glucose and fructose). When heated with them, it changes from blue to green, then yellow/orange, and finally brick-red (a precipitate of copper(I) oxide).
• Non-reducing sugars such as sucrose give no reaction directly. This is why step I gives no change, and the solution stays blue.
• To detect a non-reducing sugar, you first break it into its reducing monosaccharides by acid hydrolysis.

Step-by-step:

1. Step I: Benedict's solution is added to the sugar, with no colour change (it stays blue). This confirms the sugar is not a reducing sugar.
2. Step II: Boiling with dilute HCl hydrolyses sucrose into glucose and fructose, which are reducing sugars.
3. Step III: Sodium hydroxide neutralises the acid. Benedict's reagent works only in alkaline conditions, so this step is essential.
4. Step IV: Benedict's solution is added to the cooled solution (and heated). The glucose and fructose now reduce the copper(II) sulphate, giving a brick-red precipitate.

Eliminating the other options:

• Blue-black is the colour for the starch and iodine test.
• Purple or violet is the colour for the biuret test (proteins).

Answer: B. brick-red

Tip: Remember the sequence for non-reducing sugars: Acid, Alkali, Benedict's, meaning hydrolyse, neutralise, then test. Benedict's gives blue → green → yellow → orange → brick-red as the reducing sugar concentration increases. Also remember that the iodine test gives blue-black and the biuret test gives purple or violet.