When a mixture of a food substance and Benedict's solution was warmed, the solution changed from blue to brick-red. This indicates the presence of?

Step 1: Analyze the question The question asks which substance is present when Benedict's solution, warmed with a food sample, turns from blue to brick-red.

Step 2: The concept Benedict's test detects reducing sugars. Benedict's solution contains copper(II) sulphate (blue), which is alkaline. Reducing sugars have a free aldehyde or ketone group that can donate electrons. When heated, they reduce blue copper(II) ions (Cu²⁺) to copper(I) oxide (Cu₂O), which is an insoluble brick-red precipitate.

The colour change depends on the amount of reducing sugar:

• Blue: none present
• Green: trace
• Yellow/orange: moderate
• Brick-red: large amount

Examples of reducing sugars: glucose, fructose, galactose, maltose, lactose.

Step 3: Check each option

• A. Reducing sugar: Correct. This is exactly what produces the brick-red precipitate.
• B. Fatty acid: Fats are tested with the ethanol emulsion test or the Sudan III test, not Benedict's.
• C. Sucrose: Sucrose is a non-reducing sugar, so it gives no colour change (it stays blue). It only reacts if first boiled with dilute hydrochloric acid to break it into glucose and fructose, then neutralised.
• D. Amino acid: Amino acids and proteins are tested with the Biuret test (violet/purple) or Ninhydrin, not Benedict's.

Step 4: Answer The answer is A. reducing sugar.

Exam tip: Remember "Benedict's: Blue → Brick-red = Reducing sugar". Also remember that sucrose is the classic trap in JAMB. It is a sugar, but it is non-reducing, so it will not give a positive Benedict's test unless hydrolysed first.