Which of the following food substances would give a negative reaction with Benedict's reagent?

Step 1: Analyze the question Benedict's reagent tests for reducing sugars. A positive result is a color change from blue to green, yellow, orange, or brick-red precipitate on heating. A negative reaction means the solution stays blue. We need the sugar that is NOT a reducing sugar.

Step 2: The concept A reducing sugar has a free aldehyde group or a free ketone group (which can tautomerize to an aldehyde in alkaline conditions). This lets it donate electrons and reduce blue copper(II) sulphate to red copper(I) oxide.

• Monosaccharides (glucose, fructose, galactose) are all reducing sugars.
• Most disaccharides (maltose, lactose) are reducing sugars.
• Sucrose is the exception. It is formed from glucose and fructose joined through their reducing (anomeric) carbons, a glycosidic bond between C1 of glucose and C2 of fructose. This leaves no free aldehyde or ketone group, so sucrose is a non-reducing sugar.

Step 3: Check each option

• A. Fructose: monosaccharide, reducing, positive test
• B. Glucose: monosaccharide, reducing, positive test
• C. Galactose: monosaccharide, reducing, positive test
• D. Sucrose: non-reducing, negative test (stays blue)

Step 4: Answer The correct option is D. Sucrose.

Extra note: If you boil sucrose with dilute hydrochloric acid first, it hydrolyzes into glucose and fructose. After neutralizing with sodium hydroxide or sodium hydrogencarbonate, it will then give a positive Benedict's test. WASSCE often asks about this.

Exam tip: Remember "Sucrose is the Stubborn Sugar - it says NO to Benedict's." Among common carbohydrates, the non-reducing ones are sucrose and polysaccharides like starch. Everything else (glucose, fructose, galactose, maltose, lactose) is reducing.