Which of the following substances would not produce ethanol when fermented?

Analyze the question: We need to find the substance that cannot be fermented to give ethanol.

Concept: Fermentation is the breakdown of carbohydrates by enzymes (from yeast or other microorganisms) to give ethanol and carbon dioxide.

• Cane sugar (sucrose) is hydrolysed by invertase into glucose and fructose, and then zymase converts these to ethanol.
• Glucose is converted directly by zymase: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
• Starch is first hydrolysed by diastase (amylase) to maltose, then maltase converts it to glucose, and then zymase converts the glucose to ethanol.
• Vinegar is dilute ethanoic acid (CH₃COOH). It is not a carbohydrate. It is actually a product of oxidising ethanol, so it cannot be fermented back into ethanol.

Step-by-step solution:

1. A: Cane sugar → glucose + fructose → ethanol. It produces ethanol.
2. B: Glucose → ethanol + CO₂. It produces ethanol.
3. C: Starch → maltose → glucose → ethanol. It produces ethanol.
4. D: Vinegar is ethanoic acid, which is already an oxidation product of ethanol. It produces no ethanol.

Correct answer: D (Vinegar)

Tip: Fermentation needs a carbohydrate (sugar or starch) as raw material. Remember the direction: ethanol → (oxidation) → ethanoic acid (vinegar). It does not go backwards by fermentation. If an option is an acid or not a carbohydrate, it is usually the "odd one out."