The by-product of fermentation of sugar is

Analyze the question: Fermentation of sugar is a reaction in which enzymes break sugar down. The question asks for the by-product, not the main product.

Explain the concept: Fermentation is the breakdown of sugars (such as glucose) by enzymes in yeast. The enzyme zymase converts glucose into ethanol (the main product) and carbon(IV) oxide (the by-product).

Step-by-step solution:

1. Sugar (sucrose) is first hydrolysed by the enzyme invertase into glucose and fructose: C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆

2. Zymase in yeast then converts glucose to ethanol and carbon(IV) oxide: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂

3. Ethanol (C₂H₅OH) is the main product, and CO₂ is the by-product, which is the gas that bubbles out. This is why dough rises in baking and why palm wine froths.

Checking the options:

• A. Carbon(IV) oxide is correct, as it is released alongside ethanol.
• B. Ethanoic acid forms only if ethanol is further oxidised, as when wine turns sour. It is not the by-product of the fermentation itself.
• C. Propanol is not formed.
• D. Propan-1,2,3-triol (glycerol) is a by-product of saponification or fat hydrolysis, not sugar fermentation.

**Answer: A. carbon(IV) oxide.

Exam tip: Remember "Fermentation = Ethanol + CO₂". Ethanol is the main product, CO₂ is the by-product. You can confirm the CO₂ in the lab because it turns lime water milky.