Chemistry: Organic Chemistry: UPSC Previous Year Questions (Science and Technology)

Explanation

The distinctive pungent odour of garlic is due to sulphur-containing compounds, mainly allicin. These compounds are formed when alliin is converted by the enzyme alliinase during chopping or crushing.

Explanation

Eugenol is the active component in clove oil, contributing to its distinct aroma and analgesic properties. It is widely used in dental care and as a natural preservative.

Explanation

Mustard gas (sulfur mustard) was widely used as a chemical weapon during World War I. It causes blistering of the skin and mucous membranes, along with severe respiratory damage.

Explanation

Option (a) is correct: Hydrogen has the highest calorific value of around 120-150 MJ/kg, making it an efficient and eco-friendly fuel. Combustion of hydrogen produces only water, contributing to clean energy initiatives. Options (b), (c) and (d) are incorrect:

Charcoal: Has a calorific value of 30 MJ/kg(approx.), significantly lower than hydrogen. Natural gas: Primarily composed of Methane, With a fuel value of around 55 MJ/kg(approx.), it is lower than hydrogen but widely used due to availability. Gasoline: Though energy-dense around 47 MJ/kg (approx.), but lower than hydrogen.

Explanation

Option (d) is correct: Ethylene glycol is a widely used anti-freeze for automobile engines. It effectively lowers the freezing point of the coolant, preventing it from freezing in cold climates. Additionally, it raises the boiling point of the coolant, ensuring efficient heat transfer during engine operation. Options (a), (b) and (c) are incorrect:

Propyl alcohol is rarely used as an anti-freeze due to its lower efficiency and higher cost compared to ethylene glycol. While ethanol has some anti-freeze properties, it is volatile and less efficient than ethylene glycol. Although methanol can be used as an anti-freeze, it is highly toxic and less efficient compared to ethylene glycol.