Which of the following substances is the most volatile at room temperature?

Step 1: Analyze the question Volatility describes how easily a liquid evaporates at a given temperature. A highly volatile liquid has weak intermolecular forces and a low boiling point, so it turns into vapour quickly at room temperature.

Step 2: Explain the concept

• Weak intermolecular forces mean easy evaporation, so the liquid is more volatile.
• Strong intermolecular forces or large molecules mean slow evaporation, so the liquid is less volatile.
• For hydrocarbon fuels, the smaller the molecules (shorter carbon chains), the more volatile the fuel.

Step 3: Compare the options

| Substance | Approx. composition / boiling range | Volatility | |---|---|---| | Water | Boils at 100 °C, with strong hydrogen bonding | Low | | Diesel | About C₁₂–C₂₀, boils roughly 200–350 °C | Very low | | Petrol | About C₄–C₁₂, boils roughly 30–200 °C | Very high | | Kerosine | About C₁₀–C₁₆, boils roughly 150–275 °C | Low to moderate |

Petrol has the smallest molecules and the lowest boiling range. This is why petrol vapour can ignite so easily, and why petrol stations carry strict safety warnings. It evaporates quickly if left in an open container. Diesel and kerosine are heavier and evaporate much more slowly. Water has a higher boiling point and strong hydrogen bonds, so it is also less volatile.

Step 4: Answer Petrol is the most volatile at room temperature, which is option C.

Exam tip: For fuels, remember the order of volatility: petrol > kerosine > diesel. Think of the fractional distillation of crude oil. Fractions that come off at the top of the column (lighter, lower boiling point) are the most volatile.