WAEC Chemistry 2022 Paper

Concept:A liquid at room temperature must have a melting point below room temperature and a boiling point above room temperature. Rapid evaporation in air indicates high volatility, which is linked to a low boiling point.Explanation:Room temperature is about $25^{\circ}\mathrm{C}$.Check each substance at this temperature:P: melting point $-78^{\circ}\mathrm{C}$, boiling point $-25^{\circ}\mathrm{C}$. Since $-25^{\circ}\mathrm{C} < 25^{\circ}\mathrm{C}$, P is a gas at room temperature.Q: melting point $-8^{\circ}\mathrm{C}$, boiling point $40^{\circ}\mathrm{C}$. So Q is liquid between $-8^{\circ}\mathrm{C}$ and $40^{\circ}\mathrm{C}$, so it is liquid at $25^{\circ}\mathrm{C}$.R: melting point $-6^{\circ}\mathrm{C}$, boiling point $150^{\circ}\mathrm{C}$. R is also liquid at room temperature.S: melting point $44^{\circ}\mathrm{C}$. Since $44^{\circ}\mathrm{C} > 25^{\circ}\mathrm{C}$, S is solid at room temperature.Both Q and R are liquids, but Q has a much lower boiling point ($40^{\circ}\mathrm{C}$) than R ($150^{\circ}\mathrm{C}$).A lower boiling point means a liquid evaporates more quickly when exposed to air.Therefore, Q is the substance that is liquid at room temperature and rapidly evaporates on exposure to air.Answer:B. Q