One special advantage of alcohol over mercury as a thermometric liquid is its?

Analyze the question: The question asks for an advantage that alcohol has over mercury as a thermometric liquid. We need to compare the physical properties of the two liquids.

Explain the concept: A thermometric liquid should stay liquid over the temperature range being measured. The two liquids differ as follows:

• Mercury: freezing point about -39 °C, boiling point about 357 °C.
• Alcohol (ethanol): freezing point about -114 °C, boiling point about 78 °C.

Because alcohol freezes at a much lower temperature, it can measure very cold temperatures, such as in polar regions, where mercury would already be solid. Mercury is better for high temperatures because of its high boiling point.

Step-by-step check of the options:

• A. Low density: Alcohol does have a lower density, but this is not the special advantage in thermometry. Low density has no direct benefit for measuring temperature.
• B. Low freezing point: Correct. Alcohol stays liquid down to about -114 °C, so it can measure very low temperatures that mercury cannot.
• C. High specific (heat capacity): The option is incomplete, and a high specific heat capacity would actually be a disadvantage. It would make the thermometer slow to respond to temperature changes.
• D. Low boiling point: This is a disadvantage, not an advantage, because it limits alcohol to low-temperature measurements.

Answer: The correct option is B. low freezing point.

Tip: Remember "Alcohol for the cold, mercury for the hot." Alcohol has a low freezing point, so it is used for low temperatures. Mercury has a high boiling point, so it is used for high temperatures. JAMB often asks about the advantages of mercury (opaque, uniform expansion, high boiling point, does not wet glass) and of alcohol (low freezing point, expands more), so learn both lists.