In very cold countries, alcohol is often used as a thermometric liquid in thermometers instead of mercury mainly because of which of the following reasons?

Introduction / Context:
This question relates to the choice of thermometric liquids in thermometers and how physical properties of liquids affect their usefulness. In extremely cold regions, thermometers must measure temperatures well below the freezing point of water and also below the freezing point of mercury. Using a liquid that solidifies at a higher temperature would make the thermometer useless in such conditions. Alcohol based thermometers solve this problem and are common in very cold climates. Understanding the key property that makes alcohol suitable is an important concept in basic physics and everyday science.

Given Data / Assumptions:

• We are considering thermometers used in very cold countries.
• Alcohol is compared with mercury as a thermometric liquid.
• Options mention conductivity, colouring, freezing point and sticking to glass.
• We must identify the most important reason for preferring alcohol in this specific context.

Concept / Approach:
A thermometric liquid should remain in liquid form over the temperature range to be measured. If it freezes within that range, it cannot expand or contract properly and the thermometer fails. Mercury freezes at a temperature around -39 degrees Celsius. In some very cold regions, air temperatures can drop below this limit, risking solidification of mercury. Alcohol (such as ethanol) has a much lower freezing point, around -114 degrees Celsius, and therefore remains liquid at temperatures where mercury would freeze. While it is true that alcohol can be coloured for visibility and has other convenient properties, the main reason it is favoured in very cold climates is its very low freezing point.

Step-by-Step Solution:
Step 1: Identify the requirement: a thermometric liquid that remains liquid at very low temperatures common in cold countries. Step 2: Recall that mercury freezes near -39 degrees Celsius, which is insufficient for extremely low temperature measurements. Step 3: Note that alcohol, such as ethanol, has a much lower freezing point (well below -100 degrees Celsius), so it remains liquid in extreme cold. Step 4: Recognise that this very low freezing point is the primary property that allows alcohol thermometers to function where mercury thermometers would fail. Step 5: Therefore, among the options, the correct main reason is that alcohol has a very low freezing point.

Verification / Alternative check:
To verify, imagine using a mercury thermometer in a polar region where air temperatures might fall below -40 degrees Celsius. Once mercury freezes, it cannot expand or contract, and the thermometer reading becomes stuck. In contrast, alcohol remains fluid at these temperatures and can continue to respond to temperature changes. Other properties such as ease of colouring and wetting of glass are helpful for readability and construction, but they do not address the fundamental requirement of staying liquid. Hence, the low freezing point is the decisive factor in this context.

Why Other Options Are Wrong:
Alcohol being a poor conductor of heat is not particularly relevant; thermometer response actually benefits from reasonable thermal conduction to reach equilibrium quickly. Alcohol can indeed be coloured to see its level more easily, but this is a secondary convenience, not the main reason for its use in very cold countries. The idea that alcohol does not stick to the glass tube is not the primary consideration; both mercury and alcohol can be made to wet or not wet glass with proper design.

Common Pitfalls:
Students sometimes latch onto secondary advantages like easy colouring or visibility and choose those options, forgetting the fundamental requirement that the thermometric liquid must not freeze within the measurement range. Others may focus on conductivity without realising that the key limiting factor in extreme cold is freezing point. To avoid this, always consider the operational temperature range first when evaluating why one liquid is chosen over another in thermometer design.

Final Answer:
Alcohol is used as a thermometric liquid in very cold countries mainly because alcohol has a very low freezing point and remains liquid at temperatures where mercury would freeze.