Question

The table below shows the melting point of three liquids.

| Melting Point| Boiling Point
---|---|---
Alcohol| $- 115^\circ C$ | $78^\circ C$
Mercury| $- 39^\circ C$| $357^\circ C$
Water| $0^\circ C$| $100^\circ C$

Which liquid is most suitable to be used in a thermometer for measuring a temperature range of $- 10^\circ C$ to $95^\circ C$.

Answer 1

Hint
To solve this problem, we need to find a suitable liquid from the three liquids given in the table that remains in its liquid form in the given temperature range as in the question. The liquid that fulfills both the conditions will give us the answer.

Complete Step by Step Solution
The given temperature range in the question is $- 10^\circ C$ to $95^\circ C$. So, we need to find a suitable substance that can be used in the thermometer that can measure temperature in between that range.
This means that the substance inside the thermometer has to remain in the liquid state to measure the temperature properly even at the highest and the lowest extremes.
Now, in the case of alcohol, it has a melting point up to $- 115^\circ C$. This means that alcohol will remain in its liquid state at a temperature of $- 10^\circ C$. But alcohol also has a boiling point of $78^\circ C$. Then, at the temperature of $95^\circ C$ the alcohol will turn into vapour and we won’t get a reading on the thermometer.
Similarly, in the case of water, it has a boiling point $100^\circ C$. So, it will easily stay liquid at $95^\circ C$. But the melting point of water is $0^\circ C$. So, at the temperature of $- 10^\circ C$ the water will freeze inside the thermometer, and we won’t get a reading.
But for mercury, its melting point is $- 39^\circ C$ and the boiling point is $357^\circ C$. So the range of temperature given in the question is well within the range in which mercury remains liquid.
So mercury is the only liquid that is suitable to be used in the thermometer to measure temperature between the range of $- 10^\circ C$ to $95^\circ C$.

Note
Even though water has a melting and boiling point quite close to the range, other properties make it unsuitable to be used in a thermometer. Few of them are:
(i) Its transparent colour makes it quite difficult to be seen while taking a reading.
(ii) It sticks to the walls of the capillary tube.