Question

Which liquid metal is used to make thermometers?
(A) Mercury
(B) Bromine
(C) Iodine
(D) Cadmium

Answer 1

Hint: Thermometers are the instruments to measure the temperature. Most of the metals are good conductors of heat. But the one which has a high coefficient of thermal expansion is always preferred to use in thermometers. The mercury has a low melting and boiling point.it has a high thermal coefficient of expansion or volumetric expansion. The mercury expands on the rise of temperature.

Complete step by step solution:
Most of the metals are good conductors of heat and they are normally found as a solid at room temperature. The atoms of such metals are closed bonded and thus they exist as a compact solid.
However, the mercury atoms do not share the valence electron. In short, the neighbouring atoms are not closely packed. Rather the electrons in the mercury are more tightly bound to the nucleus thus they exhibit the relativistic effect. They can move easily concerning each other. The electrons behave as the larger but slow-moving electrons. Due to this weak bonding between the mercury atom t, mercury has a low boiling point as well as a melting point. We can say that the amount of energy required to break the weak bond is easily acquired at room temperature.
Let’s first define the coefficient of thermal expansion.
The coefficient of thermal expansion is defined as the expansion in the size of the object with the temperature change. It scribes the fractional change in size per degree change associated with the temperature at constant pressure.
Volumetric expansion is the most common type of thermal expansion. The mercury is liquid at room temperature. Mercury has a coefficient expansion of \[0.00018\]. It expands to $0.18{\scriptstyle{}^{0}/{}_{0}}$ in volume for every degree rise in temperature. It has a high thermal expansion coefficient. Therefore, the temperature rise can be easily detected in the presence of mercury.
In thermometers, the mercury is stored in a reservoir that is placed at the bottom of the thermometer. This bulb or reservoir is connected to the thin glass capillary. With the increase in temperature, the liquid mercury rises in the glass capillary. The volume of mercury is used to determine the temperature of the system.
The mercury is liquid and has a high coefficient of expansion and it is widely used in thermometers.

Hence, (A) is the correct option.

Note: Bromine is liquid at room temperature but it is not used in the thermometers. This is because the mercury has low specific heat and it does not stick to the walls of the capillary. However, the bromine does not exhibit these properties. There are non-mercury based thermometers. The ethanol filled thermometers have given the preference over the mercury in meteorological instruments which can show the temperature range up to $-{{70}^{0}}\text{C or -9}{{\text{4}}^{\text{0}}}\text{F}$.