4.1.3 Thermometer

Liquid in Glass Thermometer

  1. Liquid in glass thermometer works on the principle that liquid expands as the temperature increases and contracts as the temperature decreases.
  2. The most commonly used liquids in such thermometers are
    1. Mercury
    2. Alcohol

Q & A

Q: State the advantageous and disadvantageous of using mercury as the liquid in a liquid in glass thermometer.

A:
Advantageous:
  1. Doesn’t wet the wall of the capillary tube.
  2. Can be seen easily
  3. Expand uniformly when heated
  4. Good heat conductor

Disadvantageous:
  1. Freezing point = -39°C. Not suitable to measure temperature lower than -39°C.
  2. Poisonous
  3. Expensive

Q & A

Q: State the advantageous and disadvantageous of using alcohol as the liquid in a liquid in glass thermometer.

A:
Advantageous:
  1. Freezing point = -115°C. Suitable for measuring low temperature.
  2. Expands greater than mercury.

Disadvantageous:
  1. Transparent. Difficult to be seen. Need to be coloured.
  2. Always cling the wall of the capillary tube.
  3. Has tendency to break the tube at high temperature.


Q & A

Q: State the characteristics of the liquid used in a liquid in glass thermometer.

A:
  1. Easily visible
  2. Good conductor of heat
  3. Expand and contract rapidly over a wide range of temperature
  4. Does not cling to the wall of the capillary tube of the thermometer.

Q & A

Q: State and explain how the sensitivity of a liquid in glass thermometer can be increased.
A:
  1. The sensitivity of a mercury thermometer can be increased by using a smaller mercury bulb, thinner wall and smaller bore.
  2. A smaller bulb contains less mercury and hence absorbs heat in shorter time. As a result it can response faster to temperature change.
  3. A glass bulb with thinner wall can transfer heat to the bulb easier. Therefore, the thermometer can response quickly to small changes of temperature near the surrounding.
  4. Capillary with narrow bore produces a greater change in the length of the mercury column. Therefore a small change in temperature can be detected easily.