Heat (Part 2) class 7



Heat


Q. What is thermometer? Explain it's working.
Ans. A thermometer is a device used for measuring the temperature of an object. Generally, mercury is used in thermometers. Mercury exists as liquid at room temperature. The property of expansion and contraction of mercury due to heating and cooling is made use in thermometers. It's expansion and contraction points out the temperature at the scale of thermometer. 
These days, digital thermometers are available which do not use mercury.

Q. What are the advantages of using mercury in thermometers?
Ans. Advantages of using mercury in thermometers are:
—  Mercury exist as liquid at room temperature.
—  It is opaque, shiny and visible.
—  It does not stick to the glass.
—  It is a good conductor of heat.

Q. What are the Limitations/Disadvantages of using mercury in thermometers?
Ans. Limitations/Disadvantages of using mercury in thermometers are:
—  It is a toxic substance and is very difficult to dispose off if a thermometer breaks.
—  It can not measure the small changes in temperatures.
—  Freezing point of mercury is about -40°C, it is not suitable for measuring much low temperatures. 

Q. We can't use water in place of mercury in thermometers. Give reason?
Ans. 
  • Boiling point and freezing point of water has a very short range as compared to mercury.
  • Water does not show expansion and contraction property like mercury.
  • Water droplets stick to the glass of the thermometers.
  • Water is not easily observable like mercury.






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