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Gas thermometers work on the principle that gas pressure or volume changes proportionally with temperature. Option A (Helium gas thermometer) is incorrect — helium thermometers are used for extremely low temperatures, especially below -200°C, because helium remains gaseous even near absolute zero (it boils at -269°C). Helium is used for the range below -200°C, not -200°C to 500°C. Option B (Nitrogen gas thermometer) is incorrect — nitrogen liquefies at -196°C, so it cannot be used as a gas thermometer below that temperature. Nitrogen thermometers are used for specific narrower ranges. Option C (Hydrogen gas thermometer) is CORRECT — hydrogen gas thermometers are used in the range of -200°C to 500°C. Hydrogen has a very wide gaseous range and expands uniformly with temperature, making it suitable for this broad measurement span. It is one of the most accurate gas thermometers for this range. Option D (Oxygen gas thermometer) is incorrect — oxygen liquefies at -183°C, making it unsuitable for gas thermometry at temperatures below -183°C.
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Heat is a form of energy that transfers between objects or systems due to a temperature difference, moving from a hotter body to a cooler one until thermal equilibrium is reached.
