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Very low temperature measurement requires gases that remain in gaseous state even at extremely low temperatures. Option A (Hydrogen gas thermometer) is incorrect — while hydrogen has a very low boiling point (-253°C), hydrogen gas thermometers are standardly used in the range of -200°C to 500°C, not specifically below -200°C. Also, at temperatures approaching -253°C, hydrogen itself starts to liquefy. Option B (Nitrogen gas thermometer) is incorrect — nitrogen liquefies at -196°C. Therefore, nitrogen cannot be used as a gas thermometer below -196°C, making it completely unsuitable for temperatures below -200°C. Option C (Helium gas thermometer) is CORRECT — helium is used for measuring temperatures below -200°C (i.e., in the cryogenic range). Helium has the lowest boiling point of any element at -269°C (4 K). Since it remains gaseous at near absolute zero temperatures, it is the ideal gas for cryogenic thermometry. Liquid helium itself is used to achieve near absolute zero temperatures in laboratories. Option D (Oxygen gas thermometer) is incorrect — oxygen liquefies at -183°C, making it completely useless as a gas thermometer at -200°C or below. Remember: Helium is the only noble gas with such an extremely low boiling point, making it uniquely suitable for cryogenic measurements.
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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.
