Why mercury is used in barometers: Select the best reason(s) that justify mercury as the working fluid in standard barometers.
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AIts vapour pressure is practically zero at ambient conditions
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BBecause of high density, the barometer column height is small
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CIt is the “best” liquid by all measures regardless of conditions
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DBoth (a) and (b)
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EBoth (b) and (c)
Answer
Correct Answer: Both (a) and (b)
Explanation
Introduction / Context:Barometers measure atmospheric pressure via the height of a liquid column. The choice of liquid affects instrument size, sensitivity, and accuracy. Mercury has been the classical choice dating back to Torricelli’s experiment.
Given Data / Assumptions:
- Ambient temperatures where mercury remains liquid.
- Standard gravity; vertical tube evacuated above the column.
- Clean, non-wetting glass conditions typical for barometers.
Concept / Approach:
Two key properties make mercury ideal: extremely low vapour pressure at room temperature (minimizing vapour above the column so pressure equals atmospheric) and high density (~13,600 kg/m^3), which keeps the column height manageable (~760 mm at 1 atm) and reduces instrument size compared with water (~10.3 m column).
Step-by-Step Solution:
Identify impact of vapour pressure: negligible vapour pressure prevents significant vapour cushion errors.Recognize effect of density: high density shortens the required column height to achieve hydrostatic balance p = ρ g h.Hence, statements (a) and (b) together justify mercury’s use.Verification / Alternative check:
Compute water column for 1 atm: h = p/(ρ g) ≈ 101325/(1000*9.81) ≈ 10.3 m, impractical indoors; mercury reduces this to ~0.76 m.
Why Other Options Are Wrong:
(c) is vague and untrue universally; (e) includes (c), which is unjustified; only (d) provides the standard reasons.
Common Pitfalls:
Ignoring vapour pressure effects; assuming any dense liquid works equally well despite volatility and wetting issues.
Final Answer:
Both (a) and (b)