Why mercury is used in barometers: Select the best reason(s) that justify mercury as the working fluid in standard barometers.

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:

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)