Question

A barometer correctly reads the atmospheric pressure as 76 cm of mercury. Water droplets are slowly introduced into the barometer tube by a dropper. The height of the mercury column first decreases and then becomes constant. If the saturation vapour pressure at the atmospheric temperature is 0.80 cm of mercury, find the height of the mercury column when it reaches its minimum value.

Answer 1

Pressure is minimum when the vapour present inside is at saturation vapour pressure. As this is the maximum pressure which the vapours can exert.

Hence the normal level of mercury drops down by 0.8 cm

$\therefore$ The height of HG column

(Given SVP at atmospheric temperature = 0.08 cm of Hg)

= 76-0.80 cm

= 75.2 cm of Hg