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.
Solution
Atmospheric pressure = 76 cm Hg
SVP = 0.80 cm Hg
When water is introduced into the barometer, water evaporates.
Thus, it exerts its vapour pressure over the mercury meniscus.
As more and more water evaporates, the vapour pressure increases that forces down the mercury level further.
Finally, when the volume is saturated with the vapour at the atmospheric temperature, the highest vapour pressure, i.e. SVP is observed and the fall of mercury level reaches its minimum. Thus,
Net pressure acting on the column = 76 - 0.80 cmHg
Net length of Hg column at SVG = 75.2 cm
