ICSE Class 9 Answered

Water is not a suitable barometric liquid because:

(i) The density of water is low (10³ kg m^-3). Water barometer will support 10.4m of water at sea level. It is impractical to have such a long tube.

(ii) Water vapourises under vacuum conditions and hence water barometer will never show true atmospheric pressure.

(ii) Water sticks with the glass tube and wets it, so the reading becomes inaccurate.

Mercury is preferred as a barometric liquid because:

(i) The vapour pressure of mercury is almost negligible under vacuum conditions. Thus mercury barometer shows true atmospheric pressure.

(ii) Mercury is the densest liquid (13.6 g cm^-3 ) at room temperature. Thus, a short column of mercury can exert as much pressure as atmosphere.

(iii) Mercury does not wet the sides of glass and can be had in pure state.