There are few reasons why mercury is used in liquid barometer instead of water:
1. Mercury (13.69 g/cm³) is more dense than water (1.00 g/cm³), so if we use water in barometar, length of barometer would be much longer.
2. Mercury has lower vapor pressure than water.
3. Mercury has lower freezing point (-38.83°C) than water (0°C).
