The sentence with the right answer should read: Water in a tube culd be used to make a barometer, but the tube would have to be very, very long.
The reason is the density of water compared with the density of mercury for example.
Density of mercury is about 13.5 g/cm^3, this is 13.5 times the density of water.
The pressure of the atmosphere has been measured about 76 cm of mercury (that is the atmosphere pressure at sea level).
That means that a column of 76 cm of mercury exerts the same pressure that all the atmosphere. And the pressure is proportional to the weight which is proportional to the density. So, if you have that the density of mercury is 13,5 times the density of water the column of water that equals the pressure of 76 cm of mercury is:
Column of water * density of water = column of mercury * density of mercury
=> column of water = column of mercury * density of mercury / density of water
=> column of water = 76 cm * 13.5 g/cm^3 / 1 g/cm^3 = 1026 cm = 10, 26 m
Then you get that a barometer made of water would need to be more than 10 meters tall.
