Physical Properties of Pure Water:
(a) Nature- Pure water is a colourless, tasteless and odourless liquid. Distilled water is pure. Drinking water has a pleasant taste because dissolved minerals and air in the water make it taste better.
(b) States- Pure water exists in all the three states as solid (ice), liquid (water) and gaseous (steam or water vapour).
(c) Freezing Point of Water- Pure water freezes to solid ice at 0°C (at 760 mm of mercury pressure).
If the water contains impurities, it freezes at a temperature less than 0°C. Presence of impurities lowers the freezing point. Also at a higher pressure, water freezes at a temperature slightly less than 0°C.
That is why, on mountains (at a lower atmospheric pressure), water freezes at a temperature slightly higher than 0°C.
(d) Boiling point of water- Pure water boils at 100°C (at 760 mm of mercury pressure).
At high pressure, the boiling point is more than 100°C. When pressure is high, molecules of water vapour cannot escape easily from the surface. This is why food cooks faster in a pressure cooker.
At low pressure (or on high mountains), water boils at a temperature slightly less than 100°C. This is so because water molecules are more free to escape from the water surface.
(e) Stable Substance- Water is a stable substance. It is broken into its components H2 and O2 when we heat water to beyond 500°C. Alternatively, electrolysis breaks molecules of water into H2 and O2.
During freezing and boiling, the molecules of water are not broken down.
(f) Anomalous Expansion- Normally, a substance expands on heating and contracts on freezing. But with water, it is just the opposite, when heated or cooled between 4°C and 0°C.
- On cooling below 4°C, water expands and hence its volume increases. So, the volume of ice at 0°C is greater than the volume of water at 0°C. Water contracts from 0°C to 4°C and then expands as usual with rise in temperature.
- On heating water at 0°C, water contracts and hence its volume decreases. This happens up to 4°C. On freezing the water, its volume increases by about 10%.
(g) Density- Density of solid water, i.e. ice is lower than the density of liquid water at room temperature. That is why ice cubes float in water. Ice is lighter than water.
Density of a substance is defined as its mass per unit volume. Mathematically,
Density = mass / volume
Pure water has the maximum density at 4°C and minimum density at 0°C. Its density decreases by 10%. Ice becomes lighter and floats in water.
Hence, pure water has a minimum volume at 4°C and maximum volume at 0°C. Very cool water at around 4°C is the most packed arrangement.
It means that for the same mass of ice and water, the volume of ice is more than that of water. In other words, size of water increases on solidification while mass remains the same.
Therefore, water expands in volume when it is solidified to ice. But with ghee or oil, it is just the opposite; they expand on heating and contract on freezing. Normally 1 cm3 (c.c.) of water has a mass of 1 gram. That is, the density of water is 1g/cm3.
(h) Heat capacity of water- Heat capacity of a substance is the heat required to raise the temperature by one degree.
Heat capacity of water is the highest among all liquids.
(i) A good solvent- Water is an excellent solvent. It dissolves many substances forming aqueous solutions. Therefore, it is known as ‘universal’ solvent. A solution of a substance in water is known as an aqueous solution (aqueous means watery).
(j) Pure water is a bad conductor of heat and electricity. Molecules of water attract each other. This attraction between the molecules is partly electrical in nature.
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