Water : Water, Physical Properties

Water exists in three states: solid (ice), liquid (water), and vapour (steam). Steam consists almost wholly of H₂O molecules. There are several varieties of ice, and liquid water probably contains polymerised molecules (H₂O)_n.

Liquid water possesses a faint though distinct blue colour (the colour of liquid oxygen is blue), which is seen when light is passed through a tube of water 2 m. long, closed at the ends with pieces of plate glass. Ice shows the same colour in large masses, as in crevices of glaciers or icefloes. The deep blue colour of some clear lakes appears to be due to light scattered from fine particles of solid matter in suspension.

Liquid water is only slightly compressible; between 1 and 25 atm. an increase of pressure of 1 atm. reduces the volume by only 5 parts in 100,000. The expansion of water by heat is peculiar. From 0° to 3.98°, the liquid contracts; beyond 3.98° it expands: at 3.98° water is in a state of maximum density. Owing to this property, exposed water freezes only on the surface; the water sinks as it reaches 3.98°, and forms a heavier layer beneath the upper crust of ice, through which heat is only very slowly transmitted.

The volume of 1 kgm. of water at 4° weighed in vacua is denned as the standard litre; it occupies 1000.028 cm3. The volume of 1 kgm. of water at 15°, weighed in air, is Mohr's litre; it occupies 1001.98 cm.

The density of ice at 0° is 0.9168; it floats on water, which expands on freezing. The liquid may be supercooled to about -4°. Water pipes are burst on freezing; the result is obvious when a thaw sets in. Cast-iron bottles filled with water and closed with screw plugs burst when immersed in a freezing mixture.

The densities of water, referred to the weight in grams of one-thousandth of a standard litre (1 ml.) at 4° as unity, are as follows:

Temperatre	Density
-5°	0.99930
0°	0.99987
4°	1.00000
8°	0.999808
10°	0.99973
20°	0.99823
100°	0.9584
150°	0.9173
250°	0.794

The amount of heat required to raise the temperature of 1 gm. of water from 14&fraCl2;° to 15&fraCl2;°, i.e., through 1°, is called the gram calorie (g. cal.). This varies slightly with the temperature of the water; at 0° and 100° it is slightly greater than at 15°. The corresponding amount for 1 kilogram of water is the kilogram calorie: 1 k.cal. = 1000 g. cal. This heat may be generated by stirring the water, and the number of units of work spent in generating 1 g. cal. is the mechanical equivalent of heat, 4.184x10⁷ ergs per 15° g. cal.

The number of g. cal. required to raise the temperature of 1 gram of a substance through 1° under specified conditions is the specific heat: the specific heat of ice is 0.502.

When ice is converted into water a considerable absorption of heat takes place, although the temperature remains constant at o°. This heat, which amounts to 79.74 g. cal. per gram of ice, is the latent heat of fusion of ice (or latent heat of water). Other pure substances possess characteristic latent heats. Similarly, when water at its boiling point is converted into steam a large absorption of heat occurs. For i gm. this amounts to 539.1 g. cal., called the latent heat of evaporation of water (or the latent heat of steam). For evaporation at other temperatures it possesses different values. In the reverse changes of solidification or liquefaction exactly the same quantities of heat are evolved.

The vapour density of water just above the boiling point is slightly greater than corresponds with the formula H₂O. When this is corrected for deviations from Boyle's laws the results show that little, if any, association exists in the vapour (the experimental results are conflicting) and that steam consists almost entirely of H₂O molecules (hydrone, or hydrol). The properties of liquid water, its high surface tension, high dielectric constant and great tendency to promote ionisation of dissolved electrolytes, as well as its abnormal physical properties (high boiling point as compared with hydrogen sulphide, H₂S), the expansion on solidification, and the existence of a maximum density above the freezing-point, all seem to point to an association of H₂O molecules in the liquid.

There are different varieties of ice formed from ordinary ice under high pressures (Tammann; Bridgman).

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