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# Physical Properties of Water

Main Physical Properties of Water is that water may be cooled below the freezing-point without crystallisation, it is not possible to raise the temperature of ice, under ordinary pressures, above 0° C. If, therefore, heat is supplied to the system ice-vapour, the temperature remains perfectly constant at almost exactly 0° C. until the whole of the ice has melted. As already explained, the temperature is not exactly 0° C., for this is by definition the temperature at which ice melts under atmospheric pressure, and since increase of pressure lowers the melting-point by 0.0076° C. per atmosphere, the melting-point of ice under its own vapour (pressure 0.4579 cm.) is +0.0076° C. During melting a contraction occurs, 10.90 volumes of ice yielding 10.00 volumes of water.

If the supply of heat is continued, further slight contraction ensues, but accompanied by rise of temperature until 3.98° C. is reached. This is the point of maximum density of water, further rise in temperature now resulting in expansion until the boiling-point is attained.

In the accompanying table are given (1) the density or mass in grams of 1 c.c. of water, and (2) the specific volume or the volume in c.c. of 1 gram of water at various temperatures.

Density and Specific volume of water between o° and 100° C.

 Temperature °C. Density. Grams Specific Volume, c.c. 0 0.999868 1.000132 1 0.999927 1.000073 2 0.999968 1.000032 3 0.999992 1.000008 4 1.000000 1.000000 5 0.999992 1.000008 6 0.999968 1.000032 7 0.999929 1.000071 8 0.999876 1.000124 9 0.999808 1.000192 10 0.999727 1.000273 11 0.999632 1.000368 12 0.999525 1.000476 13 0.999404 1.000596 14 0.999271 1.000729 15 0.999126 1.000874 16 0.998970 1.001031 17 0.998801 1.001200 18 0.998622 1.001380 19 0.998432 1.001571 20 0.998230 1.001773 21 0.998019 1.001995 22 0.997797 1.002208 23 0.997565 1.002441 24 0.997323 1.002685 25 0.997071 1.002938 26 0.996810 1.003201 27 0.996539 1.003473 28 0.996259 1.003755 29 0.995071 1.004046 30 0.995673 1.004346 35 0.994058 1.005978 40 0.99224 1.00782 45 0.99025 1.00985 50 0.98807 1.01207 55 0.98573 1.01448 60 0.98324 1.01705 65 0.98059 1.01979 70 0.97781 1.02270 80 0.97183 1.02899 90 0.96534 1.03590 100 0.95838 1.04343

The explanation usually accepted for the anomalous behaviour of water between 0° and 4° C. is that the observed change in volume is the algebraic sum of two factors, namely, (a) normal expansion due to increased distance between the molecules in consequence of their increased energy, and (b) contraction due to depolymerisation of bulky " ice " molecules into denser " water " molecules.

Increase of pressure reduces the temperature of maximum density: the two factors being connected by the expression

tp = 3.98 – 0.0225(p - 1),

where tp is the temperature of maximum density- under a pressure of p atmospheres.

If water is cooled below 0° C. without solidifying, the expansion with fall of temperature continues, as shown below:

Density and specific volume of water between 0° and -10° C.

 Temperature, ° C. Density. Specific Volume. 0 0.99987 1.00013 -1 0.99979 1.00021 -2 0.99970 1.00031 -3 0.99958 1.00042 -4 0.99945 1.00055 -5 0.99930 1.00070 -6 0.99912 1.00088 -7 0.99892 1.00108 -8 0.99869 1.00131 -9 0.99843 1.00157 -10 0.99815 1.00186

According to the foregoing data, unit volume of water at 0° C. becomes 1.043295 volumes at 100° C.

The coefficient of expansion, a, of water, with rise of temperature from 100° to 200° C. is given by the expression:

α = 1 + 0.000,108,679t + 0.000,003,007,365t2 + 0.000,000,002,873,042t3 - 0.000,000,000,006,645,7t4.

Between 0° and 20° C., the specific volume V at temperature t, may be calculated from the equation

V = 1.00012(1 - 0.000,060t + 0.000,007,5t2),

between 0° and 33° C. by

V = 1.000132(1 - 0.464272 + 0.05850532t2 - 0.0767898t3 + 0.0950024t4),

and between -32° and 100° C. from

V = 0.999,695 + 0.000,005472t2 - 0.000,000,011,26t3,

the volume at 4° C. being taken as unity. Mendeleeff gave the formula:

which is claimed to be correct to 4 parts in 100,000.

Dissolved salts depress the temperature of the maximum density of water, the depression being directly proportional to the concentration of the salt. The depression caused by a highly ionised binary electrolyte, for example, sodium chloride, is the sum of two independent effects, namely, that due to the acid radicle, and that due to the base. It is thus possible to calculate the depression caused by such a salt if the moduli corresponding to the two ions are known.

The molecular volume of water at the boiling-point is given as 18.73.

Physical Properties of Water compressibility. Water is slightly compressible. This was first established by Canton in 1762, and has been confirmed by several investigators since that date. Owing to the exceptional difficulty of determining experimentally the extent of its compressibility, the results obtained exhibit considerable variation. The coefficient of compressibility, β, is given numerically by the change in volume induced in unit volume by unit change in pressure. Hence

where V0 and Vp are the initial and final volumes under a change of pressure, p.

The compressibility at constant temperature varies somewhat with the degree of pressure, becoming smaller as the pressure increases. The best results to illustrate this are those of Richards and Stull, together with data for mercury for the sake of comparison.

These data mean that unit volume of the liquid at 20° C. upon being subjected to a pressure of one megabar (or 0.987 atmosphere) above its original pressure, decreases in volume by the amount indicated in the same horizontal line as that giving the total pressure. Thus, for example, one litre of water at normal pressure would decrease by 0.0000452 litre or 0.0452 c.c. if the pressure were raised by one megabar.

Compressibilities of water and mercury at 20° C

 Pressure in Megabars Compressibility of Water Compressibility of Mercury. 0-100 4.52×10-5 3.88×10-6 100-200 4.41×10-5 3.82×10-6 200-300 4.18×10-5 3.79×10-6 300-400 4.11×10-5 3.76×10-6 400-500 3.94×10-5 3.71×10-6

The compressibilities of the majority of liquids at constant pressure increase with rise of temperature. Water, however, is exceptional in this respect, its compressibility falling with rise of temperature, a minimum being reached, according to Pagliani and Vicentini, in the neighbourhood of 60° C. According to Tyrer, the minimum occurs at 50° C. for the isothermal compressibility and at 70° C. for adiabatic compressibility. This is usually attributed to the diminishing number of bulky and compressible " ice " molecules as the temperature rises from 0° to 60° C. At this latter temperature their number is negligibly small, and from this point onwards water behaves as a normal liquid.

Isoterman and Adiabatic compressibilities of water between 1 and 2 atmospheres

 Temperature, ° C. Isothermal Compressibility per Atmosphere. Adiabatic Compressibility per Atmosphere. 0 5.078×10-5 5.075×10-5 10 4.843×10-5 4.838×10-5 20 4.645×10-5 4.615×10-5 30 4.520×10-5 4.452×10-5 40 4.469×10-5 4.360×10-5 50 4.462×10-5 4.302×10-5 60 4.489×10-5 4.270×10-5 70 4.544×10-5 4.260×10-5 80 4.631×10-5 4.276×10-5 90 4.743×10-5 4.305×10-5 100 4.863×10-5 4.335×10-5

Tait gives the following expression whereby the compressibilities of water between 6° and 15° C. can be calculated for pressures ranging from 150 to 500 atmospheres:

,

where V0 is the volume at t° C. under 1 atmosphere, and V the volume at t° C. under a pressure of p atmospheres.

As a general rule of Physical Properties of Water, aqueous solutions are less compressible than pure water, due, probably, to a reduction in the number of " ice " molecules in the presence of the dissolved salt. The value for sea-water at 17.5° C. is 4.36×10-5.

Although the compressibilities of natural waters are thus exceedingly small, their effect upon the distribution of land and water on the crust of the earth is important. It has been calculated that, in consequence of the compressibility of sea-water, the mean sea level is 116 feet lower than it would be if water were absolutely incompressible, with the result that two million square miles of land are now uncovered which would otherwise be submerged.

Water is not usually regarded as possessing any appreciable tensile strength. If, however, precautions are taken to prevent its diameter from varying, a cylindrical column of water may be shown to possess a high tensile strength. This was first demonstrated by Berthelot in 1850, who almost completely filled a glass tube with water, leaving a small bubble of water-vapour after sealing hermetically. The whole was then warmed until the water, possessing a higher coefficient of expansion than the glass, completely filled the tube. On cooling, the liquid continued to fill the tube, thus showing that it resisted rupture under very appreciable tension. The result was only qualitative; but repetition in a modified apparatus by Dixon and Joly indicated a tension of 7 atmospheres. Results obtained by an entirely different method led Budgett to conclude that under special conditions the tensile strength of water may amount to as much as 60 atmospheres. At about 320° C. the tensile strength becomes negligible - a result to be anticipated from its proximity to the critical point (374° C.).

The viscosity of water has been measured frequently since the classic research of Poiseuille in 1843. The most reliable data are given in the accompanying table.

Viscosity of Water in c.g.s. Units (Dynes per cm2)

 Temperature, °C. (Dynes per cm2) 0 0.01797 10 0.01301 20 0.01006 30 0.007998 40 0.006563 50 0.005500 60 0.004735 70 0.004075 80 0.003570 90 0.003143 100 (0.002993 at 95°)

Poiseuilie's results are included as illustrative of the high degree of accuracy attained by that investigator. Between 0° and 25° C. the viscosity of water may be calculated from the equation

The value for η remains constant even under exceedingly low rates of shear.

The viscosity of supercooled water is as follows:

 Temperature, ° C. 0 -4.7 -7.23 -9.3 η 0.01793 0.02121 0.02341 0.02549

Increase of pressure tends to reduce the viscosity of water at temperatures below 36° C. In this respect water differs from most liquids that have been examined, as these become more viscous under increased pressure. No doubt the explanation lies in the tendency of the higher pressures to reduce the percentage of bulky and viscous ice molecules.

The vapour pressure of water rises with the temperature, as is evident from the following data, which give the tension in millimetres of mercury:

Vpour pressure of water between -15° and 370° C

 ° C. mm. ° C. mm. ° C. mm. ° C. mm. -15 1.445 + 11 9.845 + 25 23.763 + 75 289.0 -10 2.160 12 10.519 26 25.217 80 355.1 -5 3.171 13 11.233 27 26.747 85 433.5 0 4.579 14 11.989 28 28.358 90 525.8 +1 4.926 15 12.790 29 30.052 95 634.0 2 5.294 16 13.637 30 31.834 100 760.0 3 5.685 17 14.533 35 42.188 120 1488.9 4 6.101 18 15.480 40 55.341 150 3568.7 5 6.543 19 16.481 45 71.90 200 1164.7 6 7.014 20 17.539 50 92.54 250 40.476 7 7.514 21 18.655 55 117.85 300 87.41 8 8.046 22 19.832 60 149.19 350 168 12 9 8.610 23 21.074 65 187.36 360 189.63 10 9.210 24 22.383 70 233.53 370 213.73

The vapour pressure of water between 0° and 50° C. may be calculated with great exactness by means of Thiessen's formula:

(t + 273)log p/760 = 5.409(t - 100) – 0.508 ×10-8{(365 - t)4 - 2654}.

Mention has already been made of the fact that when the temperature of pure ice is gradually raised under the ordinary atmospheric pressure, melting always takes place exactly at 0° C. The converse, however, is not equally true.

If water is cooled, solidification may not occur immediately a temperature of 0° C. is reached. Under suitable conditions it is possible to reduce the temperature many degrees below 0° without freezing taking place, even at atmospheric pressures.

This was first observed by Fahrenheit, who, in 1724, succeeded in cooling water in cleaned tubes down to 15° F. -9.4° C.) without solidification. Gay Lussac cooled water down to -12° C., the liquid condition being maintained until a fragment of solid ice was added. The surface of the water was covered with oil to prevent contamination with dust.

Dalton stated that -

"If the water be kept still, and the cold be not severe, it may be cooled in large quantities to 25° or below, without freezing; if the water be confined in the bulb of a thermometer, it is very difficult to freeze it by any cold mixture above 15° of the old scale; but it is equally difficult to cool the water much below that temperature without its freezing. I have obtained it as low as 7° or 8°, and gradually heated it again without any part of it being frozen."

Dalton also knew that -

"When water is cooled below freezing and congelation suddenly takes place, the temperature rises instantly to 32°."

Capillary Water. - Sorby pointed out that water kept in glass tubes of diameter ranging from 0.025 to 0.25 inch may easily be cooled to -5° C. without congelation, even when the tube is shaken. By keeping the tube quiet an even lower temperature may be obtained, as has been mentioned above.

When contained in capillary tubes, water offers very great resistance to freezing, unless it is in contact with ice. Thus Sorby found that no congelation took place, even upon shaking, when water was cooled to -15° C. in glass tubes of diameter 0.003 to 0.005 inch. The temperature could even be reduced to -16° C. if the tubes were kept very quiet, although at -17° C. the water froze immediately. In a tube of diameter approximately 0.01 inch the water froze at -13° C. but not at -11° C. In contact with ice, however, water freezes readily in capillary tubes, and ice thaws as usual at 0° C. when in tubes in which water will not congeal in the absence of the solid phase above -16° C.

Mtiller-Thurgau states that filter paper moistened with distilled water freezes at -0.1° C., whilst a clay sphere, under similar conditions, has been found to freeze at -0.7° C. These observations refer to the actual freezing-points under the conditions named, and are quite apart from supercooling effects, which, as shown above, may be extended to much lower temperatures.

The presence of finely divided particles of solid materials, such as ferric hydroxide, alumina, etc., causes an appreciable depression of the freezing-point.

A convenient method of cooling water below 0° C. consists in preparing a mixture of chloroform and olive-oil in such proportions that the product has the same density as water. Drops of water suspended in this mixture may be cooled down to very low temperatures (c. -20° C.) without freezing. Such water is termed supercooled or super/used, and is stable only so long as the solid phase is absent. It is therefore said to be meta-stable. The water will usually solidify immediately upon exposure to air or dust, or on the introduction of some foreign material. Even the act of scratching the inside wall of the containing vessel will suffice to induce crystallisation. If the supercooling is carried beyond a certain amount, solidification takes place spontaneously without the introduction of the solid phase or foreign material. In either case the temperature rises to 0° and remains there until solidification is complete.

The velocities of crystallisation of supercooled water, as determined in a tube 1 metre in length and 0.7 cm. in diameter, and expressed as cms. per minute, are given in the accompanying table.

Velocity of crystallisation of supercooled water

 Temperature, ° C. Velocity, cms./min. Temperature, ° C. Velocity, cms./min. -2.00 31.6 -7.10 266.7 -3.61 48.4 -7.50 308.0 -4.67 71.4 -8.19 415.2 -5.86 107.1 -8.38 513.0 -6.18 114.7 -9.07 684.0

The maximum velocity of crystallisation evidently lies below -9.07° C., but, owing to spontaneous solidification of the water, it was not found possible to make determinations at lower temperatures.

It is interesting to note that whereas ice produced with a cooling temperature within one or two degrees of the melting-point is usually clear, the product obtained with stronger cooling is milky in appearance on account of the inclusion of minute bubbles of air which was previously in solution.

The vapour pressure of supercooled water is always greater than that of ice at the same temperature. This is evident from the data in the table

Vapour pressure of supercooled water

 Temperature. Vapour Pressure in mm. Hg. Ice. Water. 0 4.579 4.579 -2 3.885 3.958 -4 3.288 3.418 -6 2.776 2.942 -8 2.337 2.525 -10 1.963 2.160 -12 1.644 1.843 -14 1.373 1.568 -16 1.253 1.331

 Vapour pressure of supercooled water.
This is shown graphically in fig. 44, the broken line indicating the vapour pressure of the supercooled water, and the continuous lines the pressures of liquid water above 0° C. and of ice I below 0° C. As already explained, in the absence of air and in presence of water-vapour only, T represents a triple point, and lies at +0.0076° C. A slight break occurs at T between curves LT and TS, but TC is a continuation of LT.

It will now be evident why TC represents a meta-stable condition of water. If a piece of ice is introduced into the same closed vessel, the vapour is supersaturated with regard to the ice, and a portion condenses. But this leads to a vapour unsaturated with respect to the supercooled liquid, which, in consequence, vaporises to a corresponding amount. This condensation on the ice and vaporisation of the liquid continues until the whole of the latter has disappeared, leaving only ice and vapour.

Physical Properties of Water related with heat conductivity. Water is generally regarded as a poor conductor of heat, although, compared with other non-metallic liquids, its conductivity is high. The thermal conductivity, K, is defined as the number of units of heat (gram calories) which will pass by conduction across unit area (sq. cm.) in unit time (second) with unit temperature gradient (1° C. per cm.). The following values for K have been obtained.

The value for K at any temperature between 7.4° and 72.6° C. may be calculated from the equation

K = 0.001325(1 + 0.002984t).

This poor conductivity manifested by water plays an important part in nature's economics.

Livingstone mentions that the temperature of the surface water of ponds in the central regions of Southern Africa may reach as high as 38° C., but, owing to the poor conductivity of heat, " deliciously cool water may be obtained by anyone walking into the middle and lifting up the water from the bottom."

A study of the specific heat of water is particularly important, since the unit of heat or gram calorie is the amount of heat required to raise 1 gram of water through 1 degree centigrade. Sometimes the gram calorie at 15° C. (calorie 15° C.) is chosen, sometimes that at 20° C. (calorie 20° C.), whilst at other times the mean value between 0° and 100° C. is adopted. These units are not identical, but the variation is small. As water, owing partly to its abundance, and partly also to the ease with which it can be obtained in a pure condition, is the standard substance for the measurement of heat quantities, it is important to determine with the utmost accuracy the variation of its specific heat with the temperature. Numerous investigations have been carried out with this object in view, very reliable data being those of Callender and Barnes. According to Callender, the specific heat of water, Qt, in terms of the calorie at 20° C., is given by the expression

Qt = 0.98536 + 0.504/(t + 20) + 0.000084t + 00000009t2

for any temperature between t = 0° and t = 100° C.

According to Narbutt, the specific heat of water for a temperature range of 0° to 100° C. may be calculated from the following empirical formula:

Specific heat = 1.00733 – 0.0007416(t - 15) + 0.000016845(t - 15)2 - 0.00000009552(2 - 15 )3

which gives values in very close agreement with the best experimental data.

The specific heat of water is abnormally high, and this fact has an enormous influence upon climate and geological phenomena. Liquid ammonia is the only liquid possessing a higher specific heat.

In the following table are the data obtained by Callender and Barnes

Specific heat of water

 Temperature, °C. Specific Heat. Temperature, °C. Specific Heat. Temperature, °C. Specific Heat. 0 1.0094 15 1.0011 30 0.9987 1 1.0085 16 1.0009 35 0.9983 2 1.0076 17 1.0007 40 0.9982 3 1.0068 18 1.0004 45 0.9984 4 1.0060 19 1.0002 50 0.9987 5 1.0054 20 1.0000 55 0.9993 6 1.0048 21 0.9999 60 1.0000 7 1.0042 22 0.9997 65 1.0008 8 1.0037 23 0.9995 70 1.0016 9 1.0032 24 0.9994 75 1.0024 10 1.0027 25 0.9992 80 1.0033 11 1.0023 26 0.9991 85 1.0043 12 1.0020 27 0.9990 90 1.0053 13 1.0017 28 0.9989 95 1.0063 14 1.0014 29 0.9988 100 1.0074

It will be observed that the gram calorie at 20° C. equals that at 60° C., whilst the mean value between 0° and 100° C. is 1.0016. A minimum value occurs at 40° C. This fluctuation in specific heat at different temperatures is usually attributed to the influence of de-polymerisation as the temperature rises. For supercooled water at -5° C. the value 1.0158 for the specific heat has been obtained in terms of the calorie at 16° C.

As the caloric is inconveniently small for some purposes, a larger unit, the Calorie, is sometimes used equal to 1000 smaller calories, the terms being frequently abbreviated into "cal." and "Cals." respectively; occasionally a unit equal to 100 calories and described as a Kalorie (Kal. or K.) is used.

The heat of formation of water from gaseous hydrogen and oxygen at 18° C. is 68.38 Calories 15° C.

The surface tension of water, like that of all liquids, diminishes with rise of temperature. A ready method of illustrating this consists in pouring water into a shallowr, clean metal plate held horizontally until a thin layer is formed. The surface is now dusted over with flowers of sulphur, and heat applied locally to a point near the centre of the under surface of the plate by means of a fine gas jet. As soon as the heat reaches the water, the sulphur is rapidly pulled away towards the circumference of the plate in consequence of the reduction in the surface tension of the warmed central liquid.

The surface tension σt at t° C. is practically a linear function of the temperature, and may be calculated according to the equation

σt = σ0(1 – αt) (1)

where σ0 is the surface tension at 0° C. and a is a constant.

The value for a in the case of water has been repeatedly determined whithin the range 0.0018-0.0023, and a mean value of 0.0020 is probably fairly accurate. Forch suggests the equation

σt = σ0(1 – 0.00190179t – 0.0000024991t2).

at for water has been determined in a variety of ways by different investigators, the results ranging from 70.6 to 78 dynes per cm. at 20° C.

The results obtained by Ramsay and Shields in their classical research, in which water was in contact with its vapour and the walls of the capillary tube only.

Whilst the foregoing results are relatively correct, it appears almost certain that their absolute values are all somewhat too low. The method adopted by Ramsay and Young consisted in measuring the difference in level between water in a capillary tube open at both ends and suspended in a wider glass tube. Richards and Coombs, however, have shown that the diameter of the wider tube must be considerably greater than has hitherto been realised if the disturbing influence of the walls is to be neglected. Thus, in a tube of diameter 2.54 ems. the water was found to lie 0.11 mm. above that in one of 3.8 mm. diameter. Experiments showed that this latter is about the limiting diameter, the effect of the surface tension under these conditions being reduced well below that of the other errors of experiment.

 Temperature, ° C. σt dynes/cm. σt (Mv)2/3 x×M 0 73.21 502.9 3.81×18 10 71.94 494.2 3.68 20 70.60 485.3 3.55 30 69.10 476.1 3.44 40 67.50 466.3 3.18 60 64.27 446.2 3.00 80 60.84 425.3 2.83 100 57.15 403.5 2.66 120 53.30 380.7 2.47 140 49.42 357.0 2.32

Taking this and several other possible sources of error into consideration, the mean value for the surface tension of water was determined experimentally as equal to 72.62 dynes per cm. at 20.00° C7

As a general rule, the presence of dissolved inorganic salts enhances the value in accordance with the simple mathematical expression

σs = σw + Rn

where σs is the surface tension of the solution, σw that of water at the same temperature, n the number of gram equivalents of the salt per litre, and R a constant, depending upon the chemical nature of the dissolved salt. For the undermentioned salts, R has the following values:

 Salt. r. NaCl 1.53 KCl 1.71 ½Na2CO3 2.00 ½K2CO3 1.77 ½ZnSO4 1.86

Organic salts, on the other hand, frequently reduce the surface tension of the solution. The oleates are cases in point.

Most other liquids, except mercury, have a lower surface tension than water. This is illustrated by the following data:

 Mercury Water Carbon disulphide Ethyl alcohol Ether Olive oil. Benzene Surface tension 547 72.6 33.6 22.0 16.5 32 29.2 Temperature, °C. 17.5 20 19.4 20 20 20 17.5

Dissolved gases tend to raise the surface tension. The following values obtained at 15° C. illustrate this:

 In vacuo Hydrogen Nitrogen Carbon monoxide Carbon dioxide Air Surface tension 71.3 72.83 73.00 73.00 72.85 73.1

Water is an excellent solvent, dissolving not only many acids, bases, and salts, but also many organic compounds, especially such as contain hydroxyl and amino-groups. The solubility of solids and liquids generally increases with rise in temperature, whilst gases, all of which are soluble in water to some extent, are invariably less soluble at higher temperatures. The presence of dissolved solids causes a depression of the freezing- and melting-point of water to an amount proportional to the concentration. This is known as Blagden's Law, and is only true provided the solutions are dilute. The extent of the depression for un-ionised substances in dilute solution is such that proportionately one gram molecular weight of the solute in 100 -grams of water would cause a depression of 18.5° C. - the molecular depression.

The molecular elevation of the boiling-point is calculated in a similar manner, and has the value 5.2° C.

The dielectric constant of water is 81.7 at the ordinary temperature. This value is a high one when compared with the same constant for other liquids and it is probably on account of its considerable dielectric power that solutions of bases, acids, and salts in water can conduct the electric current, this conduction being dependent on the electrolytic dissociation of the solute. In aqueous solution, however, some organic substances are partly associated to double or even more complex molecules.

Purified water does not appreciably conduct the electric current, so that the conductivity of a sample of water can be used as an indication of freedom from saline impurities.

The purest water hitherto obtained possessed an electrical conductivity of 0.04×10-10 mhos at 18° C, the increase in the value with rise of temperature being represented by a coefficient of 0.0532 per degree. This coefficient only holds for water of a high degree of purity, such as has not even been exposed to the atmosphere, because slight impurities have a relatively inordinate effect on the conductivity and possess a much lower coefficient of increase with temperature, namely, of the order of 0.021. Pure water, therefore, probably possesses a very slight but definite electrical conductivity which is to be attributed to the existence of a minute proportion of the water in an ionised or electrolytically dissociated condition, that is as H (positive) and OH' (negative) ions. As the conductivity due to the gram-equivalent weight of these ions in a definite volume can be calculated from the electrical conductivity of acids and alkalies respectively in dilute solution, it is possible to evaluate the proportion of pure water which is ionised at a definite temperature, the result is 6.3×10-10 grams per c.c. at 0°, the corresponding dissociation constant at 18° being 0.8×10-7. The probability of the correctness of this reasoning is borne out by the approximate agreement of the above value for the extent of dissociation with values obtained by quite different physico-chemical methods, such as the hydrolytic action of water on ammonium acetate, etc. In the presence of dissolved electrolytes the dissociation of water is still further depressed. The electrical conductivity of solutions of water in other solvents bears no simple relation to that of the pure liquid, and, indeed, varies widely according to the solvent.

The electrolytic dissociation of water increases with rise of temperature. This is unusual, but is commonly explained on the assumption that most water molecules at ordinary temperatures are polymerised, thus (H2O)x, the value for x being mainly 2, and that the existing ionisation is that of single molecules H2O. As the temperature rises the proportion of these increases through depolymerisation of the complex molecules, so that whilst the actual percentage of single molecules converted into ions may be reduced, the total number of ions is greater.

In contact with ordinary fresh air, the conductivity of water is 0.7 to 0.8×10-6 mho, the rise being due mainly to the dissolved carbon di-oxide.

Physical Properties of Water interaction with light. The absorption spectrum of water was studied by Soret and Sarasin7 in 1884, who passed a beam of light through 2.2 metres of water. A faint and narrow band was observed in the orange at a wave length of approximately 6000. This band became slightly more distinct and a general absorption of the extreme red was noticed as the thickness of the water layer was increased from 3.3 to 4.5 metres.

In small quantities water appears colourless, but in deep layers it is possessed of a bluish tinge, which tends to become greenish as the temperature is raised.

The cause of the coloured appearance of natural waters has been the subject of considerable discussion. The light blue hue of water that has been softened by Clark's process has frequently been commented upon, and points to the suggestion that the colour is due to the scattering of light by suspended particles. Threlfall found that the following solution, when viewed through a tube 18 cm. in length, matched with considerable precision the colour of a sample of water from the Mediterranean:

500 c.c. distilled water.

0.001 gr. soluble Prussian blue.

5 c.c. saturated lime water just precipitated by the smallest excess of sodium hydrogen carbonate.

Lord Rayleigh attributes the blue colour of the sea to that of the sky, seen by reflection.

According to Aitkin the Mediterranean sea owes its colour to minute suspended particles which reflect rays of all colours, whilst the water, by virtue of selective absorption, allows only the blue rays to escape. The solid particles thus determine the brilliancy of the colour, whilst the selective absorption by the water determines the colour itself. The green colour, so frequently noticed in the sea, is attributed to the presence of yellowish particles in suspension. On the other hand, the green colours of such lakes as Constance and Neuchatel are ascribed by Spring to the mixing of the natural blue of the waters with yellow produced by the presence of finely divided particles of suspended matter, which latter may themselves be quite colourless. Sometimes lakes, normally greenish in colour, become temporarily colourless. This is attributed to the presence of fine reddish mud, containing oxide of iron, which counteracts the green.

Threlfall suggests that the greenish colour of the sea off the coast of Western Australia may be due to the presence of traces of organic colouring matter dissolved out of living or dead seaweed. Buchanan attributes the green colour of Antarctic waters to diatoms and the excretions of minute animals, whilst the sea water at Mogador (Morocco) and off Valparaiso and San Francisco are believed to be coloured green by chlorophyll.

Perfectly pure water is almost a chemical impossibility, inasmuch as contact with any containing vessel must lead to contamination. Even optically pure water is difficult of attainment; it cannot be prepared by mere filtration or distillation. Hartley showed that water, obtained by distillation from acid permanganate solution and subsequent redistillation from a copper vessel in a hydrogen atmosphere, is not optically void. Tyndall obtained optically pure water by melting clear block ice in a vacuum. It showed a blue tinge when examined in a three-foot layer.

Now, if pure water is coloured slightly blue, as is generally conceded, the effect cannot be due to either of the foregoing causes, namely, the

presence of suspended or dissolved materials, which are known to accentuate, if not to be the sole cause of, the colours of natural waters. It must be due to a pure absorption effect of the water itself. The relation between colour and constitution is by no means clear, but the suggestion has been made that the greenish tinge acquired on raising the temperature is a consequence of the polymerisation of the water molecules, the polymerised molecules being bluish in colour, whilst the single monohydrols are green. This receives support from the fact that solutions of colourless salts, which may be expected to contain fewer polymerised molecules, are more green than pure water at the same temperature.

The brown colours of natural waters are sometimes due to ferrugineous suspensions, but in many cases are attributable to colloidal organic matter. Typical waters of this class occur in the uplands of Lancashire and Yorkshire. The colloid is usually electronegative in character, exhibiting electrophoresis in the direction of the anode, and being precipitable by positively charged ions and colloids, and by electrical treatment.

The refractive index for sodium light at different temperatures is as follows:

 Temperature, ° C. Refractive Index. Referred to Air at same Temperature. Referred to Vacuum. 20 1 33299 1.33335 25 1.33248 1.33284 30 1.33190 1.33225

Variation of the refractive index, n, with temperature, t, is given by the expression

n = 1.33401 - 10-7(66t + 26.2t2 - 0.1817t3 + 0.000755t4).

For the iron E line at 15° C. the refractive index is 1.335636.

The taste of water is largely dependent on its freedom or otherwise from dissolved foreign matter, especially carbon dioxide; with pure distilled water the taste is distinctly flat and unpalatable.

The magnetic susceptibility of water has frequently been determined, the value for K×106 at 20° C. being 0.7029 with a temperature coefficient of 0.00013. According to Stearns, it lies between 0.715 and 0.750 at 22° C. The dielectric constant of water at 18° C. is 81.05.

Cl in 4NHT
Cl in 4NO7
Cl in 4NML
Cl in 4NN3
Cl in 4NMW
Cl in 4NM7
Cl in 4NM5
Cl in 4NM3
Cl in 4NM0
Cl in 4NHQ