Maximum density, of water

For scientific purposes the convenient unit to employ for measuring reasonably large volumes of liquids is the cubic decimetre (dm3), or, for smaller volumes, the cubic centimetre (cm3). For many years the fundamental unit employed was the litre, based upon the volume occupied by one kilogram of water at 4 °C (the temperature of maximum density of water) the relationship between the litre... 

G. Tammann 11 found the vapour pressure of water is lowered by the soln. of ammonium bromide 2 6 mm. for 14 5 grms. of salt per 700 grms. of water 12 8 mm. for 60 82 grms. of the salt. L. C. de Coppet found the temp, of maximum density of water is lowered 8 7° per mol. per litre. 

If the supply of heat is continued, further slight contraction ensues, but accompanied by rise of temperature until 3 98° C. is reached.1 This is the point of maximum density of water, further rise m temperature now resulting in expansion until the boiling-point is attained. 

Dissolved salts depress the temperature of the maximum density of water, the depression being directly proportional to the concentration of the salt.5 The depression caused by a highly iomsed binary electrolyte, for example, sodium ehlonde, 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 arc known.6... 

In the case of water-TMS mixtures there is evidence in literature that TMS breaks down water structure in water-rich mixtures. This has been proved by studies concerning the influence of small additions of TMS on the temperature of the maximum density of water (27), the heat of mixing, and the vapor pressure of water-TMS mixtures (28). Figure 1... 

D. The maximum density of water is found in the liquid state. 

Despretz law. States that the temperature of maximum density of water is lowered from 4C on the addition of a solute by an amount proportional to the concentration of the solution. 

McBain and Kam point out that it is well known that there is a general qualitative similarity between the effect of neutral salts on such various phenomena as solubility of gases and non-electrolytes, surface tension, compressibility, maximum density of water, viscosity, dielectric capacity, imbibition and gelatimsation of gels, and increase or decrease of rate of catalysis ... 

There are several additional interesting features of the phase diagram for water. Note that the solid/liquid boundary line has a negative slope. This means that the melting point of water decreases as the external pressure increases. This behavior, which is opposite to that observed for most substances, occurs because the density of ice is less than that of liquid water at the melting point. (The maximum density of water occurs at 4°C.) Thus, when liquid water freezes, its volume increases. Also note that the solid/liquid line has no upper limit. It extends indefinitely, as indicated by the arrow in Figure 16.55. 

Detailed numerical calculations indeed justify the above simple logic. Thus, the maximum density of water at 4°C is not too much of a mystery. However, explanations of other anomalies are not that simple. 

Figure 11.13 Plot of density versus temperature for liquid water. The maximum density of water is reached at 4°C. The density of ice at 0°C is about 0.92 g/cm. ...

The maximum density of water is 1.000 g/mL at 4°C. Water has the unusual property of contracting in volume as it is cooled to 4°C and then expanding when cooled from 4°C to 0°C. Therefore, 1 g of water occupies a volume greater than 1 mL at all temperatures except 4°C. Although most liquids contract in volume all the way down to the point at which they solidify, a large increase (about 9%) in volume occurs when water changes from a liquid at 0°C to a solid (ice) at 0°C. The density of ice at 0°C is 0.917 g/mL, which means that ice, being less dense than water, will float in water. 

The list of properties that can (and have been) used to gauge water models is quite long. In addition to those just mentioned, there are many thermodynamic properties such as heat capacity, surface tension, free energy, and temperature (or pressure) where the maximum density of water occurs for a specified pressure (or temperature) and various structures for ice. There are also dynamic properties such as viscosity, orientational decay times, and vibrational density of states that can be determined using simulations. 

Most solid and liquid substances have a positive coefficient of volume expansion a. The expression drawn up indicates that the entropy S decreases with increasing pressure p. This indicates that under increased pressure, substances tend towards a state of increased molecular order. Exceptions from the rule that q > 0 can, for example, be seen for water, fn the temperature range 0 — 4 °C it is well-known that an anomaly exists for water with negative coefficient of volume expansion a so that the maximum density of water occurs at 4°C (see the table in Appendix B on Water and water vapour). 

Table D.2 Conversion factors for common quantities [132, 206]. These allow simple conversions from Gaussian cgs units into standard SI units and vice-versa, c == 2.998 X 10 represents the numerical value of the speed of light in SI units. The abbreviations used are poise (P), gauss (G), oersted (Oe), newton (N), pascal second (Pa s), tesla (T), ampere (A), volt (V), coulomb (C), joule (J). This quantity was formerly known as specific gravity it is the density of the material divided by the (maximum) density of water (given by 1 g cm , equivalent to 1000 kg m ).

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