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Physical Properties

At ordinary temperatures, pure water is a tasteless and odo(u)rless liquid ; it is colo(u)rless in moderately thin layers, but appears greenish-blue when viewed in thick layers. According to J. Aitken, the blue colo(u)r of large bodies of water – e.g., in china clay setting pits, in tanks in which water is being softened by the addition of milk of lime, etc. – is an optical effect due to the action of the fine particles suspended in the liquid on the light.

Liquids are but slightly compressible. If 1000 c.c. of water be subjected to a pressure of two atmospheres the volume will be reduced 0.05 c.c. According to P. G. Tait, this very small compressibility means that if sea-water were quite incompressible, the average level of the sea would be raised 116 feet higher than it is to-day, and 4 per cent of the present land surface would be submerged.

Non-metallic liquids are bad conductors of heat ; water is one of the best of liquids for conducting heat (mercury excepted), but even then, the thermal conductivity is small. Witness a piece of weighted ice at the bottom of a test-tube of cold water. If the test-tube be held obliquely, and heated by a Bunsen burner near the surface, the water at the surface will boil, but the ice at the bottom will remain un-melted.

Water boils at 100° under 760mm. pressure. The greater the pressure, the higher the boiling point ; and conversely, the less the pressure, *(*Roughly about 1/27 °C per mm. for a few degrees above and below 100°) the lower the boiling point. These phenomena occur with liquids generally, and it is therefore necessary to state the pressure when giving the boiling point of a liquid. If no pressure is stated “760mm.” is understood. Thus at Quito (9350 feet above sea-level), with the barometer at an average height, 525.4 mm., water boils at 90.1° ; and on the top of Mount Everest (29,002 feet), barometer at 255.3mm., water would boil at 72°. Steam or water vapo(u)r is an invisible, colo(u)rless gas which condenses to a visible cloud of small particles when it comes in contact with the atmosphere. This is readily shown by boiling water in a flask ; inside the flask, the vapo(u)r is invisible, and a cloud a minute water particles -condensed steam- appears where the steam comes in contact with the cold air.

Liquid water freezes at 0° into crystalline ice. Water vapo(u)r freezes into hoar frost and snow. The crystals of ice are extremely rare and difficult to measure. The crystals can often be seen when a piece of ice is examined with a lens while a beam of bright light is passed through it. Snow crystals are common. They appear in the form of an hexagonal (six-sided) nucleus or six-rayed star with the rays developed in bewildering complexity. The crystals are of inimitable delicacy and beauty. No two seem alike ; but all are fashioned after one definite type – the six-rayed star. Ice appears to be colo(u)rless or white when pure, but it is pale blue when seen in large masses.

By plotting the volume of a given mass of water at different temperatures, we get a curve similar to that illustrated. This curve shows that, at temperatures above 4°, water, like most liquids, expands when heated and contracts when cooled down to 4° ; but the curve below 4° is abnormal. It shows that water expands when cooled below, and contracts when heated up to 4°. If the specific gravity of water at 4° be taken as unity, it follows that water becomes specifically lighter when the temperatures is raised or lowered beyond this point. The expansion of water when cooled from 4° to 0° is very small, but that minute quantity has a very important bearing in nature. When the water on the surface of, say, a lake is cooled, it contracts. The heavier cold water sinks, and the warm water rises. This circulation cools the temperature of the whole body of water down to 4° ; any further cooling results in the formation of specifically lighter water.

Accordingly, this remains on the surface, and circulation ceases. Finally, as a result of this remarkable and abnormal property, when the temperature of the atmosphere falls to 0°, a surface film of ice is formed.* (* “Ground ice” or “anchor ice” is formed at the bottom of rapidly movin streams when the water is thoroughly mixed and does not settle in layers).

If the water did not expand in this way, as the temperature fell to 0°, the whole body of water would freeze from below upwards and produce profound climatic changes, since the larger amount of ice formed in winter would materially affect the temperature for the rest of the year. These remarks do not apply to sea-water which contracts as the temperature is lowered down to the freezing point. In the act of freezing water expands so that 100 c.c. of liquid water at 0° gives approximately 110 c.c. of ice at the same temperature. The specific gravity of ice at 0° varies with its mode of formation from 0.9159 to 0.9182 ; the specific gravity of water at 0° is 0.999867.

Accordingly, ice float on the surface of water. The expansion of water during freezing is an important factor. The expansion may burst the intercellular tissue of plants by freezing the sell-sap ; the expansion may disrupt the fibres of flesh, so that frozen meat appears rather more “pulpy” than ordinary meat. If water freezes in pipes, the expansion of water in the act of freezing may burst the pipe, and water will “leak” when the ice “thaws” ; water freezing in the surface crevices of rocks splits and widens the fissures so that the surface crust of the rock appears to disintegrate during a “thaw.” The debris collects as “talus” at the foot of the rocks, ready to be transported by water to lower levels. Hence this simple force plays an important part in the weathering and decay of rocks, building stones, etc., in countries exposed to alternate frost and thaw ; and J. Tyndall adds: “The records of geology are mainly the history of the work of water.”

The electrical properties of water. It is a poor conductor of electricity, having a specific conductivity at 25° C. of only 0.04 * 10^-6 mhos. It is, however, notable in being a good ionising solvent, that is to say, compounds like common salt and hydrochloric acid, i.e., polar compounds or compounds containing an electro-valency, dissolve in it, giving conducting solutions in which the solute has become largely dissociate into ions. Water is not the only ionising solvent known, but it is about the best.

It has been observed that solvents whose dielectric constant (or specific inductive capacity) are highest are the best ionizing solvents, and this is in agreement with what would be expected on the basis of the electronic theory of valency, according to which polar compounds, such as sodium chloride, are united by the electrical forces between the ions.

Chemical Properties

The chemical properties of water can be classified broadly under three main headings,

i.                     Reactions in which water undergoes decomposition.

ii.                    Reactions in which water acts as a catalyst.

iii.                  Reactions in which water forms addition compounds.

Decomposition Reactions:

The combination of hydrogen and oxygen to form water is attended with the evolution of a large quantity of heat, as indicated by the equation:

2H₂ + O₂ = 2H₂O + 116.2 Cals.,

the water formed remaining as steam. It therefore follows, by the application of the principle of Le Chatelier, that if the reaction is to any extent reversible, the dissociation of the steam into hydrogen and oxygen will occur most at high temperatures. Investigation shows that dissociation does in fact take place, but it is difficult to carry out such an experiment since the decomposition only takes place at high temperatures, and recombination occurs very rapidly when the temperature is lowered, so that the formation of oxygen and hydrogen is not easily detected. Table bellow gives some values of the dissociation at various temperatures.

Table – Dissociation of Water

Many elements, which are higher in the electro-chemical series than hydrogen will decompose water at a suitable temperature. The alkali metals (sodium, potassium, etc.) attack water readily at the ordinary temperature:

2Na + 2H₂O = 2NaOH + H₂

and the alkaline earth metals (calcium, strontium, etc.) behave similarly:

Ca + 2H₂O = Ca(OH)₂ + H₂

Magnesium, however, is only slightly affected by cold water, but reacts fairly readily with hot water. Magnesium, zinc and iron readily react with steam. Aluminium does not react with water in ordinary circumstances, since it is protected by a surface film of oxide, but if it is removed and some means adopted for preventing its re-formation, aluminium will decompose water in the cold. This can be effected by amalgamation with mercury.

Non-metals for the most part do not react with water ; the exceptions being carbon and silicon, and fluorine and chlorine. Carbon at a red heat decomposes steam, forming water-gas and silicon reacts similarly, but much more slowly:

C + H₂O = CO + H₂

Si + 2H₂O = SiO₂ + 2H₂

Chlorine, when passed into water, first dissolves forming a green solution, but, on standing, reacts forming hydrochloric and hypochlorous acids :

Cl₂ + H₂O = HCl + HOCl,

but in sunlight the latter acid breaks up with the formation of oxygen:

2HOCl = 2HCl + O₂

Fluorine acts somewhat similarly, but no intermediate stage can be detected, and hydrofluoric acid and oxygen (mixed with ozone) are obtained in all circumstances:

2F₂ + 2H₂O = 2H₂F₂ + O₂

Reactions involving water, but in which hydrogen is not involved are, of course, very numerous. Among these mention might now be made of hydrolytic reactions, that is to say, the decomposition of a compound by means of water. Examples are the hydrolysis of the halides of non-metals, such as the phosphorus halides:

PCl₃ + 3H₂O = P(OH)₃ + 3HCl,

in which the halogen is removed as its hydracid and the corresponding hydroxy derivative of the non-metal is formed. A similar reaction occurs with substances such as sulphuryl chloride (which may be regarded as the chloride of the “group” SO₂ – a group possessing non-metallic properties):

SO₂Cl₂ + 2H₂O = SO₂(OH)₂ + 2HCl

SO₂(OH)₂ otherwise H₂SO₄ is, of course, sulphuric acid.

Salts which are derived from acids and bases of markedly different strengths are generally hydrolysed in aqueous solution.

In some cases a basic salt is formed, for example bismuth trichloride is hydrolysed to bismuth oxychloride,

BiCl₃ + H₂O = BiOCl + 2HCl

Source: Mellor’s Modern Inorganic Chemistry, Revised and Edited by G. D. Parkes, M. A., D. Phil., Fellow of Keble College, Oxford. In collaboration with J. W. Mellor, D. Sc. With diagrams and illustrations. Longmans, Green and Co. London – New York – Toronto.

Other pages for distillation:

Yield (extract) essential oil by steam distillation in atmospheric pressure.

Purification of Water for Scientific Purposes.

Properties of Water.

Application of the Kinetic Theory to Liquids, Equilibrium and Vapo(u)r Pressure of Liquid.

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Last Update February 19^th, 2003.

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