The Relation of the Volume of a Gas to Its Temperature

If a gas is allowed to expand with increas­ing -temperature against constant pressure, one observes a very definite relation between the rise in temperature and the increase in volume. The change in volume of a fixed quantity of a gas is equal to 1/273 (one two hundred and seventy-third) of its volume at zero for each degree change in temperature, provided the pressure is constant. Should this uniform change in volume continue during the cooling of a gas to a very low temperature, the gas sample would have no volume at all at —273°, because the concentration in volume vould now be 273/273 of its volume. Could a sub­stance be cooled to —273° the molecules would be motionless. Obviously, no lower temperature than this can exist. This point is therefore called the absolute zeroi However, as all gases liquefy at temperature above —273° no gas can exist at this temperature. Absolute zero is the temperature at which a gas if it had existed, theoretically would have possessed zero volume.

The relation of the volume of a gas to its temperature is stated in the following law: the volume of a given quantity of a gas varies directly with the absolute temperature, pro­vided that there is no change in pressure. The volume which a measured gas sample would occupy at some different tem­perature may be calculated by the application of this law. However, one has to remember that this law does not hold for very low temperatures.

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Physical changes

When physical changes occur, some of the properties of a substance may be altered for a time, but no new substances are formed. The following are the examples or a physical change: 1) the melting of ice and the condensation of steam; 2) the mixing of sugar with water to form a solution; 3) the change observed when a platinum wire1 is heated to redness. In each of these there is a change in properties.

Thus, a liquid is produced from a solid when ice melts, and a gas is changed into a liquid when steam condenses.

When sugar dissolves, it, too, changes from the solid to the liquid state.

The platinum wire changes, when heated, from a silvery lustrous metal that reflects light to one that emits light. But there is no alteration of the fundamental character of any of the substances during these changes.

Sugar dissolved in water retains, its original properties and only forms a mixture from which it is readily obtained again in its crystalline form by allowing the water to evap­orate.

 

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Boyle's Law

The quantity of matter in a given body is generally de­termined by weight, but it is often convenient, when the given body is a gas or a liquid, to measure the quantity of matter indirectly by volume. Volumetric analysis is based on such measurements; and the analysis of gases is nearly always connected by volume measurements. The main advantage of measurement by volume is rapid execution; the main advan­tage of measurement by weight arises from the fact that the result is largely independent of the physical and chemical conditions of the body in question. The weight of a gas is usually so small in comparison with its volume that it is generally possible to determine the quantity of a gas more accurately by volume than by weight.

The volume of a gas is very sensitive to changes of pressure. While investigating the relation between the pres­sure and the volume of a gas, Robert Boyle found that the volume of a gas kept at one uniform temperature varies in­versely as the pressure.

This is Boyle's law. Later on some deviations from Boyle's law have been found.

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Some Physical Properties of Air

The physical properties of air were studied long before its chemical properties. Aristotle in spite of his confused ideas on the nature of gases considered that air was a material substance which possessed weight; another Qrek scientist described some experiments which were made in order to prove that air is a material substance. For instance he said: "If we invert the open end of a ves­sel, having only one opening, in water, the water will not inter; if a hole be made in the upper part of the vessel, wa ter comes in and air comes out. If we placed our hand over the opening we should feel a stream of wind which is mov­ing air."

In 1774 the French scientist Lavoisier made a large num­ber of experiments in order to prove the hypothesis that when metals are heated in air, the increase in weight is due to fixation8 of the air by the metal.

However it was the gifted6 Russian scientist M. V. Lomonosov who had come to the same conclusion in 1756, i.e. eighteen years before Lavoisier.

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Water

Water is a compound substance which consists of two ele­ments hydrogen and oxygen. It can be decomposed by the electric current. By decomposing water two volumes of hy­drogen and one volume of oxygen are obtained. That water is a compound substance may be shown by the reaction be­tween water and some metals. At ordinary temperature, pure water is a tasteless, odourless and colourless liquid. We know that water boils at 100°C under 760 mm pressure. The greater the pressure, the higher the boiling point, the less the pres­sure, the lower the boiling point. Steam or water vapour is an invisible colourless gas that condenses to a visible cloud of small particles when it comes in contact with the atmos­phere.

Liquid water freezes at 0°C into crystalline ice. The ex­pansion of water when it is cooled from 4° to 0° is very small. When the water on the surface cools 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°.

The specific gravity of ice at 0° varies with its mode of formation. The molecules in a liquid are much closer together than it is in the case with gases and they have powerful intermolecular forces. The molecules in the body of the liquid are attracted by the other molecules in all directions.