Gold . Glass (pyrex) . Lead . Similarly, we consider the fractional change in volume, ∆ V V , of a substance for temperature change ∆ T and define the coefficient of volume expansion (or volume expansivity) , as ( .
) Here α V is also a characteristic of the substance but is not strictly a constant. It depends in general on temperature (Fig . ). It is seen that α V becomes constant only at a high temperature.
Fig. . Coefficient of volume expansion of copper as a function of temperature. Table .
gives the values of coefficient of volume expansion of some common substances in the temperature range – ° C. You can see that thermal expansion of these substances (solids and liquids) is rather small, with material, a A a A a (a) Linear expansion (b) Area expansion (c) Volume expansion like pyrex glass and invar (a special iron-nickel alloy) having particularly low values of α V . From this Table we find that the value of α v for alcohol (ethanol) is more than mercury and expands more than mercury for the same rise in temperature. Table .
Values of coefficient of volume expansion for some substances Material ααααα v ( K – ) Aluminium × – Brass × – Iron . × – Paraffin . × – Glass (ordinary) . × – Glass (pyrex) × – Hard rubber .
× – Invar × – Mercury . × – Water . × – Alcohol (ethanol) × – Water exhibits an anomalous behaviour; it contracts on heating between ° C and ° C. The volume of a given amount of water decreases as it is cooled from room temperature, until its temperature reaches ° C, [Fig.
. (a)]. Below ° C, the volume increases, and therefore, the density decreases [Fig. .
(b)]. This means that water has the maximum density at ° C. This property has an important environmental effect: bodies of water, such as lakes and ponds,