Real and apparent expansion of a liquid
Unlike solids, liquids have no fixed length or surface area but always take up the shape of the containing vessel. Therefore, in the case of liquids we are concerned only with volume changes when they are heated. The real (or absolute) expansivity of a liquid is the fraction of its volume by which it expands per kelvin rise in temperature.
Any attempt at direct measurement of the expansion of a liquid is complicated by the fact that the containing vessel itself expands. However, since liquids must always be kept in some kind of vessel, it is just as useful to know the apparent expansion of. a liquid, which is the difference between its real expansion and the expansion of the vessel. The apparent expansivity of a liquid is the fraction of its volume by which the liquid appears to expand per kelvin rise in temperature when heated in an expansible vessel. In this book we shall not be concerned with methods of measuring these expansivities.