Question

how can you prove expansion of liquid expads on heating

Answer 1

If equal volumes of a liquid, a solid, and a gas are heated through the same rise in temperature, it is generally found that the liquid expands more than the solid but less than the gas. Measuring exactly how much a liquid expands with a given rise in temperature is not a completely straight forward operation. The trouble is that a liquid must have a container and the container expands when the liquid is heated. The increase in capacity of the container masks the true expansion of the liquid. For example, imagine a bottle completely full of some ideal liquid that does not expand at all. As the bottle and its contents are heated the bottle expands so that the original quantity of liquid is no longer sufficient to fill it completely. In this example the liquid would appear to have contracted. In practice the true expansion of a liquid is calculated by adding the increase in volume of the container to the apparent (or measured) expansion of the liquid.

 

One of the chief reasons why mercury is used for filling thermometers is that it expands regularly, i.e. it expands as much when its temperature rises from 1°C to 2°C as it does when its temperature rises from 101°C to 102°C.