We said that coal will sink in water because it is more dense than water. When the coal is lowered into water it pushes out (or displaces) its own volume of water. The coal is 1.3 times heavier than its own volume of water and therefore it sinks.
Imagine that the volume of water displaced was originally held up by the rest of the water. As the piece of coal is heavier than the mass of the water displaced, the rest of the water cannot support the coal, which therefore sinks.
A piece of wood is, however, less dense (or of lower relative density) than water and so, if pushed under, the wood displaces its own volume or water which is heavier than the wood itself. Hence, the rest of the water is able to hold up – or keep afloat – this wood, which is lighter than its own volume of water. In fact the wood floats when it has sunk far enough to displace its own mass of water. Less 3 Fig. 2 may help to make this clearer.
Hence we can say in general that if a substance has a lower relative density than water (ie less than 1) it will float whilst if it has a relative density of more than 1 it will sink in water. This is the basis of float and sink separation.
Turn to Test Paper 2 and answer questions 9 – 13.
Relative Density, Ash Content and Calorific Value of Coal
Relative density is an important factor in the separation of coal from shale; it is also a good guide to certain properties of the coal, such as ash content and calorific value. These will be dealt with separately.