What is a relative density?
The relative density of a substance is the number of times that a certain volume of the substance is heavier than the same volume of water. As density is mass per unit volume, it follows that relative density is equal to the density of the substance divided by the density of water, which can be written:
Relative Density = Density of Substance / Density of water
Consider clean coal. Its density is about 1.3 gram per cubic centimeter. The density of water is 1 gram per cubic centimeter. The relative density of coal is therefore: Density of Clean Coal / Density of water
Or 1.3 grams per cubit centimeters / gram per cubit centimeter
Or 1.3
Notice that the relative density has the same numerical value as the density expressed in grams per cubic centimeters but that the relative density is given is given a number only, and not by units such as g/m3.
The relative density of shale is similarly: Density of Shale / Density of Water
Or 2.4 grams per cubit centimeters / 1 gram per cubit centimeter
Or 2.4
As it says “How many times more dense,” relative density has no units. The relative density of water is the density of water divided by the density of water. So the answer is 1.
Lesson 3 Fig. 1 shows pieces of coal, middlings and shale each of the same mass. Notice how the volume of the piece diminishes as the relative density increases.
We have shown that the relative density of water, coal and shale is equal to the density when this density is measured in metric units.
Density of coal = 1.3 g/m3
Relative Density of coal = 1.3
Since coal is 1.3 times heavier than its own volume of water, coal sinks in water. We shall come back to this point later.
No units for relative density
It has already been said that relative density is the number of times a substance is more dense than water. It is a ratio of two densities and has NO UNITS.
Relative Density of water = 1.0
Relative Density of coal = 1.3
Relative Density of shale = 2.4
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