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Introducing density
We have used the term ‘density’ to describe how closely the particles in a substance are packed together; for example, hot air is less dense than cold air because the particles are more spread out. We can extend this idea to compare the densities of different substances by considering how much mass is packed into each cubic centimetre.
A technical definition of density is: the mass of a 1-centimetre cube of a substance.
By keeping the unit volume the same and measuring the mass, the density of different substances can be compared. The image below compares the density of several different substances:
Calculating Density
To calculate the density of an object we need to know two things:
- its mass
- its volume
Density = mass / volume
Density calculations are relatively straight forward for objects that are cubes or prisms as the volume can easily be determine (LxBxH).
If the object is irregular in shape (ie: a rock) the volume can be determined using a measuring cylinder. The measuring cylinder is filled with enough water to submerge the object. The initial and final volumes can be used to calculate the volume of the object:
Floating and sinking
An object will float if its density (or average density) is less than that of the liquid it is in. Pure water has a density of 1 g/cm3. This means that for objects placed in water, their density must be less than 1 if they are to float. Ice is less dense than water, so it will float. Steel is more dense than water, so it will sink in water.
Salt water is more dense than fresh water, making it easier to float in the ocean than in a freshwater lake or river. An egg will sink in fresh water, because its average density is just greater than that of the water. Mixing some salt in the water increases the density of the liquid to just greater than that of the egg. The egg’s density is now less than the liquid’s, and the egg floats.
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