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4.3 That’s heavy!
Which is heavier, 1 kg Density
of hay or 1 kg of iron We learnt in Units 2 and 3 that mass is the amount of matter a substance
nails? Which has the or object has. We also learnt that volume is the amount of space a
greater volume? Why substance or object occupies. If we take the ratio of how much matter of
textbooks nis edu kz
do you think so? a specific substance (mass) we can put into a specific space (volume) we
can start to understand a very important property called density.
To calculate the density of a substance or object we divide its mass by
its volume.
mass
density = =
volume
The density of a substance/object is represented by the Greek letter
(‘rho’). In the SI system, density is measured in kilograms per cubic metre
kg 3
( ) . If volume is measured in cm and mass is measured in g, it is
m 3 g
convenient to use grams per cubic centimetre ( ) .
cm 3
Density of gases, liquids and solids
The way we calculate density is exactly the same whether the
substance/object is a gas, a liquid or a solid. But when pure substances
change their state of matter, their density changes . This is because their
molecular arrangement changes, so the amount of matter (mass) will
occupy a different volume.
For example, the same mass of solid iron will occupy a bigger volume
when it is in liquid form. This means that when solids become liquids
their density changes, and when liquids become gases their density
changes again! This is not because their mass changes but because
the volume (space) they occupy increases. This means that the density
of one substance as a gas is lower than its density as a liquid, which is
lower than its density as a solid.
EXCEPTION TO THE RULE!
One of the few known exceptions is water. When the temperature
of water lowers to zero (and below) it freezes and becomes a solid
we call ice. Water molecules rearrange and its density changes.
Incredibly, this happens in such a way that the density of ice (solid
water) is lower than the density of liquid water. That is why ice floats
on the surface of water. The density of pure water is 1000 kg/m and
3
the density of pure ice is around 900 kg/m .
3
Spring ice drift on the Amur river in Khabarovsk, Russia.
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