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Density – Learning Outcomes
Perform simple calculations to find density.
Measure mass and volume of a variety of
solids and liquids and hence determine
their densities.
Investigate flotation for a variety of solids
and liquids in water and other liquids, and
relate the results to density.
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Density
Which is heavier, iron or paper?
This is not a fair comparison.
We need to compare the same amount
of iron and paper.
We usually compare 1 cm3 of materials,
which makes it much fairer.
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Density
e.g.
1 cm3 of iron has a mass of 7.9 g.
1 cm3 of paper has a mass of
1 cm3 of water has a mass of 1 g.
1 cm3 of lead has a mass of 11.2 g.
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Density
We call these numbers the density.
The density of a substance is its mass per
unit volume. Its unit is 𝑔/𝑐𝑚3 .
Formula: 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 =
𝑚𝑎𝑠𝑠
𝑣𝑜𝑙𝑢𝑚𝑒
also 𝑚𝑎𝑠𝑠 = 𝑑𝑒𝑛𝑠𝑖𝑡𝑦 × 𝑣𝑜𝑙𝑢𝑚𝑒
and 𝑣𝑜𝑙𝑢𝑚𝑒 =
𝑚𝑎𝑠𝑠
𝑑𝑒𝑛𝑠𝑖𝑡𝑦
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Density
e.g. A rectangular block of wood has a
mass of 30 g and a volume of 50 cm3.
What is its density?
e.g. A 178 g piece of copper is dropped
into a graduated cylinder of water. What
volume of water will be displaced if
copper has a density of 8.9 g/cm3?
e.g. Iron has density 11.2 g/cm3. What is
the mass of a rectangular block of iron
with dimensions 4 cm × 2.5 cm × 2 cm?
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Flotation
If an object is less dense than a liquid, it will float.
If an object is more dense than a liquid, it will sink.
If liquids are immiscible (i.e. don’t mix), the less
dense one will float on the other.
Ships carrying heavy cargo can float because of
all the air, making the overall density less than that
of water.
Hot air is less dense than cold air, so balloons blown
up with breath will float until they cool down.
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Flotation