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What is the scientific meaning of the word volume?
▪ the amount of (3-dimensional) space
that a substance or object occupied by
an object.
▪ The SI unit >>> cubic metre (m3)
▪ For most purposes >>> cubic centimetre (cm3)
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Calculating volume
When calculating the volume of a regular block of material,
▪ measure the lengths of the sides by ruler, metre rule &
measuring tape.
▪ measure the radius of the cylinder or sphere by Vernier calipers.
The volume of the block, V = 𝒍 b h (rectangle)
V = πr2h (cylinder)
𝟒
V = πr3 (sphere)
𝟑
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▪ If the shape of the material is irregular,
then we work out the volume using the displacement method.
▪ The object is placed into a measuring cylinder of water.
▪ The increase in volume of the water is the volume of the object.
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2 Which of these has a larger volume?
3 Which of these has a larger mass?
▪ hollow >>> a space filled with air inside them
▪ Solid >>> no space with air inside
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▪ The food containers are made from polystyrene.
▪ They feel very light, because they have a large volume
and a small mass.
▪ The nails are made from iron.
▪ The nails feel heavy because they have a small volume.
▪ The nails are also
▪ The word solid can be used as the opposite of hollow.
One of the polystyrene food containers has a larger mass than one of the nails.
However, the same volume of iron has a larger mass than the same volume of
polystyrene. We use the word density to describe this difference.
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Calculating density
What is density?
Density of a substance is defined as its mass per unit volume.
We can calculate density using the equation:
𝐦𝐚𝐬𝐬
density =
𝐯𝐨𝐥𝐮𝐦𝐞
• SI units >>> kg/m3
• For most purposes >>> g/cm3
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Comparing densities
Gases have lower densities than liquids or solids because the particles in a
gas are far apart.
As particles in a material get packed closer together, the density of the
material increases. So, the materials with the highest densities are all
solids.
For the elements, the density usually increases as the atomic number
increases.
The substance with the highest density in the universe is found in
collapsed stars.
neutronium >>> [1.0 x 1014 g/cm3]
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Floating and sinking
You can tell whether something will float or sink in water by comparing
its density with that of water.
• If something is more dense than water, it will sink in water.
• If something is less dense than water, it will float in water.
Q. How do the ship float in water?
▪ Ships are not solid blocks of steel. There are large spaces inside ships
that contain only air.
▪ That means the ship has a very large volume, so the average density
of the whole ship is less than the density of water.
-The density of this ship has become higher than the density of water.
The ship is now sitting on the bottom of the river.
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Liquids of different density
▪ When liquids are added together carefully, the less dense liquids will
float on the more dense liquids.
▪ The liquids will form separate layers.
In this container, honey is the liquid with the highest density and vegetable
oil is the liquid with the lowest density.
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Gases and liquids
▪ Gases are less dense than liquids because the
particles in a gas are much further apart than in a
liquid.
▪ Hydrogen gas is even less dense than helium, but
is not used to fill balloons because hydrogen forms
an explosive mixture with air.
Gases of different density
Helium gas is one of the least dense gases.
▪ A balloon filled with helium will float in air because the balloon and
gas is less dense than the air.
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▪ Solids and liquids are very difficult to compress because their particles
are already in contact.
▪ Gases are easy to compress because their particles are far apart.
▪ When a gas is compressed, the same number of particles is in a
smaller volume, so the density of the gas increases.
▪ If a gas expands, the same number of particles is in a larger volume,
so the density of the gas decreases.
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▪ Heat causes a gas to expand.
▪ That explains why a hot air balloon can float because the hot air
inside the balloon is less dense than the colder air outside.
▪ The hot air inside the balloon has expanded and become less dense
than the colder air outside.
