The density equation and its units, the arrangement of particles in each state, and the required practical measuring the density of regular and irregular objects.
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1.State the equation for density.
density = mass divided by volume
2.What are the units of density?
Kilograms per cubic metre, or grams per cubic centimetre.
3.A block has mass 240 g and volume 60 cm3. Calculate its density.
240 divided by 60 = 4 g/cm3.
4.Rearrange the density equation to find mass.
mass = density x volume
5.Rearrange the density equation to find volume.
volume = mass divided by density
6.Describe the arrangement of particles in a solid.
Closely packed in a regular arrangement, vibrating about fixed positions.
7.Describe the arrangement of particles in a liquid.
Closely packed but randomly arranged, able to flow past one another.
8.Describe the arrangement of particles in a gas.
Far apart, randomly arranged, moving quickly in all directions.
9.Why are solids and liquids much denser than gases?
Their particles are packed closely together, so far more mass occupies the same volume.
10.Why is a change of state a physical change?
The particles themselves are unchanged. Only their arrangement and energy change, so the material can return to its original form.
11.What happens to mass during a change of state?
It stays the same, because no particles are gained or lost.
12.What happens to density when a substance melts?
It usually decreases slightly, because the particles move a little further apart.
13.Why is water unusual?
Ice is less dense than liquid water, because the particles are held slightly further apart in the solid structure, which is why ice floats.
14.How do you find the volume of a regular solid?
Measure its dimensions and calculate, for example length times width times height for a cuboid.
15.How do you find the volume of an irregular solid?
By displacement. Lower it into a eureka can or measuring cylinder of water and measure the volume of water displaced.
16.How do you find the mass of an object in the practical?
Using a balance.
17.How do you find the density of a liquid?
Measure the mass of an empty measuring cylinder, add the liquid, measure again, and divide the mass of liquid by its volume.
18.Why must the mass of the empty container be subtracted?
Otherwise the mass of the container is included, which would make the calculated density too high.
19.Why must the object be fully submerged for displacement?
If part of it is above the water, only part of its volume displaces water and the result is too small.
20.Give a source of error in the displacement method.
Water splashing out or clinging to the object, and difficulty reading the meniscus accurately.
21.What is the volume of 1 cm3 in m3?
1 x 10 to the power minus 6 cubic metres.
22.Why does an object float?
Because it is less dense than the fluid it is placed in.
23.Two blocks have the same volume but different masses. Which is denser?
The heavier one, since density is mass per unit volume.
24.A gas is compressed into half its volume. What happens to its density?
It doubles, because the same mass now occupies half the volume.
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