MATTER Level E/F: Teacher’s Science Notes
Matter
Teacher’s Science Notes
Particle Theory
To avoid any confusion between atoms and molecules, teachers should use the term ‘particle’ as a description of the
smallest piece of any particular material for this topic. The physical properties of solids, liquids and gases can be explained
by referring to the arrangement of these particles (the chemical properties are not covered in this topic). It is no longer
appropriate to talk about particles as entities that exist but cannot be seen even with the most powerful microscope since
the invention of the atomic force microscope and the STM (Scanning Transmission Microscope). With these devices and
computer enhancement, it is possible to ‘see’ individual atoms and even the wave disturbances associated with the electron
orbits.
Even so, the particulate nature of matter is a difficult concept for pupils to understand and should be reinforced by concrete
examples. It is important that pupils have a good understanding of what is meant by solid, liquid and gas and it may be
worthwhile revisiting earlier work (Level C/D – Substances and Solutions).
Pupils find it difficult to develop a mental picture of particulate matter. They often need help in thinking about the small
solid particles separated by vacuum.
Solids, liquids and gases
Imagine a set number of particles.
In a solid, the particles are held together by strong forces. The particles in solids form tightly packed,
rigid, lattice like structures. The particles have a small amount of energy and because of the closely
packed structure can only vibrate slightly. This closely packed structure explains the fixed shape and
volume of solids. It can also be used to explain their inability to be compressed and their density.
A useful model for this might be to imagine apples packed in layers in a box.
Liquids have weaker forces holding the particles together and they have a more random arrangement.
This allows the particles to move around each other explaining the fact that they do not have a fixed
shape. Liquids tend to be less dense than solids since there are fewer particles in the same amount of
space. A model for this might be the movement of marbles in a bag or balls in a ball pool
In gases the particles are widely spaced and fast moving. They have much more energy than solids or
liquids, moving at high speeds. Since there is more space in gases they tend to have much lower
densities. The particles in gases can be compressed more easily than a solid or a liquid. A useful model
for this might be the balls moving inside a bingo machine or the lottery.
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MATTER Level E/F: Teacher’s Science Notes
Diffusion
Pupils may consider the particulate model as a static model, however
particles can move. Diffusion experiments give an idea of the relative
speed of movement of the particles i.e. slowest in the jelly and crystal
(solid moving through solid) and fastest in the brown gas and air (gas
moving through gas).
The use of two tubes of jelly with the crystals at the top of one and the
bottom of the other is used to show that the rate of diffusion is not
dependent on gravity. These experiments consolidate the idea of the
arrangement of the particles since the ones that are closer together will
find it most difficult to mix.
To confirm that particles are moving a demonstration of Brownian motion
can be set up. Place a drop of water into the cavity on a cavity slide and add a tiny pinch of powder paint. Cover with a
cover slip and view under the microscope. Pupils will be able to see the specks of powder moving about. This is caused by
the water particles hitting the specks of paint and causing them to move. It is important that in discussing what is happening
in this situation that pupils do not confuse the specks of powder with particles (in scientific terms particles are infinitely
smaller than the specks of paint).
Compression
Compression is the process of forcing particles together into a smaller space. Compression does not happen in solids or
liquids as the particles are essentially touching. In gases however there is lots of space between the particles and they can
easily be pushed together. (The relative spacing of the particles in solids, liquids and gases is approximately 1:1:10). Pupils
should know that the relative spacing in solids and liquids is about the same – it is only the arrangement of the particles
that is different but gases have much bigger spacing between the particles.
When the particles of a gas are squeezed or compressed they collide with each other and with the sides of their container.
This leads to an increase in pressure and pupils can feel this if they hold their fingers over the end of a syringe filled with air
and push the plunger. A good example of this is a blown up balloon (this could be used as a demonstration). As the balloon
is blown up, air is forced into it under pressure. As the number of air particles, and the number of collisions, increases,
pressure increases on the inside of the balloon causing a force pushing the insides outward. The balloon expands until the
elastic force of the rubber balloon skin is the same as the force from the air inside the balloon. This gives the balloon its
shape.
It is worth spending some time to make sure that pupils have the correct ideas about the spacing and arrangement of
particles in solids liquids and gases at this point before moving on to the other properties.
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MATTER Level E/F: Teacher’s Science Notes
Expansion and contraction
When asked what happens to solids when they are heated most pupils will say that they will melt. This is true depending
on the solid and how long it is heated for. The activities in this topic allow pupils to see that heating any substance can
cause it to expand. When a solid is heated, energy is passed onto the particles causing them to vibrate more. This causes
the particles to move further apart, thus the volume of the solid gets bigger. Melting occurs when we continue to heat the
solid causing the particles to vibrate faster and eventually break free of the strong forces which hold the particles together.
Once these forces are weakened or broken the particles can move more freely and it becomes a liquid.
Expansion also occurs in liquids and gases but to a far greater extent than in solids. The use of the specialised apparatus to
illustrate the expansion of solids testifies to the almost unnoticeable increase in volume. However, liquids expand much
more readily and simply heating a flask in hot water will cause expansion that is easily measured. A gas expands even
more than a liquid and warming a flask of air with your hands will cause air bubbles to be seen if the top is held under
water. As in solids, the extra heat causes the particles in liquids and gases to move faster (vibration in liquids, higher
speeds in gases); this extra movement requires extra space and leads to an increase in volume.
Gas (air) pressure
Although most gases are colourless and therefore ‘invisible’ the particles within them still have an effect on the things they
come in contact with. The moving particles of gas hit the surface of containers and objects exerting a pressure. The gas,
which affects us most, is air and therefore the experiments in this topic deal with air pressure. Air exerts a pressure on us all
the time and this pressure causes a force of 100,000 N (equivalent to 10 tonnes weight) for every square metre. We can see
the effects of air pressure when the wind blows objects along.
Air pressure could be introduced using party whistles (the rolled up paper ones) or a game of blow football and asking
pupils to try to explain what is happening using the knowledge they have gained in previous experiments.
Density
This is a very difficult concept for most pupils (and some adults!). Pupils often confuse density with thickness! For example
if asked whether cream or milk has the greater density, many pupils will say it is the cream – but the cream floats on the top
of the milk, so it is less dense than milk – oil and water have the same effect and can be demonstrated in the lab. Another
interesting question to illustrate density is, “which is the heavier, a tonne of bricks or a tonne of feathers?” This can lead to
interesting discussions about the amount of ‘stuff’ that can be packed into the same volume.
The dictionary defines density as the ratio of the mass of an object to its volume. In terms of particles the density can be
thought of as the number of particles in a given volume – the more particles there are in the same amount of space the
greater the mass will be. The more concrete examples pupils are exposed to the easier it will be for them to grasp the idea
of density. Allowing pupils to measure the mass of different materials of the same volume and showing models/diagrams
to illustrate relative numbers of particles will help.
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MATTER Level E/F: Teacher’s Science Notes
As substances change state from solid to liquid to gas the density decreases as the particles move further apart from one
another. When the particles move closer together (gas to liquid to solid) the density increases. An interesting and important
biological exception to this is water where the density decreases when the water freezes. This is due to hydrogen bonding
between the water molecules, which gives the ice a very open structure. This explains why ice floats on water. If ponds
froze from the bottom up, all life would die; because they freeze from the bottom down (and the ice layers also acts like an
insulator) life can go on.
Solubility
It is very easy for pupils to confuse diffusion with
solubility. In diffusion the particles mix without being
shaken or stirred. Dissolving is different in that the
particles of solid, (usually ionic solids), become
surrounded by water molecules which form bonds with
the ions and then ‘pull’ them apart.
By heating the water the molecules have more energy
and move faster. They will therefore have more chance
of hitting the solid and form bonds quicker. This speeds
up the rate of dissolving
Changes of state
Pupils should be familiar with the definitions of the terms melting, freezing, boiling, condensing and evaporating from
work covered at Level C/D (Substances and Solutions).
When a solid is heated the particles are given energy, which causes the particles to vibrate more, and in turn they move
apart slightly. This explains the expansion of solids when heated. If the solid receives enough energy the particles can
vibrate enough to break free of the rigid structure and are able to move around more freely. The solid has melted and
become a liquid. If this solid continues to be heated the particles gain enough energy to separate from other particles and
become a gas.
As gases are cooled they lose energy and so the particles slow down, move closer together and a liquid forms (condensation).
If the liquid is cooled further the particles lose more energy and eventually slow down so much that they move into the
particle arrangement of a solid.
Sublimation is the change of state from a solid directly to a gas. This can be demonstrated by heating a very small crystal of
iodine in a test tube with a cotton wool plug. Pupils can see that the solid takes up very little space compared to the gas,
which fills the test tube. Another observation made by the pupils is that there is no sign of melting or boiling.
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