The Particle Model of Matter, Grade 8

Practical Task 2 is formally assessed, and there is nothing practical in it.

The prescribed task asks a Grade 8 class to draw particles, label three boxes, rank three blocks and explain a balloon. All on paper, all out of 20. This page carries it in full, because it is assessed, and then does the same four ideas on a bench: diffusion timed in hot and cold water, wax melted in a water bath, air squeezed in a sealed syringe, and a metal ring whose hole gets bigger when you heat it.

The particle model, in four statements

  1. All matter is made of particles. In this topic a particle means an atom or a molecule
  2. The particles are far too small to see. No light microscope shows one
  3. There is empty space between them. Not air. Air is itself made of particles
  4. The particles attract each other, strongly in a solid and hardly at all in a gas

Four things worth getting exactly right

Often said Better
"A particle is a grain of sand, or something smaller" A grain of sand holds roughly 1019 atoms and you can see it. A particle here is an atom or a molecule
"A gas has no shape" A gas takes the shape and the volume of its container. A liquid takes the shape but keeps its own volume, and that second half is the difference
"The space between particles is air" It is empty. Even in air, the space between the molecules is empty
"Gas particles are smaller" The particles never change size or number. Only their spacing and arrangement change

States of matter

Solid Liquid Gas
Arrangement Fixed pattern, touching Close, but free to slide Far apart, random
Movement Vibrate in place Slide past each other Fast, in every direction
Energy Lowest More Most
Shape Its own Of the container Of the container
Volume Its own Its own Of the container
Can it be squeezed? Almost not at all Almost not at all Yes, there is space to close up

For what the temperature does while a substance is changing state, and why it stops rising at the melting and boiling points, see heating and cooling curves of water. That practical is where the flat sections are measured.

Where this fits in the curriculum

Subject Natural Sciences
Grade 8
Term 2
Strand Matter and materials
Topic Particle model of matter
Status CAPS Practical Task 2, formally assessed
Marks 20 prescribed. Our worksheet is 50: Part A is the prescribed 20, Part B is 30

Practical 1: diffusion, timed

You need: two 400 ml beakers, a thermometer, food colouring and a clock.

  1. Fill one beaker with cold water and one with hot water from a kettle, to the same depth, and record both temperatures
  2. Let them stand still for a minute. Moving water is not diffusion
  3. Add one drop of food colouring to each, run down the side, and do not stir
  4. Time how long each takes to colour right through, sketching both at 1, 3 and 5 minutes
Cold, about 15 °C Hot, about 60 °C
Coloured right through 10 to 20 minutes 2 to 5 minutes

Hotter particles move faster, so they spread faster. Measured, rather than asserted.

Diffusion is also faster in gases than in liquids, and the usual explanation, that the attraction between liquid particles is stronger, is only a small part of it. The real reason is room: gas particles are far apart and travel a long way between collisions, while liquid particles are shoulder to shoulder and are knocked off course constantly.

The smell that crosses the room

One spray of deodorant at the front, and hands go up at the back within a minute. The answer expected is "diffusion". Mostly it is not.

Crossing six metres by diffusion alone Hours
What it actually takes Seconds to a minute
What carried it Air currents. Draughts, and warm air rising off bodies and lights

Diffusion does the last few centimetres, into your nose. The room-crossing part is moving air, and a class that can separate the two has understood both.

One demonstration we would leave out. A version of this closes every window and door and throws ether onto the chalkboard. We refuse it. Diethyl ether catches alight from a flame across the room, its vapour is heavier than air and creeps along the bench, and it is an anaesthetic. Deodorant does the same job with the doors open, and it is the alternative the task itself offers.

Practical 2: melting wax without a flame

You need: a test tube, a 400 ml beaker, a thermometer, a piece of candle and hot water.

  1. Break a piece of candle into the test tube
  2. Stand the tube in a beaker of hot water. The water heats the wax; nothing heats the wax directly
  3. Record the temperature when the wax is completely liquid
  4. Lift the tube out and watch it set again, from the outside in

Candle wax softens over a range rather than melting at one sharp temperature, which is itself the result: a pure substance melts sharply, a mixture does not.

Never melt wax over a flame in an open dish. Wax vapour catches alight, and water thrown on a wax fire flashes to steam and throws burning wax across the room. A water bath cannot go above 100 °C, which takes the danger out of the practical completely.

Practical 3: pressure in a sealed syringe

You need: a 50 ml syringe, no needle.

  1. Pull the plunger back to 50 ml with the tip open
  2. Seal the tip hard with a fingertip
  3. Push. The plunger moves, then fights back harder and harder
  4. Let go. It springs out again
  5. Now fill the syringe with water and try again. It barely moves
Observation What the particles are doing
The gas compresses There is empty space between the particles to close up
It gets harder as you push The same particles in less space hit each square centimetre more often
The plunger springs back The squeezed gas pushes out
Water hardly moves Its particles are already touching. There is nothing to squeeze out

Pressure is not a force

Pressure = force ÷ area. Its unit is the pascal, one newton per square metre.

You will often see gas pressure defined as "the force of the particles hitting the walls". The collisions do make a force, but the pressure is that force spread over the area it lands on. The same force on a smaller area is a bigger pressure, which is why a drawing pin goes into a board and your thumb does not.

Practical 4: a balloon on a flask

You need: a wide-neck 100 ml conical flask, a party balloon, hot water and iced water.

  1. Stretch the balloon over the mouth of the empty flask
  2. Stand the flask in hot water. The balloon lifts and stands up
  3. Move it to iced water. It collapses, and is pulled down into the neck

Nothing was added and nothing was let out. The same air, warmed, moves faster and hits the balloon harder and more often. Cooled, it does the opposite.

And a balloon is hardest to blow up at the very first breath, before the rubber has stretched. You may see it claimed the other way round. It takes one balloon to settle.

Practical 5: ball and ring, both halves

You need: a ball and ring apparatus, a spirit burner, tongs and a heatproof mat.

The half everyone does

  1. Check that the cold ball passes through the ring
  2. Heat the ball for two to three minutes
  3. Try it again. It will not go through
  4. Cool it in water, and it fits once more

The half that teaches more

  1. Let everything cool, then ask the class to predict: heat the ring instead, and does the hole get bigger or smaller?
  2. Most of a class says smaller. Heat the ring for two to three minutes and try the cold ball
  3. The ball drops straight through. The hole got bigger

The metal expands in every direction, so every distance in it grows, including the distance across the hole. It is exactly how a tight metal ring is fitted to a shaft: heat the ring, drop it on, let it cool and grip.

One more thing worth doing: rest the hot ball on the cold ring and leave it. After a minute or so it falls through by itself, as the ring warms and expands while the ball cools and contracts.

Heating lowers density, too. Same particles, same mass, more volume. That is why hot air rises, and it is measured on our Grade 8 density practical.

Practical Task 2, and how the 50 marks are made up

The assessed task is reproduced in full on our worksheet as Part A, at the prescribed 20 marks, in the prescribed proportions. Part B is ours, and covers the bench work above.

Part Section Marks
A, assessed Particle diagrams of wax in three states 6
Three boxes of particles 6
Ranking three blocks by density 4
Balloon pressure 4
B, ours Diffusion 8
Melting wax 6
Pressure 8
Expansion 8

Check the block masses before you set the density question. A version in circulation gives a 1 000 cm³ wooden block a mass of 4 kg, which is 4 g/cm³: denser than aluminium, and it would sink. Pine is about 0,5 g/cm³, so 0,5 kg is the sensible figure, and that is what our sheet uses.

If it does not work

Problem Cause Fix
Both beakers colour at the same speed The water was moving Stand them still for a minute first, and do not stir or carry them
The cold beaker colours quickly The drop went in too hard Run it down the side
The wax will not melt The water is not hot enough Fresh kettle water, and five minutes
The syringe leaks The fingertip seal Press hard, or cap the tip
The balloon does not lift Poor seal, or lukewarm water Stretch it further down the neck, and use water straight from the kettle
The heated ball still fits Not heated long enough Two to three minutes on a spirit burner, not one
The ball will not drop through the heated ring The ring cooled before the test Test straight away. Metal loses heat quickly

Safety

  • Goggles for the heating practicals
  • Wax is melted in a water bath, never over a flame. If wax ever does catch alight, it gets a lid, never water
  • Kettle water scalds. Pour it for the class
  • The ball and ring stay hot long after they look cool. Tongs, a heatproof mat, and a minute before anyone touches them
  • Lower hot metal into water, never drop it. It spits
  • One balloon per learner, never shared, and ask about latex allergies first
  • One spray of deodorant, doors open, never towards a face. No ether
  • Chalk is cut with a plastic knife or a ruler edge, not a kitchen knife

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

  • Learner worksheet, 50 marks. Part A is the assessed task at its prescribed 20 marks; Part B is the bench work at 30
  • Marking memorandum, with what to accept for each answer, the drawing error that costs most learners a mark, and the five answers that look right and score nothing

Related practicals

What you need to run it

One item does the heavy lifting: the ball and ring apparatus. It is the only part of this topic a class cannot improvise, it lasts decades, and it carries the prediction question that makes the lesson.

After that it is ordinary stock: spirit burners, 400 ml beakers, thermometers at R11 each, 50 ml syringes and a wide-neck conical flask. Food colouring, a candle, balloons and a can of deodorant come from a supermarket.

The rest of the range for this topic is in heat and expansion equipment.