Pupil sheet · The particle model

Brownian motion — questions

Name ______________________ Date ____________

Try every question before you use a nudge. Write in full sentences where you are asked to explain.

1. identify

A drop of muddy river water is sealed under a thin cover slip and watched through a microscope for 20 minutes. Tiny particles of clay are seen in it. Which row describes what the clay particles do?

Row	Path of each clay particle	Neighbouring clay particles	After 20 minutes
A	a random zig-zag	each moves in its own way	still jiggling
B	a random zig-zag	all move the same way together	still jiggling
C	a smooth, straight drift	each moves in its own way	they have stopped
D	a regular back-and-forth	all move the same way together	still jiggling

Circle one answer.

Nudge (only if you are stuck): The drop is sealed and still, so no current carries the clay along. Check each column of every row against what you know about Brownian motion.

2. identify / explain

Specks of chalk dust in a sealed drop of water are watched through a microscope. Neighbouring specks jump in different directions. A speck with no other specks near it jiggles just as much. The specks are still jiggling the next day, and they jiggle in the same way in a drop of water that was boiled and then cooled. Which is the best explanation of the jiggling? Then explain which observation rules out one of the other explanations.

Circle one answer.

Currents in the water carry the specks along with them
The specks are tiny living things that swim about
Water molecules hit each speck unevenly, at random
The specks keep bumping into and pushing each other

Which observation rules out one of the other explanations?

Nudge (only if you are stuck): Test each explanation against every observation. The best explanation is the one that fits all of them.

3. identify

Ali says that air is a smooth substance, with no particles in it. Bea says that air is made of tiny particles that are always moving. Tiny droplets of mist float in a sealed glass jar of air and are watched through a microscope. Which observation supports Bea’s idea and not Ali’s?

Circle one answer.

Each droplet jiggles at random, independently of its neighbours
The droplets scatter light, so they shine on a dark background
The droplets are too small to see without using a microscope
The droplets slowly settle towards the bottom of the jar

Nudge (only if you are stuck): An observation supports Bea only if Ali’s smooth air could not explain it.

4. predict / explain

A speck of flour dust in a sealed cell of air is watched through a microscope, and its position is marked every 10 s. Joined up, the marks make a zig-zag. The experiment is repeated, with the position marked every 1 s for the same total time. Predict what the new path looks like. Then explain your prediction.

Circle one answer.

More, shorter jumps, making a more jagged zig-zag
The same lines, with extra marks dotted along them
A smooth curve, as the marks are much closer in time
Fewer jumps, each one ten times longer than before

Explain your prediction.

Nudge (only if you are stuck): Think about what the speck was doing between two of the marks made every 10 s.

5. identify

Supplement. Sort each of these into an atom or molecule, a microscopic particle, or a living cell that swims by itself.

Write the number of the matching option.

atom or molecule
microscopic particle
living cell that swims by itself

A grain of potato starch in water: ______
An argon atom in a light bulb: ______
A speck of rust in a drop of water: ______
A carbon dioxide molecule in fizzy water: ______
A tiny water droplet in a cloud: ______
A methane molecule in the gas supply: ______

Nudge (only if you are stuck): An atom or molecule is far too small to see with any light microscope. A microscopic particle can be seen through a microscope, and is made of a huge number of atoms.

6. calculate / explain

Supplement. A tiny plastic sphere used in a laboratory is 0.7 μm across. A water molecule is about 0.28 nm across. 1 μm = 1000 nm. Calculate how many times wider the sphere is than the water molecule.

Width of the sphere ÷ width of a water molecule: ______________ unit ________

Use your answer to explain why one hit by a water molecule barely moves the sphere.

Nudge (only if you are stuck): Put both sizes in the same unit first.

7. explain

Supplement. A tiny droplet of oil, about 2 μm across, is seen through a microscope in a drop of salad dressing. One hit by a water molecule barely moves it. Explain why the droplet still jiggles about enough to be seen.

Nudge (only if you are stuck): Think about how many hits the droplet gets, how fast the molecules move, and whether the hits on its sides are ever exactly equal.

8. identify / explain

Which of these observations are evidence that matter is made of tiny particles in constant random motion? Choose all that are correct, then explain one of your choices.

Tick every correct statement.

Graphite specks from a pencil jiggle at random in a still drop of water
Flakes in a shaken snow globe swirl round together, then settle
Fine specks of talc in a sealed cell of air jiggle, each on its own
A tadpole darts about in a jar of pond water, then stops still
Grains of rice dropped in water sink and lie still on the bottom

Explain one of your choices.

Nudge (only if you are stuck): Evidence for moving particles is a jiggle of tiny specks, each on its own, that no current, no living thing and no falling can explain.

9. explain

A tiny speck of brick dust in a sealed drop of water is watched through a microscope. An hour later it is jiggling just as much as at the start. Hana says that it should slow down and stop, like a football that has been kicked once. Explain why Hana is incorrect.

Nudge (only if you are stuck): A football is kicked once. How often is the speck hit, and what happens to the things that hit it?

10. identify / state / predict / explain

At an outdoor concert on a still night, with no wind, a huge inflatable ball bounces about over the crowd. People push it upwards and away whenever it comes near them. A teacher says this is a model of Brownian motion. (a) In this model, identify what stands for the speck and what stands for the molecules. (b) State two ways in which the concert model is not like real Brownian motion. (c) Supplement. A small beach ball is pushed about by the same crowd. Predict whether it jiggles about more or less than the huge ball, and explain your prediction. (d) Later, only the ball is lit, by a spotlight, and the crowd cannot be seen from the stage. Explain how watching the ball alone shows that people are there, pushing it, and how this is like the evidence that Brownian motion gives.

(a) What stands for what

(b) Two ways the model differs

(d) The ball in the dark

Nudge (only if you are stuck): In each part, compare the ball and the people with a speck and the molecules that hit it. What is the same, and what is very different?