Chapter 2 of 8

All 8 chapters
  1. 1The particle model, readunderstood
  2. 2Evidence for particles, readunderstood
  3. 3Changes of state, readunderstood
  4. 4Diffusion and temperature, readunderstood
  5. 5Gas pressure, readunderstood
  6. 6Expanding and shrinking, readunderstood
  7. 7Dissolving and solutions, readunderstood
  8. 8Density, readunderstood

S3.5.2 Chapter 2

Evidence for particles

How can I smell food from across the room when I cannot see anything travel?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Explain how a smell spreads across a room
  • Explain why a spoon of sugar seems to vanish in water

Nobody has ever seen a single particle with their own eyes. So how do we know that particles are there?

We use evidence. Evidence is what we see or measure that supports an idea. Scientists use the particle model because it explains a great deal of evidence.

Here is the first piece. Someone peels an orange at one end of a room. Soon people at the other end can smell it. Nothing you can see has crossed the room.

Draw it first

Draw a smell spreading through air

  1. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startairsmell

    1 Draw a box of air with open circles. Put the smell, dark dots, in one corner.

  2. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startsoonairsmell

    2 Draw the same box a little later. Some dark dots have moved away from the corner.

  3. Three closed boxes of gas, side by side, with a key below. Each box has 6 open circles for air and 3 dark dots for smell. In the box labelled start, the dark dots are bunched in one corner. In the box labelled soon, some dark dots have moved away from the corner. In the box labelled later, the dark dots are mixed through the whole box. Labels: start, soon, later, air, smell.startsoonlaterairsmell

    3 Draw it later still. The dark dots are mixed through the whole box.

The orange gives off particles of a gas. They mix with the air, which is a gas made of particles too. The smell particles fly about and keep bumping into air particles. Each bump sends them a new way, so they zigzag.

A closed box with open circles for air and one dark dot for a smell particle on the left. A line goes from the dark dot in straight stretches. It bends each time the dot touches an air particle, and it ends with an arrow. Labels: one smell particle.one smell particle
A smell particle flies straight until it hits an air particle. Then it goes a new way.

Slowly, the smell particles spread through the whole room. This is diffusion. In diffusion, particles spread out by themselves, from where there are many to where there are few.

In a real room, slow air currents carry most of the smell. Diffusion alone is slower, but it never stops, even in still air. Leave a drop of food colour in a glass of water. With no stirring, the colour still spreads, though it takes an hour or more.

Here is the second piece. Stir a spoon of sugar into water. Soon you cannot see it. Did the sugar vanish?

No. The water particles pull the sugar apart into separate particles. They are far too small to see. They spread out among the water particles, so the water stays clear. We say the sugar dissolves.

Three open cups of water, side by side, with a key below. Each cup has 20 open circles for water and 6 dark dots for sugar, and the circles all touch. In the cup labelled at first, the dark dots are bunched in the bottom left corner. In the cup labelled soon, some dark dots have moved away. In the cup labelled later, the dark dots are mixed through the water. Labels: at first, soon, later, water, sugar.at firstsoonlaterwatersugar
Sugar in water. The sugar particles are still there. They are spread through the water.

The sugar did not vanish. It is mixed in. Salt does the same. Leave salty water in a dish in the sun. The water goes into the air, and white salt is left on the dish.

See it three ways

With things

A drop of ink spreads through still water. Perfume spreads through a room. Sugar spreads through tea. In each one, something mixes through something else without a spoon or a wind.

As a picture

Two open cups of water, side by side, with a key below. Each cup has open circles for water and dark dots for ink. In the cup labelled start, the dark dots are bunched in one corner. In the cup labelled later, the dark dots are mixed through the water. Labels: start, later, water, ink.startlaterwaterink

In words

Particles of two materials mix by themselves. The particles move about, and they spread from where there are many to where there are few.

Try it

Time a smell across a room

You need: an orange or a lemon, a room with the door and windows shut, 3 friends, a clock, or a way to count seconds.

Be safe: Nobody should sniff the peel close up. Do not try this with anyone who is allergic to oranges or lemons.

  1. Ask your friends to stand in the room. One stands near the orange, one half way, and one at the far end. They must stay still.
  2. Peel the orange at one end of the room. Start counting at once.
  3. Ask each friend to put a hand up the moment they smell it. Write down each time.

What you will see: The friend nearest the orange smells it first. The friend at the far end smells it last. You saw nothing cross the room, but particles did. Air currents help carry them, so this test cannot show diffusion alone.

If you cannot try it, this is what happens.
A row of three people stand in a line away from an orange on the left. The nearest person smells it first, the one half way smells it second, and the one at the far end smells it third. Labels: orange, 1st, 2nd, 3rd.orange1st2nd3rd

A wrong idea many people have

A smell needs wind to travel.

Why it feels right: A breeze often brings the smell of cooking from the next house.

The test that breaks it: Leave a drop of food colour in a glass of still water. With no wind and no stirring, the colour still spreads out, though very slowly.

What is true: Wind and air currents carry a smell faster. But the particles also spread out by themselves, even in still air, only slowly. This is diffusion.

Key words

evidence
what we see or measure that supports an idea.
diffusion
particles spreading out by themselves, from where there are many to where there are few.
dissolves
mixes into a liquid so completely that the particles cannot be seen.

Back to your guess

The answer is C: Tiny particles from the food spread out through the air until some reach my nose.

Particles from the food mix into the air and spread out by themselves. Air currents carry them faster.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. Someone opens a bottle of perfume at one end of a still room. Why can a person at the other end smell it after a while?

    Tap your answer.

    1. Not this one. The particles zigzag as they bump into air particles. They spread out, and nothing pushes them in a line. Go to S3.5.2. Try another.

    2. Not this one. Wind helps, but particles spread by themselves, even in still air. Go to S3.5.2. Try another.

    3. Yes, that is right. The particles spread out. Air currents help, and diffusion alone is slower, but it needs no wind.

    See the answer

    Perfume particles fly about, bump into air particles and spread through the room.

    The particles spread out. Air currents help, and diffusion alone is slower, but it needs no wind.

    • If you chose A: The particles zigzag as they bump into air particles. They spread out, and nothing pushes them in a line. Go to S3.5.2.
    • If you chose B: Wind helps, but particles spread by themselves, even in still air. Go to S3.5.2.

  2. Why can you not see the sugar after you stir it into water?

    Tap your answer.

    1. Not this one. The sugar does not change into water. Its particles are still sugar, spread through the water. Go to S3.5.2. Try another.

    2. Yes, that is right. The sugar dissolves. Its particles are mixed among the water particles, so the water stays clear.

    3. Not this one. Nothing is destroyed. Leave the water to dry and the sugar is still there. Go to S3.5.2. Try another.

    See the answer

    The sugar breaks into particles too small to see, and they spread through the water.

    The sugar dissolves. Its particles are mixed among the water particles, so the water stays clear.

    • If you chose A: The sugar does not change into water. Its particles are still sugar, spread through the water. Go to S3.5.2.
    • If you chose C: Nothing is destroyed. Leave the water to dry and the sugar is still there. Go to S3.5.2.

  3. The picture shows 6 air particles and 3 smell particles in a box. How many particles are in the box in all?

    A closed box of gas with 6 open circles for air and 3 dark dots for smell, and a key below. Labels: air, smell.airsmell
    Tap your answer.

    1. Yes, that is right. Add the air particles and the smell particles. 6 + 3 = 9.

    2. Not this one. A smell is made of particles too. Count the dark dots as well as the open circles. Go to S3.5.2. Try another.

    3. Not this one. The air is made of particles too. Count the open circles as well as the dark dots. Go to S3.5.2. Try another.

    See the answer

    9 particles in all

    Add the air particles and the smell particles. 6 + 3 = 9.

    • If you chose B: A smell is made of particles too. Count the dark dots as well as the open circles. Go to S3.5.2.
    • If you chose C: The air is made of particles too. Count the open circles as well as the dark dots. Go to S3.5.2.

  4. In one orange test, a friend near the orange smelled it after 20 seconds. A friend at the far end smelled it after 90 seconds. How many seconds longer did the far friend wait?

    Tap your answer.

    1. Yes, that is right. The smell took longer to reach the far end. 90 − 20 = 70.

    2. Not this one. That is the far friend's whole time. The question asks how much longer than the near friend. Go to S3.5.2. Try another.

    3. Not this one. To find how much longer, take the shorter time away. Do not add the times. Go to S3.5.2. Try another.

    See the answer

    70 seconds longer

    The smell took longer to reach the far end. 90 − 20 = 70.

    • If you chose B: That is the far friend's whole time. The question asks how much longer than the near friend. Go to S3.5.2.
    • If you chose C: To find how much longer, take the shorter time away. Do not add the times. Go to S3.5.2.

  5. You stir salt into water and can no longer see it. How could you show the salt is still there?

    Tap your answer.

    1. Not this one. The salt has not gone. Its particles are still in the water, so nothing has been lost. Go to S3.5.2. Try another.

    2. Yes, that is right. The water goes into the air as a gas, and the salt particles stay behind. That shows the salt was dissolved, not destroyed.

    3. Not this one. Salt does not turn into water. It dissolves, and its particles are spread among the water particles. Go to S3.5.2. Try another.

    See the answer

    Leave the salty water in a dish in the sun. The water dries away, and white salt is left.

    The water goes into the air as a gas, and the salt particles stay behind. That shows the salt was dissolved, not destroyed.

    • If you chose A: The salt has not gone. Its particles are still in the water, so nothing has been lost. Go to S3.5.2.
    • If you chose C: Salt does not turn into water. It dissolves, and its particles are spread among the water particles. Go to S3.5.2.

  6. From an earlier chapter

    In which state do the particles touch each other but slide past each other?

    Tap your answer.

    1. Not this one. In a solid the particles touch but are held in place. Go to S3.5.1. Try another.

    2. Yes, that is right. In a liquid the particles touch, but they are free to slide. That is why a liquid flows.

    3. Not this one. In a gas the particles are far apart. Go to S3.5.1. Try another.

    See the answer

    Liquid

    In a liquid the particles touch, but they are free to slide. That is why a liquid flows.

    • If you chose A: In a solid the particles touch but are held in place. Go to S3.5.1.
    • If you chose C: In a gas the particles are far apart. Go to S3.5.1.

Keep it

Copy this into your exercise book.

We cannot see particles, so we use evidence. A smell spreads because its particles fly about and mix with air. This is diffusion. Dissolved sugar is not gone. It is spread out.

Draw: Draw two boxes of open circles. Put dark dots in one corner of the first. Spread them through the second.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (865 KB). It opens on any phone.