Chapter 4 of 8

All 8 chapters
  1. 1Elements and compounds, readunderstood
  2. 2Pure substances and mixtures, readunderstood
  3. 3Separating mixtures, readunderstood
  4. 4Physical and chemical changes, readunderstood
  5. 5Rusting and other reactions, readunderstood
  6. 6Burning and fire safety, readunderstood
  7. 7Acids and alkalis, readunderstood
  8. 8Neutralisation, readunderstood

S3.6.4 Chapter 4

Physical and chemical changes

Ice melting and wood burning are both changes. Are they the same kind of change?

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

  • Sort changes into physical and chemical
  • List signs of a chemical change, such as gas, heat or colour

Things change all the time. Ice melts. Milk turns sour. Paper burns. Sugar vanishes in tea. In science we sort these changes into two kinds. The question to ask is whether a new substance is made.

Draw it first

Draw ice melting

  1. Two boxes of circles side by side. In the box labelled ice, the circles sit in neat rows and touch. In the box labelled water, the circles touch but are jumbled at the bottom, under a flat top. Labels: ice, water.ice

    1 Draw the particles of ice, in neat rows. They hold each other in place.

  2. Two boxes of circles side by side. In the box labelled ice, the circles sit in neat rows and touch. In the box labelled water, the circles touch but are jumbled at the bottom, under a flat top. Labels: ice, water.icewater

    2 Draw the same particles as water. They are jumbled and slide past each other. They are the same particles.

Melting is a physical change. The particles are the same before and after. No new substance is made. Only the way the particles are arranged has changed.

Three boxes of circles in a row, labelled solid, liquid and gas. Arrows above point from solid to liquid, labelled melting, and from liquid to gas, labelled boiling, each with energy in. Arrows below point from gas to liquid, labelled condensing, and from liquid to solid, labelled freezing, each with energy out.solidliquidgasmeltingenergy inboilingenergy incondensingenergy outfreezingenergy out
Every change of state can go both ways. Melting can be reversed by freezing.

A physical change can often be turned back, so we say it is reversible. Freeze the water and you get ice again. Dissolve sugar in water and you can get the sugar back by evaporating. Crushing a can or cutting paper are also physical changes.

In a chemical change, the atoms are joined in new ways, and a new substance is made. The new substance has new properties. A chemical change is not easy to turn back.

Two boxes with an arrow between them, and a key under them. The box labelled before holds 2 pairs of small circles with the letter H and 1 pair of large circles with the letter O, none joined to another pair, all far apart in a closed box. The box labelled after holds 2 water groups, each one large circle with the letter O joined to two small circles with the letter H, far apart as steam. The key names the circles: hydrogen and oxygen.OOHHHHbeforeOOHHHHafterHhydrogenOoxygen
Hydrogen burns in oxygen and makes water. There are 6 atoms before and 6 atoms after. The atoms are not lost. They join in new groups.

How can you tell if a change is chemical? Look for signs. These are clues that a new substance may be forming.

A table with the columns sign and example. A new gas: rising bread. Heat or light: candle flame. Colour change: cut apple. A new solid: rust on iron.signexamplea new gasrising breadheat or lightcandle flamecolour changecut applea new solidrust on iron
Four signs of a chemical change. A cut apple turns brown because it reacts with oxygen in the air.

But a sign alone is not proof. Some physical changes show the same signs. The real test is whether a new substance has been made.

A kerosene lamp burns its fuel to give light and heat, and that is a chemical change. An electric bulb gives light and heat with no new substance, and that is physical.

A table with the columns sign and but physical. A gas: boiling water. Heat or light: electric bulb. Colour change: mixing paint. A solid: ice forming.signbut physicala gasboiling waterheat or lightelectric bulbcolour changemixing painta solidice forming
Each of these looks like a sign, but no new substance is made.

See it three ways

With things

Ask two questions. Is a new substance made? Can it be turned back easily? Melting ice: no new substance, and it turns back. Burning wood: new substances, and no way back.

As a picture

A table with the columns change and kind. Melting ice: physical. Burning wood: chemical. Dissolving sugar: physical. A nail rusting: chemical. Cooking an egg: chemical. Crushing a can: physical.changekindmelting icephysicalburning woodchemicaldissolving sugarphysicala nail rustingchemicalcooking an eggchemicalcrushing a canphysical

In words

In a physical change no new substance is made, and it can often be reversed. In a chemical change a new substance is made, and it is hard to reverse.

Try it

Fizz or no fizz

You need: two large cups, a plate, a spoon of sugar, a spoon of baking soda, water, vinegar.

Be safe: Use a big cup, because the fizz can spill over. Keep your face back from the cup. If any splashes in your eyes, rinse them with plenty of clean water. Do not taste anything. Do not seal the mixture in a bottle.

  1. Cup A: stir a spoon of sugar into half a cup of water. Watch for bubbles and for any change in heat.
  2. Cup B: put a spoon of baking soda in a large cup. Slowly pour in a little vinegar.
  3. Watch cup B. Then pour a thin layer of the sugar water from cup A onto the plate. Leave it in the sun until the water has gone. What is left?

What you will see: In cup A the sugar vanishes with no bubbles. The water evaporates and a sticky layer of sugar is left on the plate. So dissolving is a physical change. In cup B a gas forms and fizzes. The gas is carbon dioxide, a new substance, so this is a chemical change.

If you cannot try it, this is what happens.
A table with the columns mixture and new gas? Sugar in water: no. Soda and vinegar: yes.mixturenew gas?sugar in waternosoda and vinegaryes

A wrong idea many people have

If something changes colour, it must be a chemical change.

Why it feels right: Burning wood goes black and rust is brown. So colour seems to be the sign that matters.

The test that breaks it: Stir a drop of food colouring into a glass of water. The water changes colour. Now leave it in the sun until the water has gone. The colouring is left, unchanged. Nothing new was made.

What is true: A colour change is only a clue. Ask if a new substance has been made, and if the change can be turned back.

Key words

physical change
a change where no new substance is made.
reversible
able to be turned back.
chemical change
a change where atoms join in new ways and a new substance is made.

Back to your guess

The answer is B: No. Melting makes no new substance. Burning makes new substances that you cannot easily turn back.

Melting is a physical change. The water is still water. Burning is a chemical change. It makes new substances, such as ash and gases, that you cannot easily turn back.

Check yourself

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

  1. Which of these is a physical change?

    Tap your answer.

    1. Not this one. Burning makes new substances, such as carbon dioxide and water vapour. That is a chemical change. Go to S3.6.4. Try another.

    2. Not this one. Cooking changes the substances in the egg for good, and you cannot turn it back. That is a chemical change. Go to S3.6.4. Try another.

    3. Yes, that is right. Ice and water are the same substance. Melting only changes how the particles are arranged, and freezing turns it back.

    See the answer

    Ice melts into water.

    Ice and water are the same substance. Melting only changes how the particles are arranged, and freezing turns it back.

    • If you chose A: Burning makes new substances, such as carbon dioxide and water vapour. That is a chemical change. Go to S3.6.4.
    • If you chose B: Cooking changes the substances in the egg for good, and you cannot turn it back. That is a chemical change. Go to S3.6.4.

  2. Which of these is a chemical change?

    Tap your answer.

    1. Yes, that is right. New substances are made, with new properties, and you cannot turn them back into wood.

    2. Not this one. The sugar is still there, spread through the tea. You can get it back by evaporating. That is physical. Go to S3.5.7. Try another.

    3. Not this one. Steam is still water. This is a change of state, which is physical. Go to S3.5.3. Try another.

    See the answer

    Wood burns to ash and gases.

    New substances are made, with new properties, and you cannot turn them back into wood.

    • If you chose B: The sugar is still there, spread through the tea. You can get it back by evaporating. That is physical. Go to S3.5.7.
    • If you chose C: Steam is still water. This is a change of state, which is physical. Go to S3.5.3.

  3. You heat a pan of water. Bubbles rise and steam leaves. Is this a chemical change?

    Tap your answer.

    1. Not this one. A gas is only a clue. Here the gas is water vapour, which is still water. It is not a new substance. Go to S3.6.4. Try another.

    2. Yes, that is right. Boiling is a change of state. Water vapour is water as a gas. A sign alone is not proof of a chemical change.

    3. Not this one. Heat also melts ice and boils water, which are physical changes. Heat alone is not proof. Go to S3.6.4. Try another.

    See the answer

    No. The bubbles are water vapour, so no new substance is made.

    Boiling is a change of state. Water vapour is water as a gas. A sign alone is not proof of a chemical change.

    • If you chose A: A gas is only a clue. Here the gas is water vapour, which is still water. It is not a new substance. Go to S3.6.4.
    • If you chose C: Heat also melts ice and boils water, which are physical changes. Heat alone is not proof. Go to S3.6.4.

  4. Baking soda is mixed with vinegar, and a gas fizzes out. Which sign of a chemical change is this?

    Tap your answer.

    1. Not this one. The mixture does not change colour. What you see is bubbles of gas. Go to S3.6.4. Try another.

    2. Not this one. No solid forms. The fizz is a gas leaving the mixture. Go to S3.6.4. Try another.

    3. Yes, that is right. The fizz is carbon dioxide gas, a new substance. A new gas is one of the signs of a chemical change.

    See the answer

    A new gas is made.

    The fizz is carbon dioxide gas, a new substance. A new gas is one of the signs of a chemical change.

    • If you chose A: The mixture does not change colour. What you see is bubbles of gas. Go to S3.6.4.
    • If you chose B: No solid forms. The fizz is a gas leaving the mixture. Go to S3.6.4.

  5. What is the best test for a chemical change?

    Tap your answer.

    1. Yes, that is right. The signs are clues. The real test is whether a new substance with new properties has been made.

    2. Not this one. Mixing paint changes colour with no new substance. Colour is only a clue. Go to S3.6.4. Try another.

    3. Not this one. An electric bulb gets hot, and so does a pan on a stove, with no new substance. Heat is only a clue. Go to S3.6.4. Try another.

    See the answer

    A new substance is made.

    The signs are clues. The real test is whether a new substance with new properties has been made.

    • If you chose B: Mixing paint changes colour with no new substance. Colour is only a clue. Go to S3.6.4.
    • If you chose C: An electric bulb gets hot, and so does a pan on a stove, with no new substance. Heat is only a clue. Go to S3.6.4.

  6. In the picture, hydrogen and oxygen change into water. Which sentence is true?

    Two boxes with an arrow between them, and a key under them. The box labelled before holds 2 pairs of small circles with the letter H and 1 pair of large circles with the letter O, none joined to another pair, all far apart in a closed box. The box labelled after holds 2 water groups, each one large circle with the letter O joined to two small circles with the letter H, far apart as steam. The key names the circles: hydrogen and oxygen.OOHHHHbeforeOOHHHHafterHhydrogenOoxygen
    Tap your answer.

    1. Not this one. Atoms are not lost in a chemical change. There are 6 atoms before and 6 after. Go to S3.6.4. Try another.

    2. Not this one. An atom stays the same kind. The water groups still hold hydrogen atoms and oxygen atoms. Go to S3.6.1. Try another.

    3. Yes, that is right. The atoms are only rearranged. They join in new groups, and the new groups are a new substance, water.

    See the answer

    The same atoms join in new groups, and a new substance forms.

    The atoms are only rearranged. They join in new groups, and the new groups are a new substance, water.

    • If you chose A: Atoms are not lost in a chemical change. There are 6 atoms before and 6 after. Go to S3.6.4.
    • If you chose B: An atom stays the same kind. The water groups still hold hydrogen atoms and oxygen atoms. Go to S3.6.1.

  7. After the change there are 2 water groups, and each group has 3 atoms. How many atoms are there in all?

    See the answer

    6 atoms

    2 groups with 3 atoms each. 2 × 3 = 6. This is the same number of atoms as before.

  8. From an earlier chapter

    Which method gets the salt back from a dish of salty water?

    Tap your answer.

    1. Not this one. Dissolved salt passes through filter paper with the water. Go to S3.6.3. Try another.

    2. Not this one. A sieve separates pieces of different size. Dissolved salt is spread through the water and passes through. Go to S3.6.3. Try another.

    3. Yes, that is right. The water evaporates and the salt does not, so the salt is left in the dish.

    See the answer

    Evaporate the water in the sun.

    The water evaporates and the salt does not, so the salt is left in the dish.

    • If you chose A: Dissolved salt passes through filter paper with the water. Go to S3.6.3.
    • If you chose B: A sieve separates pieces of different size. Dissolved salt is spread through the water and passes through. Go to S3.6.3.

Keep it

Copy this into your exercise book.

A physical change makes no new substance. A chemical change makes a new substance. Gas, heat or light, colour or a new solid are clues, not proof.

Draw: Draw two boxes, before and after, with atoms joined in new groups. Label it chemical change.

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 (936 KB). It opens on any phone.