KS3 · Chemistry

Characteristics of chemical reactions

You can un-melt an ice cube by freezing it again. Can you un-toast toast? That one question splits all changes into two kinds.

Chemistry · Chemical reactions

Chemical reaction or physical change?

Sort each everyday change. Commit first, then see what evidence decides it.

Still to sort

Chemical reaction (0)

A new substance is made

Where the line is: Look for evidence that new products have formed. Atoms have been rearranged.

Physical change (0)

No new substance is made

Where the line is: If there is no evidence of a reaction, we probably assume it is physical. It usually involves a change of state.

13 of 13 still to sort.

Pick a change, pick a box. Then read the reason: it names the one thing that decides it.

Watch out: A change can look dramatic and still be physical. Ask whether a new substance has been made, not whether something visibly happened.

Predict, then check

Most people expect reactions to get hot. Let's test that.

Iron filings added to copper sulphate solution make the temperature go up. Now citric acid is mixed with sodium bicarbonate, and the mixture fizzes. What do you predict happens to the temperature?

Inside a reaction: methane and oxygen

Tap each molecule and count its atoms, then add up both sides.

Tap a molecule to see what it does in the reaction.

Two kinds of change, side by side

Chemical reactionvsPhysical change

Start with the question that splits them best: can you go back?

Focus

Can it be reversed?

Chemical reaction

Generally irreversible: you can't go back to the original reactants

Physical change

Can be reversed: wax freezes again, steam condenses back to water

The insight

A reaction is generally irreversible because the atoms are now bonded to different atoms and new products have been made.

Are atoms rearranged?

Chemical reaction

Yes: atoms join to different atoms

Physical change

No: atoms are not rearranged

Are new products made?

Chemical reaction

Yes: at least one new substance

Physical change

No new substance

Is there a change of state?

Chemical reaction

Not the test: look for new products instead

Physical change

Usually: solid to liquid, liquid to gas, or gas to liquid

Chemistry · Chemical reactions

Whose idea is closest to yours?

Four learners are arguing about what makes something a chemical reaction.

Which idea is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

One question sorts almost every change you'll meet: has a new substance been made?

What you need to know

  • In a chemical reaction, atoms rearrange themselves to form new substances called products; the substances you start with are the reactants.
  • Signs of a reaction are colour changes, bubbling (effervescence), temperature changes, a smell, light, sound, or a precipitate, a solid forming from solutions.
  • Atoms are never destroyed in a reaction. They are bonded to different atoms, which is why a reaction is generally irreversible.
  • A physical change makes no new substance, can be reversed and usually involves a change of state. If there is no evidence of a reaction, assume a physical change.

The big picture

A chemical reaction rearranges atoms to make new products, and it is generally irreversible. A physical change makes no new substance, can be reversed and usually involves a change of state. Signs such as bubbles, colour changes, temperature changes, smells, light, sound or a precipitate are clues that a new substance has formed.

Key points

1Chemical reaction: atoms are rearranged, new products are made, and it is generally irreversible.
2Physical change: atoms are not rearranged, no new products are made, it can be reversed and it usually involves a change of state.
3Reactants are the substances before the reaction (left of an equation); products are formed afterwards (right of an equation).
4Bubbles (effervescence) show a new gas has been made; a temperature change in either direction can also show a reaction.
5Dissolving sugar in water is physical, because the sugar can be recovered by evaporating the water.

Worked example

Problem

Solid silver is heated until it melts to molten silver. When it cools, it freezes back to solid silver. Is this a chemical reaction or a physical change? Give your evidence.

⚠ Watch out

Thinking that anything dramatic is a chemical reaction. Ice melting, water boiling and wax melting all look like big changes, but no new substance forms, so they are physical. The question to ask is: has a new product been made?

🧠

Memory hook

New stuff or same stuff? Picture a dance floor: in a reaction the same dancers (atoms) swap partners, so the pairs are new but nobody leaves. The picture only goes so far: atoms don't choose partners.

✓

Check yourself

Burning wood is a reaction but melting ice isn't. What is the difference? Answer: burning makes new products; melting ice only changes state.

Flashcards

(13)
What is a chemical reaction?
A change in which atoms are rearranged to make new substances (new products), and the changes can be observed.
What are reactants?
The elements or compounds before a reaction takes place, found on the left-hand side of a chemical equation.
What are products?
The elements or compounds formed after a reaction has taken place, found on the right-hand side of a chemical equation.
Can atoms be destroyed in a chemical reaction?
No. They stop being bonded to the same atoms and are rearranged to form the products.
Name signs that a chemical reaction is taking place.
Colour change, bubbling, temperature change, a smell, light, sound, or a precipitate.
What is effervescence?
Bubbling or fizzing. The bubbles show that a new gas, and so a new product, has been made.
What is a precipitate?
A solid that forms from solutions during a reaction.
Why is a chemical reaction generally irreversible?
The atoms have bonded to different atoms and new products have been made, so we can't go back to the original reactants.
What is a physical change?
A change with no new substances made. It can be reversed and usually involves a change of state.
Why is dissolving sugar in water a physical change?
The sugar can be recovered by evaporating all the water.
Can a temperature change show a reaction?
Yes, in either direction: iron filings in copper sulphate warm up; citric acid with sodium bicarbonate bubbles and cools down.
What do we assume if there is no evidence of a reaction?
We probably assume it is a physical change.
When methane reacts with oxygen, what is made, and what happens to the atoms?
Carbon dioxide and water. There are exactly the same number of atoms, but they are all changed around.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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