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SavannahYear 8
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ScienceChemical reactions, acids and alkalis30 min★★★ difficulty

Chemical reactions and balancing equations

In a chemical reaction atoms are never created or destroyed; they are rearranged. Once that is fixed in your head, balancing equations becomes a counting job rather than a mystery.

Part of your national curriculum
  • Chemistry: chemical reactions: Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations

Lesson overview

What you'll learn in this lesson

Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations

Key learning points

  • • Signs a reaction has happened
  • • Word equations
  • • Balancing: the method
  • • Practice that always works

This lesson at a glance

  • 30 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

chemicalreactionsrearrangementrepresentedsymbol

Scroll down — the lesson carries on below

Part 1 of 19 · Discover5%
1

Watch & discover

Part 1 of 19

Watch first

Chemical reactions and balancing equations

An English teaching video on this topic from Archie Science Show. Watch it, then scroll down for the lesson.

Do it for real

The practicals below use what the film shows, with a full method you can follow at home.

Making your own pH indicator

In a nutshell

Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations

2

What you already know

Part 2 of 19

Before we start

What you already know

You should be able to count atoms in a formula, including brackets.

Recap: Pure substances and separating mixtures
3

Key words

Part 3 of 19

Key words

Words you'll need today

Chemical reaction
A change in which atoms are rearranged to form new substances.
Conservation of mass
The total mass of reactants equals the total mass of products.
Word equation
A reaction written using names, with an arrow to the products.
Symbol equation
A reaction written using formulae, balanced on both sides.
Balanced
The same number of each type of atom on each side of the equation.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Rearrangement and word equations

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

Signs a reaction has happened

New substances have been made if you see a colour change, a gas produced, a precipitate forming, or a temperature change, and if the change cannot easily be reversed. Melting and dissolving are physical changes: the substance is still the same, so they are not reactions.

6

Explore the idea

Part 6 of 19

Learn

Word equations

Magnesium + oxygen produces magnesium oxide. Reactants on the left, products on the right, arrow in between - never an equals sign. Mass is conserved: if 2.4 g of magnesium combines with 1.6 g of oxygen, the oxide has a mass of 4.0 g. Apparent mass loss in an open container usually means a gas escaped.

7

Quick check

Part 7 of 19

Quick check

In a chemical reaction, atoms are...

8

Quick check

Part 8 of 19

Quick check

2.4 g magnesium + 1.6 g oxygen produces how much magnesium oxide?

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Balancing symbol equations

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Balancing: the method

Take H2 + O2 gives H2O. Oxygen: two on the left, one on the right, so write 2H2O. Now hydrogen is two on the left, four on the right, so write 2H2. Final answer: 2H2 + O2 gives 2H2O. Only change the big numbers in front - changing a formula changes the substance itself.

12

Explore the idea

Part 12 of 19

Learn

Practice that always works

Write the atom counts underneath each side, adjust one big number, then recount everything. Leave any element that appears in several compounds until last, and if you get stuck with a stubborn odd number, double the whole equation. Two or three attempts is normal; nobody balances every equation on sight.

13

Quick check

Part 13 of 19

Quick check

Balance: H2 + O2 gives H2O

14

Quick check

Part 14 of 19

Quick check

Which is NOT a chemical reaction?

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Never change the small numbers in a formula to balance - only the big numbers in front.
  • Melting and dissolving are physical changes, not chemical reactions.
  • Mass is conserved; a mass drop means a gas escaped.
  • Use an arrow, not an equals sign, in an equation.
16

Challenge round

Part 16 of 19

Game · Sort it

Which of these are true?

Drag each card into the right column. Tap a card first if dragging is fiddly.

True

Not true

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

Melting and dissolving are physical changes: the substance is still the same, so they are not ____.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Chemical reactions and balancing equations

6 questions, marked with the reasoning shown. No timer.

  1. 1. An equation uses which symbol between reactants and products?

  2. 2. To balance an equation you change...

  3. 3. A reaction in an open flask appears to lose mass. Most likely...

  4. 4. Which shows a reaction has happened?

  5. 5. In CH4 + 2O2 → CO2 + 2H2O, oxygen atoms on the left number...

  6. 6. Conservation of mass means...

19

Lesson round-up

Part 19 of 19

Lesson round-up

Ready when you are

Quiz score

Not sat

Games

Not played

Points this lesson

0

Best run

0 in a row

Luna: 0 out of 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do the work

Your written work for this lesson

This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.

Case study

Why the school greenhouse failed

A science club grew tomato plants in the school greenhouse. In June the plants grew well: leaves were dark green and the fruit set quickly. In July the club went away for two weeks and left the greenhouse door shut. When they came back, the leaves were curled and pale and most of the fruit had dropped.

The club had a data logger inside. It showed the temperature reached 46 °C on several afternoons and the soil moisture fell close to zero. The club knew photosynthesis needs light, water and carbon dioxide, and that enzymes in a plant work best in a narrow range of temperature. Above that range the enzymes stop working properly.

The club repeated the summer with the door propped open and a water bottle drip on each pot. Temperature peaked at 31 °C and the plants survived.

Written for Good Learning Co.

Reading questions

  1. Name the variable the club changed the second time, and one variable they kept the same.
  2. Explain, using the words photosynthesis and enzymes, why the plants were damaged in July.
  3. Write a conclusion for the club's report, saying how far the evidence supports it.

  • Method and results

    Writing

    Write the aim, what you changed, what you measured and what you kept the same, then record your results in a table.

    1 page + results table

  • Conclusion

    Writing

    Explain what your results show and why, using the scientific words from the lesson. Say how you know you can trust them.

    6–8 sentences

  • Labelled diagram

    Diagram

    Draw the apparatus or the process in pencil, label it fully, and photograph it.

    1 labelled diagram

  • Reading and notes

    Reading

    Read the lesson text again and write the key ideas in your own words.

    Half a page of notes

Saved as you type.

Part 1 of 19 · Watch & discover

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    Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations

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