Chemical reactions and equations
In a chemical reaction, no atoms are created or destroyed: they are rearranged into new substances. Everything about equations follows from that single idea.
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
- • Evidence of a chemical change
- • Word and symbol equations
- • Balancing
- • Conservation of mass
This lesson at a glance
- 30 minutes
- 21 parts to scroll through
- 4 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 21
Visual introductionPicture this
Chemical reactions and equations
In a chemical reaction, no atoms are created or destroyed: they are rearranged into new substances. Everything about equations follows from that single idea.
In a nutshell
Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations
Learning cycle
Part 2 of 21
Learning cycle 1 of 2
Part 1 · Evidence of a chemical change
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 3 of 21
Learn
Evidence of a chemical change
A colour change, a gas released, a temperature change or a new solid forming all suggest a chemical reaction. The decisive test is whether new substances with different properties are made and whether the change is difficult to reverse; melting ice is a physical change, burning magnesium is not.
Reset break
Part 4 of 21
Pause
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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.
Explore the idea
Part 5 of 21
Learn
Word and symbol equations
Reactants are written on the left, products on the right: magnesium + oxygen → magnesium oxide, or 2Mg + O₂ → 2MgO. State symbols (s), (l), (g) and (aq) add useful detail about the conditions of the reaction.
Quick check
Part 6 of 21
Quick check
Part 7 of 21
Reset break
Part 8 of 21
Pause
That's sitting 2 of 5 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.
Learning cycle
Part 9 of 21
Learning cycle 2 of 2
Part 2 · Balancing
A short piece of teaching, then a check to make sure it has landed.
Explore the idea
Part 10 of 21
Learn
Balancing
Because atoms are only rearranged, each element must appear in equal numbers on both sides. Balance by changing the large numbers in front of formulae, never the subscripts inside them: changing O₂ to O₃ would change the substance itself rather than the quantity.
Explore the idea
Part 11 of 21
Learn
Conservation of mass
The total mass before a reaction equals the total mass after. If a burning metal appears to gain mass, it has combined with oxygen from the air; if a reaction in an open beaker appears to lose mass, a gas has escaped. Repeating the experiment in a sealed container shows the mass is unchanged.
Reset break
Part 12 of 21
Pause
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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.
Quick check
Part 13 of 21
Quick check
Part 14 of 21
Explore the idea
Part 15 of 21
Worked example
Worked answer: 4.8 g of magnesium burns to form 8.0 g of magnesium oxide. Explain the mass increase (3 marks)
Mass is conserved, so no atoms have been created (1). The magnesium has reacted with oxygen from the air, and 8.0 − 4.8 = 3.2 g of oxygen has combined with it (1). The oxygen atoms are now part of the solid product rather than in the air, so the solid on the balance is heavier while the total mass of everything involved is unchanged (1).
Reset break
Part 16 of 21
Pause
That's sitting 4 of 5 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.
Challenge round
Part 17 of 21
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
Challenge round
Part 18 of 21
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
A colour change, a gas released, a temperature change or a new solid forming all suggest a ____ reaction.
Challenge round
Part 19 of 21
Game · Recall cards
What happens to atoms during a chemical reaction?
Card 1 of 4
Mastery quiz
Part 20 of 21
Marked quiz
End of lesson quiz: Chemical reactions and equations
4 questions, marked with the reasoning shown. No timer.
1. What happens to atoms during a chemical reaction?
2. In 2Mg + O2 → 2MgO, how many magnesium atoms are on each side?
3. Why must chemical equations be balanced?
4. Balancing an equation means changing
Lesson round-up
Part 21 of 21
Lesson round-up
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Understand chemical reactions as the rearrangement of atoms, represented by word and symbol equations