Chemical Reactions: Reactants, Products and Equations
Chemical Reactions
This KS3 chemistry lesson explores Chemical Reactions: Reactants, Products and Equations. Students identify evidence of chemical change, distinguish reactants from products and represent simple reactions using word equations. It provides a clear introduction to the topic and can be used for classroom teaching, homework or revision.
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Chemical Reactions
This KS3 chemistry lesson explores Chemical Reactions: Reactants, Products and Equations. Students identify evidence of chemical change, distinguish reactants from products and represent simple reactions using word equations. It provides a clear introduction to the topic and can be used for classroom teaching, homework or revision.
Chemical Reactions, KS3 Science, Chemistry, Reactants, Products, Word Equations, Conservation of Mass, Endothermic, Exothermic, Chemical Change, MCQs, Open Questions, Polls, Emoji Polls, Exit Tickets
What are some chemical reactions you use in your everyday life?
Use this to surface prior ideas such as atoms, change, burning, bubbles, products, heat, colour, or new substances. Pick out misconceptions too, such as 'melting' or 'boiling', to revisit later.
What is a chemical reaction?
A chemical reaction is a rearrangement of atoms.
In a reaction, reactants change into products. The atoms are not lost or created; they are joined in new combinations to make new substances.
Emphasise particle-level reasoning: the key idea is not that matter disappears, but that atoms are rearranged into different substances.
Reactants and products
Reactants are the starting substances
Products are the substances formed
Reactions can be shown with a word equation
Example: magnesium + oxygen → magnesium oxide
Use the example to link language to structure: left side means reactants, right side means products. Avoid going into ionic detail unless needed.
Atoms are rearranged
In a chemical reaction, the types of atoms stay the same, but the way they are grouped changes.
This is why the products have different properties from the reactants: the atoms are now bonded in a different arrangement.
A common misconception is that atoms turn into different atoms. Stress that chemical reactions rearrange atoms; they do not change one element into another.
New substances form
A reaction makes one or more new substances
New substances have their own properties
This is the main difference between a reaction and a simple change of state
You can compare burning magnesium with melting ice: both involve change, but only one forms a new substance.
Chemical or physical change?
A chemical reaction forms new substances.
A physical change does not form a new substance. The material may change state, shape or size, but it is still the same substance.
Keep the distinction crisp. Students often confuse dramatic physical changes, such as boiling, with chemical change because they can see bubbles.
Comparing the two
Physical change
New substance formed
Atoms rearranged
Often difficult to reverse
Example: iron rusting
Chemical reaction
No new substance formed
Same particles remain
Often easier to reverse
Example: ice melting
The word 'often' matters here. Avoid absolute statements about reversibility, because some physical changes are hard to reverse and some reactions can be reversed.
A useful test
To decide whether a change is chemical or physical, ask:
Has a new substance formed?
If the answer is yes, it is a chemical reaction.
This gives students a practical rule they can apply across examples. Link back to reactants and products as evidence of new substances.
Which statement best describes a chemical reaction?
Atoms rearrange
Atoms disappear
Mass is destroyed
Elements change
Tests the core idea from slides 2–4. Distractors target common misconceptions: atoms are not lost, mass is not destroyed, and chemical reactions do not turn one element into another.
Complete the summary
In a chemical reaction, reactants change into products. The atoms are rearranged, and a new substance forms. In a physical change, no new substance is made.
Echoes the exact language of slides 2, 5 and 6. Each blank is load-bearing and has one clear position.
Signs of a reaction
Common evidence that a chemical reaction may have happened includes:
gas production
colour change
temperature change
precipitate formation
These signs suggest that new substances have formed.
Use the wording 'may have happened' carefully. A visible sign is evidence, but students should still connect it to the formation of new substances.
Gas production
Bubbles or fizzing can show that a gas is being formed
The gas is a product of the reaction
This is different from boiling, where no new substance is made
This is a key misconception: bubbles do not always mean a chemical reaction. Contrast gas from a reaction with water vapour formed during boiling.
Colour and temperature change
A reaction may produce a clear colour change
Energy can be transferred, causing a temperature change
The mixture may become warmer or cooler as products form
Do not overcomplicate with exothermic and endothermic unless useful as extension language. The main point is that temperature change can be evidence of reaction.
Precipitate formation
A precipitate is an insoluble solid formed when two solutions react.
If a solid suddenly appears in a liquid mixture, this is strong evidence that a new substance has been made.
Students may not know 'insoluble'. Briefly gloss it as 'does not dissolve in the liquid'.
Which is the strongest evidence that a new substance has formed?
Colour change
Gas produced
Precipitate forms
All can show it
Builds from slides 9–12. The key idea is that several signs can suggest a reaction, not just one favourite clue.
Complete the reaction evidence summary
A chemical reaction may be shown by gas production, a colour change, or a temperature change. A precipitate is an insoluble solid that appears when two solutions react.
The distractors target confusion between evidence of reaction and physical changes such as boiling or melting.
Why do bubbles not always mean a chemical reaction?
Expected elements: students should explain that bubbles alone are not enough, because boiling also makes bubbles without forming a new substance. Good answers may mention checking whether a new substance has formed.
Mass is conserved
In a chemical reaction, atoms are rearranged
Because the atoms are still there, total mass is conserved
In a closed system, the mass before and after the reaction stays the same
Make the closed system explicit. Students often think mass changes because gas escapes from an open container.
Why a closed system matters
If gas is produced in an open system, it can escape, so the measured mass may seem to decrease.
In a closed system, all reactants and products stay inside, so the total measured mass remains constant.
This is the main explanatory turn for apparent mass loss. Keep the distinction between actual mass and measured mass clear.
Word equations and mass
Word equations show which reactants change into which products.
Example:
magnesium + oxygen → magnesium oxide
The equation shows a new substance forming, while the total atoms must still balance.
Bridge from names to particle conservation. The word equation does not show numbers of atoms, but it helps students identify reactants and products.
Balanced symbol equations
Symbol equations show formulas instead of names.
Example:
2Mg + O₂ → 2MgO
A balanced equation has the same number of each type of atom on both sides.
Explain that the large numbers in front are coefficients. Avoid changing the small numbers in formulas, because that would change the substance.
Checking an equation
Left side
2Mg + O₂
magnesium atoms: 2
oxygen atoms: 2
Right side
2MgO
magnesium atoms: 2
oxygen atoms: 2
Use this slide to show that balancing is evidence for conservation of mass at the particle level, not just a mathematical rule.
Why does mass stay the same in a closed system?
Atoms escape slowly
Atoms are rearranged
Products weigh less
Energy adds mass
Links slides 13–17. Students should connect conservation of mass to atoms still being present, just in new arrangements.
Complete the conservation summary
In a closed system, total mass stays the same because atoms are not lost. A balanced symbol equation has the same number of each type of atom on both sides.
Reinforces the exact meaning of closed system and balanced equation from slides 13–17.
Why can measured mass fall in an open system?
Expected elements: gas can escape from an open system, so the measured mass can seem to decrease even though total mass is conserved overall.
How confident do you feel with chemical reactions now?
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Glossary 1
chemical reaction - a process in which atoms are rearranged and reactants are changed into products
reactant - a starting substance in a chemical reaction
product - a substance formed in a chemical reaction
physical change - a change that does not form a new substance
word equation - an equation that shows reactions using the names of substances
symbol equation - an equation that shows reactions using chemical formulae
balanced equation - an equation with the same number of each type of atom on both sides
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Glossary 2
conservation of mass - the idea that total mass stays the same in a chemical reaction
closed system - a system where no substances can enter or leave
gas production - evidence of a reaction where a gas is formed
colour change - a visible change in colour that can show a new substance has formed
temperature change - warming or cooling during a reaction as energy is transferred
precipitate - an insoluble solid formed when solutions react
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