KS3 · Chemistry
Conservation of mass and balanced equations
Put a solid in a flask, let it react, and the scales don't move. So where did the solid go? The answer is hiding in the atoms.
Chemistry · Reaction balancer
Atom audit: tip the equation into balance
A reaction only rearranges atoms, so every element must add up the same on both sides of the arrow. Press + or − to change the big number in front of a whole formula, and watch the tally under it.
Hydrogen and oxygen make water. This time one fix upsets the other element, so you will go round the count-and-add loop more than once. Notice that the small numbers inside H₂, O₂ and H₂O never move.
Reading an equation
Start with the word equation: sodium + bromine → sodium bromide. Reactants are on the left, products are on the right, the + means 'adding' and the arrow means 'changes into'. Now the symbol equation, which uses formulae. Tap each part to decode it.
Tap a molecule to see what it does in the reaction.
Predict, then check
Commit to an answer before you open the reveal.
Calcium carbonate and hydrochloric acid are in a conical flask, with a balloon on top to collect the carbon dioxide gas that is made. The flask, balloon and chemicals read 100 g on a balance at the start. What will the balance read once the reaction has finished?
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
What you need to know
- In a reaction, atoms are rearranged to make new substances. None are created or destroyed, so total mass of reactants = total mass of products.
- A subscript (small number) counts the atoms of the element it follows. A coefficient (big number in front) multiplies the whole formula.
- To balance an equation, change coefficients only until each element has the same number of atoms on both sides of the arrow.
The big picture
In a chemical reaction atoms are rearranged, never created or destroyed, so the mass of the products equals the mass of the reactants. A balanced symbol equation shows the same number of atoms of each element on both sides of the arrow. You balance by changing coefficients (the big numbers in front of formulae), never subscripts.
Key points
Worked example
Problem
127 g of copper reacts with 32 g of oxygen to make copper oxide. What mass of copper oxide is formed?
⚠ Watch out
Changing a small number to make the atoms match. Writing H₂O₂ instead of H₂O 'fixes' the oxygen count, but H₂O₂ is hydrogen peroxide, not water. Put a big number in front of the whole formula instead.
Memory hook
Big numbers are dials, small numbers are name tags. Turn the dials as much as you like, but never touch a name tag. (The comparison breaks down in one place: change a subscript and you don't get a relabelled substance, you get a different one.)
Check yourself
From memory: a reaction makes 80 g of product from two reactants, and one reactant is 35 g. What is the other? (Answer: 45 g, because the reactants must total 80 g.)
Flashcards
(14)In a chemical reaction, what happens to the atoms?
What do + and → mean in a word equation?
What does a subscript tell you? (the 2 in H₂O)
What does a coefficient tell you? (the 2 in 2MgO)
How many atoms of each element are in CuSO₄?
When balancing, which numbers can you change?
Why can't you change H₂O to H₂O₂ to balance an equation?
What does 'balanced' mean for a symbol equation?
What is the method for balancing a symbol equation?
How many atoms of each element are on each side of 2Na + Br₂ → 2NaBr?
A reaction makes 256 g of a single product. What mass of reactant was there?
How do you find a missing mass in a reaction?
In the calcium carbonate and acid flask with a balloon, why did the balance still read 100 g?
What does the symbol Σ mean?
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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How this lesson was checked. This KS3 Chemistrylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 2 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.