Balancing Chemical Equations: Match Every Atom
Understand that atoms are conserved in a chemical reaction, so a correct equation has the same number of each element on both sides, achieved by changing coefficients and never subscripts.
Before you try
What if you just changed H₂O into H₃O to make the numbers fit? Would the equation still describe water?
Try it out
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What happened?
Move a control to see what changes and why.
Balancing settles the counts only. It does not tell you whether a reaction really happens, how fast it goes, or what conditions it needs.
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Try these ideas
- Balance hydrogen and oxygen into water and count the oxygen atoms on each side.
- Try methane burning, and leave oxygen until last.
- Set every coefficient to double the correct answer and read the warning.
Why it works
In a chemical reaction the atoms are rearranged into new combinations, but not a single one is created or destroyed. That is conservation of mass, and it is why a correct equation must have the same number of each element on the left and the right. The big numbers written in front of a formula are coefficients, and they multiply the whole molecule. The small subscripts inside a formula say how many atoms are bonded together, and changing one would give an entirely different substance.
In this simulator you pick a reaction and move one slider per species, watching the atom counts update in a table and the molecules redraw as coloured, lettered atoms. The panel tells you which element is short and on which side. The equation is judged balanced only when every element matches, and it also warns you when all your coefficients share a common factor.
Check your understanding
Question 1 of 4
Why must a chemical equation be balanced?
Which number are you allowed to change when balancing?
How many water molecules balance 2H₂ + O₂ → ?H₂O?
You balance an equation as 4H₂ + 2O₂ → 4H₂O. What is wrong?
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
What will you discover next?
Try Atomic Structure: Make Elements from Particles next.
For parents & teachers
- Count the atoms in a burning candle reaction together on paper.
- Ask where the mass of a burnt matchstick has gone.
- Discuss why a sealed jar does not change mass when a reaction happens inside it.