Subscripts and coefficients look alike but do different jobs. A subscript says what the substance is. A coefficient says how much of it there is.
This page helps when balancing goes wrong because a formula was changed along the way. The skill of balancing itself is taught in balancing chemical equations.
What does it sound like when this happens?
Students describe it in similar words.
- “I changed the 2 in H₂O to make the oxygen match.”
- “I keep adjusting the little numbers until it looks right.”
- “The count works but my teacher says the equation is wrong.”
What is usually behind it?
Both numbers are small and sit next to the symbols, so they feel like the same kind of number. The pressure to get the atom counts equal makes changing a subscript look like the fastest fix.
The deeper cause is not seeing a formula as one fixed substance. Once the formulae are locked, the only thing left to change is how many of each substance you have, and that is the coefficient.
What does each number do?
| Number | Where | What it does | Can it change when balancing? |
|---|---|---|---|
| Subscript | Small, inside the formula | Shows atoms in one unit of the substance | No |
| Coefficient | Large, in front | Shows how many units of the substance | Yes |
Worked example: hydrogen and oxygen, wrong and right
Start with H₂ + O₂ → H₂O. The left has 2 O, the right has 1 O.
Wrong: change H₂O to H₂O₂ so that there are 2 O on each side. The atom counts now match, but H₂O₂ is hydrogen peroxide, not water.
Right: keep H₂O as written and put a 2 in front, giving 2H₂O. Now there are 2 O and 4 H on the right, so put a 2 in front of H₂ on the left.
2H₂ + O₂ → 2H₂O. Both sides have 4 H and 2 O.
Worked example: balancing with brackets
Balance Mg(OH)₂ + HCl → MgCl₂ + H₂O.
The 2 outside the bracket doubles both O and H inside it. So Mg(OH)₂ has 1 Mg, 2 O and 2 H.
| Atom | Left (before) | Right (before) |
|---|---|---|
| Mg | 1 | 1 |
| Cl | 1 | 2 |
| H | 2 + 1 = 3 | 2 |
| O | 2 | 1 |
Put a 2 in front of HCl (2 Cl) and a 2 in front of H₂O (2 O). Then H is 2 + 2 = 4 on the left and 4 on the right.
Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O.
What can you do this week?
- Write every formula in the equation first, then underline the subscripts.
- Promise yourself that the underlined numbers will not change.
- Make a small atom-count table for each equation.
- Adjust one coefficient at a time and update the table.
The chemical equation balance checker lets you test yours. For equations that include charges, see writing ionic equations with correct charges.
When might one-to-one tuition help?
If you still touch a subscript when the clock is running, a teacher can balance live equations with you and stop you the moment it happens. That is how the habit forms.
See online one-to-one Chemistry tuition for how starting works. The mistake log shows whether the error repeats.