How to Balance Chemical Equations — CBSE Class 10 Guide
A Step-by-Step Method to Apply the Law of Conservation of Atoms in Board Exams
You have probably counted things to make sure nothing was missing — rotis on a plate, chairs before a function, students before a school trip.
Balancing a chemical equation is the same instinct. Atoms go into a reaction. Every single one must come out the other side. Not one extra. Not one is missing.
The Dal That Would Not Add Up
Meena’s mother runs a small dhaba on the highway outside Varanasi. Every morning, Meena watches her mother measure out dal, water, and spices into a pressure cooker. The dal softens. Steam rises. The smell changes. But her mother never throws anything away mid-cook and never adds mystery ingredients halfway through.
“Whatever goes in,” her mother says, “comes out in some form.”
Meena accepted this without thinking much about it. Food goes in. Food comes out. Different shape, different smell, same ingredients underneath.
Chemists found that the same rule holds for every reaction in the universe. Atoms that enter a reaction must all appear in the products. They rearrange. They combine differently. But not one atom disappears, and not one new atom appears from nowhere.
That rule has a name. And now that you understand what it does — you are ready to hear it.
What the Rule Is Actually Called
The rule Meena’s mother followed without knowing it is called the Law of Conservation of Mass.
For atoms specifically: the number of each type of atom on the left side of a chemical equation must equal the number of the same type of atom on the right side.
When that condition is met, the equation is balanced.
An unbalanced equation is not wrong chemistry — it is incomplete accounting. The reaction happened correctly. The equation has not been written correctly yet.
One term to hold onto: the numbers placed in front of chemical formulas to balance an equation are called stoichiometric coefficients — or simply, coefficients. You adjust these numbers. You never change what is inside the formula itself.
How the Balancing Actually Works — Step by Step
Take this reaction: hydrogen gas combines with oxygen gas to form water.
Unbalanced: H₂ + O₂ → H₂O
Step 1 — Count every atom on both sides.
| Atom | Left Side | Right Side |
|---|---|---|
| H | 2 | 2 |
| O | 2 | 1 |
Hydrogen matches. Oxygen does not. One oxygen atom has gone missing on the right side. This equation is unbalanced.
Step 2 — Start with the most complex molecule.
Water (H₂O) has two elements. Start there. Place a coefficient of 2 in front of H₂O.
H₂ + O₂ → 2H₂O
Step 3 — Recount.
| Atom | Left Side | Right Side |
|---|---|---|
| H | 2 | 4 |
| O | 2 | 2 |
Oxygen now matches. But hydrogen does not — 2 on the left, 4 on the right.
Step 4 — Adjust the left side to match.
Place a coefficient of 2 in front of H₂.
2H₂ + O₂ → 2H₂O
Step 5 — Final count.
| Atom | Left Side | Right Side |
|---|---|---|
| H | 4 | 4 |
| O | 2 | 2 |
Every atom matches. The equation is balanced.
2H₂ + O₂ → 2H₂O
Meena’s mother would approve. Everything that went in came out.
The Formula Rule — Don’t Panic
Every symbol in a balanced equation is something you can count. You are not doing advanced mathematics. You are doing careful counting with a system.
The coefficient rule:
Coefficient × Subscript = Total atoms of that element in that molecule
So in 2H₂O:
- Coefficient = 2
- Subscript of H = 2
- Total hydrogen atoms = 2 × 2 = 4
- Subscript of O = 1
- Total oxygen atoms = 2 × 1 = 2
This one formula does all the counting work. Every time you feel lost in a balancing problem, come back to it: coefficient × subscript = total atoms.
And the rule that governs everything else:
Atoms on left = Atoms on right (for every element)
You are not solving for x. You are adjusting coefficients until both sides of that equation hold true.
A Harder Example — Iron and Oxygen
Now try the reaction from the last article. Iron rusts when it reacts with oxygen.
Unbalanced: Fe + O₂ → Fe₂O₃
Count first:
| Atom | Left | Right |
|---|---|---|
| Fe | 1 | 2 |
| O | 2 | 3 |
Neither element matches. Start with the more complex molecule — Fe₂O₃.
Round 1: To get 3 oxygen atoms on the left from O₂ molecules, you need a fraction — or you can scale up. Multiply Fe₂O₃ by 2 to make oxygen easier to work with.
Fe + O₂ → 2Fe₂O₃
New oxygen on right: 6. So place 3 in front of O₂.
Fe + 3O₂ → 2Fe₂O₃
New iron on right: 4. So place 4 in front of Fe.
4Fe + 3O₂ → 2Fe₂O₃
Final count:
| Atom | Left | Right |
|---|---|---|
| Fe | 4 | 4 |
| O | 6 | 6 |
Balanced. Every atom accounted for. Nothing lost. Nothing invented.
Where You See This Beyond Exams
Cooking fuel in a gas cylinder: The LPG in your kitchen cylinder is mostly butane — C₄H₁₀. When it burns, it reacts with oxygen to produce carbon dioxide and water. The balanced equation for that combustion tells engineers exactly how much oxygen is needed per unit of fuel. Get the ratio wrong in an industrial burner, and either fuel is wasted or combustion is incomplete.
Making medicines in bulk: Pharmaceutical factories produce compounds through chain reactions. Each reaction must be balanced to calculate exactly how many kilograms of starting material produce how many kilograms of final product. A 2% error in stoichiometry at an industrial scale means tons of material wasted or lost.
Water treatment plants: Chlorine is added to water in precise quantities to kill bacteria. Too little — bacteria survive. Too much — the water becomes harmful. The correct quantity is calculated from a balanced equation. The plant outside Patna that supplies drinking water to surrounding villages uses this calculation every single day.
Where Students Drop Marks
Mistake 1 — Changing the subscript instead of the coefficient. A student sees Fe + O₂ → Fe₂O₃ and writes Fe + O₃ → Fe₂O₃ to “fix” the oxygen count. This changes the substance entirely. O₂ and O₃ are different molecules — oxygen gas and ozone. You adjust what is in front of the formula, never what is inside it.
Mistake 2 — Counting atoms before applying the coefficient. Students often count H₂O as 1 hydrogen and 1 oxygen, forgetting the coefficient in front. A common confusion here is treating 2H₂O the same as H₂O. The coefficient doubles everything inside. Always multiply first, then count.
Mistake 3 — Stopping after one element matches. Students balance oxygen, see it matches, and stop. Then they submit an equation where hydrogen is still wrong. Check every element, every time. One table, every atom, before calling it balanced.
Summary
- Law of Conservation of Mass: atoms are neither created nor destroyed in a chemical reaction
- Balanced equation: same number of each atom on both sides
- Coefficient: number placed in front of a formula — adjusts atom count without changing the substance
- Subscript: number inside a formula — never change this
- Key formula: Coefficient × Subscript = Total atoms of that element
- Sequence: count → identify mismatch → adjust coefficient of complex molecule → recount → repeat until balanced
In Five Sentences, Three, and One
In five sentences: A chemical equation must show the same number of each atom on both sides. This follows the Law of Conservation of Mass — atoms cannot appear or disappear during a reaction. To balance an equation, you adjust the coefficients in front of each formula. You never change the subscripts inside a formula, as that would change the substance itself. Balance one element at a time, starting with the most complex molecule, until every atom matches.
In three sentences: Balancing a chemical equation means making sure every atom that enters a reaction also appears in the products. You do this by adjusting coefficients — the numbers in front of formulas. Subscripts inside formulas are never touched.
In one sentence: A balanced equation is proof that every atom entering a reaction has been honestly accounted for on the other side.
Practice Questions
- Balance this equation: Mg + HCl → MgCl₂ + H₂
- Balance this equation: CH₄ + O₂ → CO₂ + H₂O
- A student writes: H₂ + O₃ → H₂O to balance a water-formation equation. What error have they made?
- How many atoms of oxygen are present in 3Fe₂O₃? Show your working using the coefficient × subscript method.
- Why is it incorrect to say that atoms are “destroyed” during a chemical reaction?
Students Ask These Questions
Q: Do I always start with the most complex molecule? It is the most reliable starting point for Class 10 equations. The most complex molecule usually has the most elements, so fixing it first reduces the adjustments needed elsewhere. For very simple equations with only two elements, it matters less — but the habit protects you in harder problems.
Q: Can coefficients be fractions? While working, yes — fractions are sometimes used as a temporary step. But a final balanced equation must use whole numbers only. If you end up with fractions, multiply every coefficient in the equation by the denominator to clear them.
Q: What if I try a coefficient and it makes things worse? That is part of the process. Balancing is trial and adjustment, not a one-shot calculation. Professional chemists use the same trial method for simple equations. The step-by-step count after each adjustment tells you whether you moved in the right direction.
Q: Is the equation for a reaction always unique? The simplest whole-number ratio of coefficients is the standard form. You could technically multiply every coefficient by 2, and the equation would still be balanced — but convention requires the smallest possible whole numbers. Examiners expect the simplified form.
Q: Why can I never change a subscript? Because the subscript defines what the substance is. H₂O is water. H₂O₂ is hydrogen peroxide. They are chemically different and behave completely differently in reactions. Changing a subscript to balance an equation is like changing the ingredients of a recipe — you have not fixed the cooking, you have made a different dish entirely.
Related reading: Chemical Reactions and Equations — Class 10 CBSE | Oxidation and Reduction Reactions — Class 10 | Mole Concept and Stoichiometry — Class 11
