On this page
You can probably balance Mg + O₂ → MgO in seconds. The trouble starts with equations like barium chloride plus aluminium sulphate, where every change you make seems to break something else. This post gives you one fixed routine that works on every equation in the chapter, three worked examples taken from the NCERT exercises, an algebra method for the stubborn cases, and the mistakes that cost marks.
What does the Class 10 syllabus ask you to do?
The CBSE Class X Science syllabus for 2026–27 lists "Chemical reactions, Chemical equation, Balanced chemical equation" in Unit I, before the types of reactions. Chapter 1 of the NCERT Class 10 Science textbook (Reprint 2026–27) explains why: by the law of conservation of mass, the number of atoms of each element must be the same before and after a reaction. NCERT calls its approach the hit-and-trial method. The routine below is the same idea with the trials put in a sensible order. If you want the reaction types, read our guide to types of chemical reactions in Class 10. Check the syllabus for your own exam year before you revise.
What is the routine, step by step?
Write correct formulas first, then freeze them. NCERT suggests drawing a box around each formula. Nothing inside a box may change.
Make an atom table. List every element with its count on the left and right.
Treat a polyatomic ion as one unit if it appears unchanged on both sides, such as SO₄ or NO₃. Count "SO₄" instead of counting S and O separately.
Balance compounds first, free elements last. An element on its own (O₂, H₂, Fe, Zn) appears in only one place, so changing its coefficient cannot disturb anything else. Save it for the end.
Allow a fraction, then clear it. If oxygen needs 3/2 O₂, write it, finish, then multiply every coefficient by 2.
Recount every element and add state symbols if the question asks for them.
Worked example 1: keep the sulphate together
From NCERT Exercise 5(c): BaCl₂ + Al₂(SO₄)₃ → BaSO₄ + AlCl₃
SO₄ appears unchanged on both sides, so treat it as a unit. Left: 3 SO₄. Right: 1. Write 3BaSO₄.
Barium is now 3 on the right, so write 3BaCl₂.
Chlorine is now 6 on the left. Each AlCl₃ has 3, so write 2AlCl₃.
Aluminium: 2 on the left, 2 on the right.
Balanced: 3BaCl₂ + Al₂(SO₄)₃ → 3BaSO₄ + 2AlCl₃. Check: Ba 3 and 3, Cl 6 and 6, Al 2 and 2, SO₄ 3 and 3.
Worked example 2: let oxygen go last
From Exercise 5(b), hydrogen sulphide burning in air: H₂S + O₂ → H₂O + SO₂
Sulphur: 1 and 1. Hydrogen: 2 and 2. Leave O₂ alone for now because it is a free element.
Oxygen on the right: 1 in H₂O plus 2 in SO₂ makes 3. To get 3 oxygen atoms from O₂ you need 3/2 O₂.
Multiply everything by 2: 2H₂S + 3O₂ → 2H₂O + 2SO₂.
Check: S 2 and 2, H 4 and 4, O 6 on the left and 2 + 4 = 6 on the right.
Worked example 3: an odd number of hydrogen atoms
From Exercise 5(d): K + H₂O → KOH + H₂
Hydrogen is 2 on the left but 1 + 2 = 3 on the right, an odd number. Balance the compounds first (K, O and the H in KOH match with one of each), then fix the free element: K + H₂O → KOH + ½H₂. Clear the fraction: 2K + 2H₂O → 2KOH + H₂. Check: K 2 and 2, H 4 and 4, O 2 and 2.
What if hit-and-trial goes in circles?
Use algebra, which always reaches an answer. Put a letter in front of each formula: a H₂S + b O₂ → c H₂O + d SO₂. Write one equation per element:
Sulphur: a = d
Hydrogen: 2a = 2c, so c = a
Oxygen: 2b = c + 2d = 3a, so b = 3a/2
Choose a = 2 to avoid fractions: a = 2, b = 3, c = 2, d = 2, the same answer as before. You will rarely need this in Class 10, but knowing it exists stops panic in the exam hall. In Class 11 you will meet redox equations where a related, more structured method helps; our post on balancing redox reactions shows it.
Which mistakes cost the most marks?
Changing a subscript. Writing H₂O₂ to "fix" oxygen turns water into hydrogen peroxide. In a study of 156 students, Sanger (2005) found that confusing subscripts with coefficients was among the most common errors, and students who made it did worse on the follow-up stoichiometry calculations.
Writing an unreacted substance into the equation. The same study found students adding leftover reactant to both sides, which led to wrong reacting ratios.
Missing atoms inside brackets. Ca(NO₃)₂ has 2 nitrogen and 6 oxygen atoms, not 1 and 3.
Not reducing. 4H₂S + 6O₂ → 4H₂O + 4SO₂ balances, but the smallest whole numbers are 2, 3, 2, 2.
Writing O instead of O₂ for oxygen gas, or H instead of H₂.
If you keep getting stuck at the same step, the Fermi Problem Library has worked problems with a step-by-step solver that explains the reasoning, so you can see where your order of steps went wrong.
A practice routine for this week
Balance the four equations in NCERT Exercise 6 using the six steps and an atom table. Answers to check against: 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O; 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O; NaCl + AgNO₃ → AgCl + NaNO₃ (already balanced); BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl.
Do Exercise 7 the next day without looking at your notes.
Pick one equation and balance it by algebra as well, to see both methods agree.
After a week, mix five equations from the whole chapter and time yourself.
Tonight, take Exercise 5(b) and balance it twice: once with the routine and once with letters.
