A Chemical Equation Must Balance Because Atoms Cannot Vanish
Recognise from observation that a chemical reaction has taken place, turn a word equation into a balanced one with physical states, and balance a skeletal equation by hit and trial while checking that mass is conserved.
Why can a chemical equation never be left unbalanced?
Burn a magnesium ribbon and the white ash left behind weighs more than the ribbon did. That looks like matter being created out of nothing.
It is not. The extra mass is oxygen, taken from the air, and if the whole experiment is done in a sealed container the total mass does not change at all. Atoms are rearranged in a chemical reaction, never created and never destroyed — that statement is the law of conservation of mass, and it is the reason equations must balance.
So an equation has to satisfy a strict condition: every element must have the same number of atoms on both sides. Write
and you have claimed that two oxygen atoms went in and one came out. One atom has vanished, which is impossible. The corrected version is
where magnesium counts on each side and oxygen counts on each side.
An unbalanced equation is not merely untidy — it is a false statement about matter. That is worth holding on to, because everything in this chapter follows from it: the coefficients you put in front of formulas are the numbers that make the sentence true.
This page covers the first part of the CBSE Class 10 Science chapter on chemical reactions and equations: recognising a reaction from observation, writing balanced equations with physical states, and balancing by hit and trial.
It is not. The extra mass is oxygen, taken from the air, and if the whole experiment is done in a sealed container the total mass does not change at all. Atoms are rearranged in a chemical reaction, never created and never destroyed — that statement is the law of conservation of mass, and it is the reason equations must balance.
So an equation has to satisfy a strict condition: every element must have the same number of atoms on both sides. Write
and you have claimed that two oxygen atoms went in and one came out. One atom has vanished, which is impossible. The corrected version is
where magnesium counts on each side and oxygen counts on each side.
An unbalanced equation is not merely untidy — it is a false statement about matter. That is worth holding on to, because everything in this chapter follows from it: the coefficients you put in front of formulas are the numbers that make the sentence true.
This page covers the first part of the CBSE Class 10 Science chapter on chemical reactions and equations: recognising a reaction from observation, writing balanced equations with physical states, and balancing by hit and trial.
How can you tell from looking that a chemical reaction has happened?
Watch for five observable changes: a change of state, a change of colour, the evolution of a gas, a change of temperature, or the formation of a precipitate.
Change of state. A magnesium ribbon burns in air with a dazzling flame, and a solid metal plus a gas becomes a white powdery solid:
Change of colour. Drop a clean iron nail into blue copper sulphate solution. Within minutes the solution fades towards pale green and the nail carries a reddish-brown deposit:
Evolution of a gas. Add dilute sulphuric acid to zinc granules and bubbles rise steadily:
Change of temperature. Add water to quicklime and the container becomes hot enough to be uncomfortable to hold — energy is released, so the reaction is exothermic:
Formation of a precipitate. Mix lead nitrate solution with potassium iodide solution and a bright yellow solid appears at once:
Now the important limitation. None of these five signs proves a chemical reaction on its own, because physical changes can show them too. Ice melting is a change of state; dissolving potassium permanganate turns water purple; boiling water gives off steam; dissolving common salt cools the water slightly.
The real test is whether a new substance has been formed — one with properties different from the starting materials, and one that cannot be recovered by simple physical means such as filtering, evaporating or cooling. Magnesium oxide cannot be turned back into magnesium ribbon by cooling it, while melted ice becomes ice again in a freezer. That difference, not the observation, is what makes a change chemical.
Change of state. A magnesium ribbon burns in air with a dazzling flame, and a solid metal plus a gas becomes a white powdery solid:
Change of colour. Drop a clean iron nail into blue copper sulphate solution. Within minutes the solution fades towards pale green and the nail carries a reddish-brown deposit:
Evolution of a gas. Add dilute sulphuric acid to zinc granules and bubbles rise steadily:
Change of temperature. Add water to quicklime and the container becomes hot enough to be uncomfortable to hold — energy is released, so the reaction is exothermic:
Formation of a precipitate. Mix lead nitrate solution with potassium iodide solution and a bright yellow solid appears at once:
Now the important limitation. None of these five signs proves a chemical reaction on its own, because physical changes can show them too. Ice melting is a change of state; dissolving potassium permanganate turns water purple; boiling water gives off steam; dissolving common salt cools the water slightly.
The real test is whether a new substance has been formed — one with properties different from the starting materials, and one that cannot be recovered by simple physical means such as filtering, evaporating or cooling. Magnesium oxide cannot be turned back into magnesium ribbon by cooling it, while melted ice becomes ice again in a freezer. That difference, not the observation, is what makes a change chemical.
How do you turn a word equation into a balanced one with states?
Write the correct formulas first, then balance, and add the physical states last. Doing it in that order stops you from fiddling with formulas while balancing.
The four state symbols: for solid, for liquid, for gas and for a substance dissolved in water.
Worked example 1. Barium chloride solution reacts with sodium sulphate solution to give a white precipitate of barium sulphate and a solution of sodium chloride.
Formulas: , , , . Now balance — chlorine is on the left, so sodium chloride needs a :
Check: Ba and ; Cl and ; Na and ; the sulphate group and .
Worked example 2. Carbon dioxide passed through lime water gives a precipitate of calcium carbonate and water.
Check: Ca ; C ; H and ; O on the left, and on the right.
Notice the two different symbols for water. Here it is because liquid water is produced. In the quicklime reaction the calcium hydroxide was because it dissolves in the water present. ** means the pure liquid; means dissolved in water — and using one for the other is a standard way to lose a mark.
Worked example 3 — showing the conditions.** Calcium carbonate on strong heating gives calcium oxide and carbon dioxide.
Conditions such as heat, light, pressure or a catalyst are written above or below the arrow, never as a reactant. Heat is not a substance, so it cannot appear on the left of the arrow with the chemicals.
Worked example 4 — precipitate and gas arrows. A downward arrow after a formula marks a precipitate, and an upward arrow marks a gas escaping. Either is an alternative to the state symbol, not an addition to it:
Why the states are worth the trouble. An equation with states tells you what you would actually see — a solid forming, a gas bubbling off, a clear solution staying clear. The same reaction written without states is chemically correct but experimentally silent, which is why the syllabus asks for them and why examiners award them separately.
The four state symbols: for solid, for liquid, for gas and for a substance dissolved in water.
Worked example 1. Barium chloride solution reacts with sodium sulphate solution to give a white precipitate of barium sulphate and a solution of sodium chloride.
Formulas: , , , . Now balance — chlorine is on the left, so sodium chloride needs a :
Check: Ba and ; Cl and ; Na and ; the sulphate group and .
Worked example 2. Carbon dioxide passed through lime water gives a precipitate of calcium carbonate and water.
Check: Ca ; C ; H and ; O on the left, and on the right.
Notice the two different symbols for water. Here it is because liquid water is produced. In the quicklime reaction the calcium hydroxide was because it dissolves in the water present. ** means the pure liquid; means dissolved in water — and using one for the other is a standard way to lose a mark.
Worked example 3 — showing the conditions.** Calcium carbonate on strong heating gives calcium oxide and carbon dioxide.
Conditions such as heat, light, pressure or a catalyst are written above or below the arrow, never as a reactant. Heat is not a substance, so it cannot appear on the left of the arrow with the chemicals.
Worked example 4 — precipitate and gas arrows. A downward arrow after a formula marks a precipitate, and an upward arrow marks a gas escaping. Either is an alternative to the state symbol, not an addition to it:
Why the states are worth the trouble. An equation with states tells you what you would actually see — a solid forming, a gas bubbling off, a clear solution staying clear. The same reaction written without states is chemically correct but experimentally silent, which is why the syllabus asks for them and why examiners award them separately.
How do you balance a skeletal equation by hit and trial?
Count the atoms of each element on both sides, then adjust the coefficients — never the subscripts — until every count matches. Start with the most complicated formula, leave hydrogen and oxygen for last.
Worked example 1. Balance .
The most complex formula is , so work from it. It needs iron and oxygen atoms:
- Put before on the left
- Put before to supply oxygen atoms
- That gives hydrogen atoms on the left, so put before
Check every element.
- Fe: on the left, on the right
- O: on the left, on the right
- H: on the left, on the right
Now verify that mass is conserved, using the atomic masses Fe , H , O :
The two totals agree, which is the law of conservation of mass demonstrated on a single equation — and a far better check than recounting the atoms.
Worked example 2. Balance .
Chlorine appears as on the left and on the right, so make both : put before and before . Then aluminium needs and copper needs :
Check: Al and ; Cu and ; Cl and .
The trick was the lowest common multiple of the two chlorine counts. Whenever one element appears with different subscripts on the two sides, take their LCM and the coefficients follow.
Worked example 3. Balance .
The nitrate group is on the left and on the right, so put before . That gives hydrogen atoms from the acid plus from the hydroxide, so in total, needing water molecules:
Check by group rather than by atom: the nitrate group and ; Ca and ; H and ; O counts and . Treating a polyatomic group as a single unit is much faster than counting its nitrogen and oxygen separately.
Worked example 4 — a combustion. Balance .
Carbon gives carbon dioxide molecules; hydrogen gives water molecules; the oxygen needed is then atoms, that is molecules:
The rule you must never break. Balancing changes only the big numbers in front. Turning into to get an extra oxygen would balance the count and destroy the chemistry — hydrogen peroxide is a different substance from water. Subscripts are part of a substance's identity and are fixed; coefficients are how many of it there are, and they are yours to choose.
Worked example 1. Balance .
The most complex formula is , so work from it. It needs iron and oxygen atoms:
- Put before on the left
- Put before to supply oxygen atoms
- That gives hydrogen atoms on the left, so put before
Check every element.
- Fe: on the left, on the right
- O: on the left, on the right
- H: on the left, on the right
Now verify that mass is conserved, using the atomic masses Fe , H , O :
The two totals agree, which is the law of conservation of mass demonstrated on a single equation — and a far better check than recounting the atoms.
Worked example 2. Balance .
Chlorine appears as on the left and on the right, so make both : put before and before . Then aluminium needs and copper needs :
Check: Al and ; Cu and ; Cl and .
The trick was the lowest common multiple of the two chlorine counts. Whenever one element appears with different subscripts on the two sides, take their LCM and the coefficients follow.
Worked example 3. Balance .
The nitrate group is on the left and on the right, so put before . That gives hydrogen atoms from the acid plus from the hydroxide, so in total, needing water molecules:
Check by group rather than by atom: the nitrate group and ; Ca and ; H and ; O counts and . Treating a polyatomic group as a single unit is much faster than counting its nitrogen and oxygen separately.
Worked example 4 — a combustion. Balance .
Carbon gives carbon dioxide molecules; hydrogen gives water molecules; the oxygen needed is then atoms, that is molecules:
The rule you must never break. Balancing changes only the big numbers in front. Turning into to get an extra oxygen would balance the count and destroy the chemistry — hydrogen peroxide is a different substance from water. Subscripts are part of a substance's identity and are fixed; coefficients are how many of it there are, and they are yours to choose.
Exam tip
What layout keeps an equation question from losing marks?
Write the skeletal equation, then the atom count, then the balanced equation, then add states. Four short lines, each separately creditable.
- Get the formulas right first. A balanced equation built on earns nothing, however neat the arithmetic. Recall the valencies before you start
- Balance the most complex formula first, then metals, then non-metals, then hydrogen, then oxygen. Oxygen last saves the most recounting
- Treat polyatomic groups as single units — nitrate, sulphate, carbonate, hydroxide — as long as the group survives the reaction unchanged
- Use the LCM when one element has different subscripts on the two sides
- Never touch a subscript. Only the coefficients may change
- Write the atom count as a short list under the equation: *Fe and ; O and ; H and .* It shows the balancing was checked rather than guessed
- Add the state symbols last, and use for dissolved and for a pure liquid
- Put conditions above the arrow, not among the reactants
One extra check worth the thirty seconds. Add up the formula masses on each side, as in worked example 1. If the two totals differ, the equation is unbalanced no matter how many times your atom count said otherwise — and the mass check catches an error your counting has already missed once.
- Get the formulas right first. A balanced equation built on earns nothing, however neat the arithmetic. Recall the valencies before you start
- Balance the most complex formula first, then metals, then non-metals, then hydrogen, then oxygen. Oxygen last saves the most recounting
- Treat polyatomic groups as single units — nitrate, sulphate, carbonate, hydroxide — as long as the group survives the reaction unchanged
- Use the LCM when one element has different subscripts on the two sides
- Never touch a subscript. Only the coefficients may change
- Write the atom count as a short list under the equation: *Fe and ; O and ; H and .* It shows the balancing was checked rather than guessed
- Add the state symbols last, and use for dissolved and for a pure liquid
- Put conditions above the arrow, not among the reactants
One extra check worth the thirty seconds. Add up the formula masses on each side, as in worked example 1. If the two totals differ, the equation is unbalanced no matter how many times your atom count said otherwise — and the mass check catches an error your counting has already missed once.
Did you know
Why does a burning magnesium ribbon get heavier?
Weigh a magnesium ribbon, burn it, and weigh the white ash. The ash is heavier — noticeably so. For a substance that has just been destroyed by fire, that is a surprising result, and working out why is a small exercise in conservation of mass.
The reaction is
Every magnesium atom, of mass , has picked up an oxygen atom of mass . So the product has formula mass for every of metal consumed:
**So g of ribbon should leave g of ash, an increase of two-thirds. The mass did not come from nowhere — it came from the air, which is why the gain is invisible in the accounting unless you weigh the oxygen too.
Now the reverse case. Burn a candle and the mass clearly falls, because the products, carbon dioxide and water vapour, escape into the room. Seal the same candle inside a closed container on a balance and the reading does not change at all: what leaves the wax stays in the container.
So the apparent direction of the change depends entirely on what you remember to weigh. A reaction that takes something from the air seems to gain; a reaction that releases a gas seems to lose; a reaction in a sealed vessel shows the truth, which is that the total never moves.
That is why balancing an equation is not a bookkeeping ritual.** It is the arithmetic of a genuine physical law, and the coefficients you write are a prediction: from this much reactant you get exactly that much product — a prediction you can test on a balance.
The reaction is
Every magnesium atom, of mass , has picked up an oxygen atom of mass . So the product has formula mass for every of metal consumed:
**So g of ribbon should leave g of ash, an increase of two-thirds. The mass did not come from nowhere — it came from the air, which is why the gain is invisible in the accounting unless you weigh the oxygen too.
Now the reverse case. Burn a candle and the mass clearly falls, because the products, carbon dioxide and water vapour, escape into the room. Seal the same candle inside a closed container on a balance and the reading does not change at all: what leaves the wax stays in the container.
So the apparent direction of the change depends entirely on what you remember to weigh. A reaction that takes something from the air seems to gain; a reaction that releases a gas seems to lose; a reaction in a sealed vessel shows the truth, which is that the total never moves.
That is why balancing an equation is not a bookkeeping ritual.** It is the arithmetic of a genuine physical law, and the coefficients you write are a prediction: from this much reactant you get exactly that much product — a prediction you can test on a balance.
Exam relevance
Why do JEE and NEET keep coming back to balanced equations?
This is foundation work for the single most calculation-heavy topic in Class 11 Chemistry.
Where it leads. The Class 11 chapter Some Basic Concepts of Chemistry turns a balanced equation into a calculating tool: the coefficients become mole ratios, so an equation tells you how many grams of product a given mass of reactant can yield. Every stoichiometry question in JEE Main and NEET begins by balancing an equation, and a wrong coefficient makes the whole numerical wrong while every later step looks correct.
Worth seeing now. In worked example 1, the coefficients say that moles of iron need moles of steam. With Fe , that is g of iron reacting with g of water to give g of the oxide and g of hydrogen. That calculation is Class 11 stoichiometry, done with nothing but the equation you balanced here.
Where it goes next. Balancing becomes systematic in Class 11 and 12 Redox Reactions, where the oxidation-number and half-reaction methods replace hit and trial for equations too complicated to balance by eye. Those methods are examined directly, and they are built on the same conservation requirement — with charge conserved as well as atoms.
Question types to expect. At this level: write, balance, add states, and identify observations. In competitive papers: mass and mole calculations from a balanced equation, limiting-reagent problems, and assertion-reason items on conservation of mass.
The single trap that costs marks. Changing a subscript to force a balance. It converts the substance into a different compound, and in a competitive paper the resulting answer will match one of the distractors. Subscripts are identity; coefficients are quantity.
A second trap. Forgetting that a gas taken from or released into the air still counts. Limiting-reagent questions are built on exactly the magnesium-ribbon surprise: the oxygen is easy to leave out of the accounting.
Board versus competitive emphasis. The CBSE paper marks the balanced equation, the state symbols and the observation; a competitive paper marks a mass or a number of moles that depends on the coefficients being right. The transferable habit is the mass check — add the formula masses on both sides, and an unbalanced equation cannot hide.
Where it leads. The Class 11 chapter Some Basic Concepts of Chemistry turns a balanced equation into a calculating tool: the coefficients become mole ratios, so an equation tells you how many grams of product a given mass of reactant can yield. Every stoichiometry question in JEE Main and NEET begins by balancing an equation, and a wrong coefficient makes the whole numerical wrong while every later step looks correct.
Worth seeing now. In worked example 1, the coefficients say that moles of iron need moles of steam. With Fe , that is g of iron reacting with g of water to give g of the oxide and g of hydrogen. That calculation is Class 11 stoichiometry, done with nothing but the equation you balanced here.
Where it goes next. Balancing becomes systematic in Class 11 and 12 Redox Reactions, where the oxidation-number and half-reaction methods replace hit and trial for equations too complicated to balance by eye. Those methods are examined directly, and they are built on the same conservation requirement — with charge conserved as well as atoms.
Question types to expect. At this level: write, balance, add states, and identify observations. In competitive papers: mass and mole calculations from a balanced equation, limiting-reagent problems, and assertion-reason items on conservation of mass.
The single trap that costs marks. Changing a subscript to force a balance. It converts the substance into a different compound, and in a competitive paper the resulting answer will match one of the distractors. Subscripts are identity; coefficients are quantity.
A second trap. Forgetting that a gas taken from or released into the air still counts. Limiting-reagent questions are built on exactly the magnesium-ribbon surprise: the oxygen is easy to leave out of the accounting.
Board versus competitive emphasis. The CBSE paper marks the balanced equation, the state symbols and the observation; a competitive paper marks a mass or a number of moles that depends on the coefficients being right. The transferable habit is the mass check — add the formula masses on both sides, and an unbalanced equation cannot hide.
Key takeaways
What should you be able to do with chemical equations before Part 2?
One physical law, and the writing conventions that respect it.
- Atoms are rearranged, never created or destroyed, so every element must have equal counts on both sides
- Five observable signs of a reaction: change of state, change of colour, evolution of a gas, change of temperature, formation of a precipitate
- None of them proves a chemical change alone — the test is whether a new substance has formed that simple physical means cannot reverse
- Write formulas first, balance second, add states last
- State symbols: , , and — with for a pure liquid and for dissolved
- Conditions such as heat or light go above the arrow, never among the reactants
- Balance the most complex formula first, then metals, non-metals, hydrogen and oxygen last
- Treat polyatomic groups as units and use the LCM when subscripts differ across the arrow
- Only coefficients may change; subscripts are fixed — they are part of the substance's identity
- Verify with formula masses on both sides, not just by recounting atoms
The quickest self-test is worked example 1. Balance from a blank page, then add the formula masses on both sides and check that both come to .
- Atoms are rearranged, never created or destroyed, so every element must have equal counts on both sides
- Five observable signs of a reaction: change of state, change of colour, evolution of a gas, change of temperature, formation of a precipitate
- None of them proves a chemical change alone — the test is whether a new substance has formed that simple physical means cannot reverse
- Write formulas first, balance second, add states last
- State symbols: , , and — with for a pure liquid and for dissolved
- Conditions such as heat or light go above the arrow, never among the reactants
- Balance the most complex formula first, then metals, non-metals, hydrogen and oxygen last
- Treat polyatomic groups as units and use the LCM when subscripts differ across the arrow
- Only coefficients may change; subscripts are fixed — they are part of the substance's identity
- Verify with formula masses on both sides, not just by recounting atoms
The quickest self-test is worked example 1. Balance from a blank page, then add the formula masses on both sides and check that both come to .