The core idea
Chemical reactions rearrange atoms into new chemical combinations. Balanced equations conserve each kind of atom, and a closed system conserves total mass during ordinary chemical changes.
1. Ask whether chemical identity changed
A physical change can alter shape, size or state while the chemical substances remain the same. Breaking a chalk piece makes smaller pieces; freezing water rearranges water molecules. In a chemical change, new substances form because atoms are connected or distributed differently. Iron reacting with oxygen and water can form rust, whose composition and properties differ from iron. Reversibility alone is a poor test: breaking a cup is difficult to reverse but is not thereby a chemical reaction, and some chemical reactions can reverse. The useful question is what substances were present before and after, and what evidence supports that identification.
2. Observations are clues that need interpretation
Gas formation, a lasting colour change, an insoluble precipitate, light or a temperature change can provide clues to a reaction. A precipitate is a solid formed from substances in solution, not merely solid powder that was added. No one clue is conclusive in every situation: boiling produces bubbles without making a new substance, and mixing two coloured liquids may change colour without reaction. Stronger evidence identifies products or combines several observations with an explanation. In this lesson, study written records and particle diagrams. There is no need to mix household cleaners, heat unknown materials or smell a reaction product to learn how evidence works.
3. Equations name substances and compare amounts
Reactants are written to the left of an arrow and products to its right. The arrow means “forms” rather than ordinary mathematical equality. A plus sign separates substances present together. State labels may include (s) for solid, (l) for liquid, (g) for gas and (aq) for dissolved in water. A coefficient multiplies a complete formula; a subscript is part of that substance’s identity. A balanced equation gives a ratio of participating atoms, molecules or formula units, not equal masses of every substance and not a reaction speed. Conditions such as light or a catalyst can be shown near the arrow because the same starting materials do not react identically under all conditions.
Sources: NCERT: Chemical Reactions and Equations ↗ · NCERT via IIT Kanpur SATHEE: Atoms and Molecules ↗
4. Choose the boundary before weighing
In an ordinary chemical reaction, atoms are rearranged rather than created or destroyed. Total reactant mass therefore equals total product mass when all matter is included. A closed system prevents matter crossing its boundary; energy may still cross. An open container can gain oxygen from air or lose a gaseous product, changing the mass on a balance. Rusting iron can gain mass because oxygen enters the solid. A gas-producing reaction can leave a lighter open vessel because gas escapes. Neither observation contradicts conservation. Track the vessel, contents and exchanged gases consistently; do not compare a complete starting system with only a selected final part.
Balance atoms, not subscripts
2H₂ + O₂ → 2H₂O
| Atom | Before | After |
|---|---|---|
| Hydrogen | 2 × 2 = 4 | 2 × 2 = 4 |
| Oxygen | 1 × 2 = 2 | 2 × 1 = 2 |
5. Bonds and energy explain why conditions matter
Breaking chemical bonds requires energy; forming new bonds releases energy. The overall balance determines whether a reaction transfers energy to its surroundings or takes energy from them. An exothermic reaction releases heat overall; an endothermic reaction absorbs heat overall. Even an energy-releasing reaction may need an initial input to begin, because reactants must pass an energy barrier. A catalyst provides a different reaction pathway with a lower barrier and is regenerated overall. It does not supply missing atoms or make an unbalanced equation correct. Reaction type and energy change are different classifications: “combination” describes joining reactants, while “exothermic” describes heat transfer.
Sources: NCERT: Chemical Reactions and Equations ↗ · IIT Kanpur SATHEE: Chemical Reactions and Equations—energy and catalysts ↗
6. Worked example: balance with an atom ledger
Paper example only: balance Al + O₂ → Al₂O₃, where Al is aluminium and O is oxygen. Oxygen appears in groups of two on the left and three on the right. Six is a shared total, so write 3O₂ and 2Al₂O₃. The right now contains four aluminium atoms, requiring 4Al on the left. The result is 4Al + 3O₂ → 2Al₂O₃. Check independently: Al is four on both sides and O is six on both sides. Coefficients 4, 3 and 2 are the smallest whole-number ratio. Changing Al₂O₃ to AlO would change the product rather than balance the stated reaction.
Sources: NCERT: Chemical Reactions and Equations ↗ · NCERT via IIT Kanpur SATHEE: Atoms and Molecules ↗
7. Worked example: account for escaping gas
Illustrative measurement record: a reaction vessel and all its starting contents have mass 186.0 g. After a gas-producing reaction, the open vessel and remaining contents measure 181.6 g. If no liquid splashed out and no other matter entered or left, escaped gas mass is 186.0 − 181.6 = 4.4 g. Add it back to the final accounting: 181.6 + 4.4 = 186.0 g. If the record instead came from a genuinely closed apparatus, the discrepancy would call for checking leaks, measurement errors or the stated boundary. Conservation guides the investigation; it does not justify silently altering a measured value to match expectations.
PUT IT INTO PRACTICE
Apply your understanding
- Balance the paper equation N₂ + H₂ → NH₃. N is nitrogen and H hydrogen. Count each kind of atom before choosing coefficients.
- For illustrative masses, 14 g nitrogen reacts completely with 3 g hydrogen. Predict product mass if all product is retained and nothing else reacts.
- Check: N₂ + 3H₂ → 2NH₃ and 17 g product. Explain why the coefficients 1:3:2 are particle ratios, while 14:3:17 are the stated mass amounts.
Check your understanding
Do bubbles always establish a chemical reaction?
No. Boiling or release of previously dissolved gas can make bubbles without forming a new substance. Identify what the gas is and how it formed.
Why does a balanced equation keep subscripts fixed?
They specify the substances. Coefficients change how many units participate while preserving identity; changing subscripts can invent different reactants or products.
Why can a rusted object be heavier?
Oxygen from outside joins the material. Include that incoming matter in the system balance; the solid’s mass alone is not the total starting mass.
Can heat be counted as extra atoms when balancing?
No. Energy transfer and atom conservation are separate parts of the explanation. Heat is not a chemical element and cannot replace missing atoms.
Why may a reaction that releases energy still need to be started?
Reactants must first cross an activation barrier. Overall energy release describes the difference between starting and final states, not the absence of an initial barrier.