Chemical Reactions and Equations — Complete Grade 10 Chemistry Guide
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| Topic | Chemical Reactions and Equations |
| Age group | 14–16 years (Grade 9–10) |
| Prerequisites | Atoms, elements, compounds, chemical formulae |
| Core skill | Balancing equations using the law of conservation of mass |
| Syllabus base | NCERT Science, Class 10 (India) — see note at the end |
1. Which syllabus are you studying?
This topic appears in almost every school chemistry course worldwide, but the names used for the same reactions differ. Students lose marks simply because a question says "single replacement" when they learned "displacement". Use this table before you begin.
| Concept | India — NCERT / CBSE | UK — GCSE / IGCSE | USA — NGSS / Honors |
|---|---|---|---|
| Two reactants form one product | Combination | Combination / Synthesis | Synthesis |
| One reactant splits into many | Decomposition | Decomposition | Decomposition |
| A metal pushes out another metal | Displacement | Displacement | Single replacement |
| Two compounds swap ions | Double displacement | Double displacement / Precipitation | Double replacement / Metathesis |
| Loss of electrons | Oxidation (taught via oxygen) | Oxidation (electrons) | Oxidation (electrons) |
| Fats going stale | Rancidity | rarely examined | Oxidative spoilage |
2. What is a chemical reaction?
A chemical reaction is a change in which one or more substances are converted into new substances with different properties. The starting substances are reactants; the new substances are products.
Figure 1 — Reactants are rearranged into products. Atoms are never created or destroyed.
Five signs that a reaction has happened:
- Change of state — a candle burning
- Change of colour — blue copper sulfate turning pale green
- Gas evolved — bubbles when zinc meets dilute acid
- Temperature change — the beaker becomes warm or cold
- Precipitate forms — an insoluble solid settles out
3. Writing a chemical equation
Step 1 — word equation
Step 2 — symbol equation
There are two oxygen atoms on the left and only one on the right. Fixing this is called balancing.
4. Balancing equations — step by step
The law of conservation of mass states that matter is neither created nor destroyed in a chemical reaction. Every atom present at the start must still be present at the end — so the count of each element must match on both sides.
Figure 2 — The pans balance only when each element has the same number of atoms on both sides.
Worked example: balance Fe + H2O → Fe3O4 + H2
| Step | Action | Equation |
|---|---|---|
| 1 | Count atoms — Fe 1:3, H 2:2, O 1:4 | Fe + H2O → Fe3O4 + H2 |
| 2 | Start with the element in greatest number (O). Put 4 on the left. | Fe + 4H2O → Fe3O4 + H2 |
| 3 | H is now 8 on the left. Put 4 on the right. | Fe + 4H2O → Fe3O4 + 4H2 |
| 4 | Finish with Fe — put 3 on the left. | 3Fe + 4H2O → Fe3O4 + 4H2 |
| 5 | Check — Fe 3=3, H 8=8, O 4=4 | ✓ balanced |
Order that works almost every time: metals → non-metals → hydrogen → oxygen.
5. State symbols and conventions
| Symbol | Meaning |
|---|---|
| (s) | solid |
| (l) | liquid |
| (g) | gas |
| (aq) | aqueous — dissolved in water |
| ↑ | gas is given off |
| ↓ | a precipitate forms |
| Δ | heat is applied (written above the arrow) |
| hν / light | light drives the reaction |
| ⇌ beyond NCERT | reversible reaction — both directions occur |
6. Try it yourself
Change the coefficients below until the equation balances. The balance scale and bar chart tell you instantly whether you are right. Ten minutes here is worth an hour of reading.
7. Types of chemical reaction
Figure 3 — The four core reaction types, with the US alternative names shown.
(a) Combination (synthesis)
Two or more reactants form a single product.
2H2(g) + O2(g) → 2H2O(l)
C(s) + O2(g) → CO2(g)
2Mg(s) + O2(g) → 2MgO(s)
(b) Decomposition
A single reactant breaks into two or more products. Energy must be supplied, so these are usually endothermic.
Thermal decomposition
2FeSO4(s) Δ→ Fe2O3(s) + SO2(g) + SO3(g)
2Pb(NO3)2(s) Δ→ 2PbO(s) + 4NO2(g) + O2(g)
Electrolytic decomposition
Photolytic decomposition
2AgBr(s) light→ 2Ag(s) + Br2(g)
(c) Displacement (single replacement)
A more reactive element displaces a less reactive one from its compound. Use the reactivity series to predict whether a reaction will happen.
Figure 4 — An iron nail in copper sulfate solution. The blue solution turns pale green and reddish-brown copper deposits on the nail.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)
(d) Double displacement (double replacement)
Two compounds exchange their ions. When one product is insoluble, a precipitate forms.
Barium sulfate forms as a white precipitate. Such reactions are also called precipitation reactions.
8. Exothermic and endothermic reactions
Figure 5 — In an exothermic reaction the products sit lower in energy; in an endothermic reaction, higher.
| Exothermic | Endothermic |
|---|---|
| Energy is released | Energy is absorbed |
| Container feels warm | Container feels cold |
| Respiration, combustion, slaking of lime | Photosynthesis, thermal decomposition |
Endothermic: CaCO3(s) + heat → CaO(s) + CO2(g)
9. Oxidation and reduction (redox)
Figure 6 — Oxidation and reduction always happen together.
• CuO is the oxidising agent — it oxidises something else and is itself reduced
• H2 is the reducing agent — it reduces something else and is itself oxidised
ZnO(s) + C(s) → Zn(s) + CO(g)
The oxygen-and-hydrogen definition above is the one used in NCERT Class 10 and it is perfectly correct. Most international syllabuses add a second, more general definition based on electrons:
- Oxidation = loss of electrons
- Reduction = gain of electrons
In Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s), zinc loses two electrons (oxidised) and copper ions gain two (reduced). Both definitions describe the same event.
10. Corrosion and rusting
When a metal is slowly eaten away by air, moisture or acids around it, the process is called corrosion. It is a form of oxidation.
Figure 7 — Corrosion products on three common metals.
Iron rusts only when both water and oxygen are present. Remove either one and rusting stops — which is exactly why an oiled or painted surface survives.
Ways to prevent corrosion:
- painting, oiling or greasing
- galvanising — coating iron with zinc
- electroplating with chromium, nickel or tin
- alloying — for example stainless steel
11. Rancidity
Foods containing fats and oils are slowly oxidised when stored for long periods. Their taste and smell deteriorate. This is called rancidity — fried food going stale after a few days is the everyday example.
How it is prevented:
- adding antioxidants
- storing in airtight containers
- flushing packets with nitrogen gas to displace oxygen — this is why a crisp packet is puffed up
- refrigeration, which slows the oxidation down
12. Laboratory activities
| Experiment | What you observe | Conclusion |
|---|---|---|
| Burning magnesium ribbon | Dazzling white flame, white ash | Combination; clean the ribbon with sandpaper first |
| Lead nitrate + potassium iodide | Yellow precipitate | Double displacement (precipitation) |
| Zinc + dilute sulfuric acid | Bubbles; flask warms up | Displacement and exothermic |
| Electrolysis of water | Gas in both tubes, ratio 2:1 | Twice as much H2 as O2 |
| Silver chloride in sunlight | White turns grey | Photolytic decomposition |
| Iron nail in copper sulfate | Blue → pale green; brown coating | Displacement |
| Heating copper powder | Black coating; turns brown again in H2 | Oxidation then reduction |
13. Worked exercises
Q. Why is a magnesium ribbon cleaned before burning?
A. A white layer of magnesium oxide forms on the surface and prevents the metal from burning properly. Rubbing with sandpaper exposes the clean metal to air.
Q. Balance: (i) hydrogen + chlorine → hydrogen chloride (ii) barium chloride + aluminium sulfate
A. (i) H2 + Cl2 → 2HCl
(ii) 3BaCl2 + Al2(SO4)3 → 3BaSO4↓ + 2AlCl3
Q. Potassium reacts with water to give potassium hydroxide and hydrogen. Write the balanced equation with state symbols.
A. 2K(s) + 2H2O(l) → 2KOH(aq) + H2(g)
Q. What is a precipitation reaction? Give an example.
A. A reaction in which an insoluble solid forms and settles out of solution.
Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq)
Q. Explain oxidation and reduction in terms of gain and loss of oxygen.
A. Gain of oxygen is oxidation; loss of oxygen is reduction.
In 2Cu + O2 → 2CuO copper is oxidised. In CuO + H2 → Cu + H2O copper oxide is reduced.
Q. Why does a copper vessel lose its shine, and why does polishing restore it?
A. Moisture and carbon dioxide in the air form a green layer of copper carbonate on the surface. Rubbing with lemon or tamarind, both acidic, dissolves this layer and the shine returns.
Q. Why are oily and fatty foods packed in nitrogen?
A. Nitrogen is unreactive. Replacing the oxygen inside the packet stops oxidation, so rancidity is delayed and the food stays fresh longer.
14. Summary table
| Type | How to spot it | Example |
|---|---|---|
| Combination | many → one | CaO + H2O → Ca(OH)2 |
| Decomposition | one → many | CaCO3 → CaO + CO2 |
| Displacement | one element pushed out | Fe + CuSO4 → FeSO4 + Cu |
| Double displacement | ions exchanged | Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl |
| Exothermic | heat released | respiration, combustion |
| Endothermic | heat absorbed | photosynthesis |
| Redox | both at once | CuO + H2 → Cu + H2O |
15. Equation bank
Revise this single block the night before the exam.
3Fe + 4H2O → Fe3O4 + 4H2
CaO + H2O → Ca(OH)2
CaCO3 Δ→ CaO + CO2
2FeSO4 Δ→ Fe2O3 + SO2 + SO3
2Pb(NO3)2 Δ→ 2PbO + 4NO2 + O2
2H2O electricity→ 2H2 + O2
2AgCl light→ 2Ag + Cl2
Fe + CuSO4 → FeSO4 + Cu
Zn + CuSO4 → ZnSO4 + Cu
Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl
2K + 2H2O → 2KOH + H2
CuO + H2 → Cu + H2O
2Cu + O2 → 2CuO
ZnO + C → Zn + CO
16. Common mistakes in exams
- Leaving the equation unbalanced. Always recount both sides after writing it.
- Omitting state symbols when the question asks for them.
- Changing a subscript instead of adding a coefficient.
- Reversing oxidising and reducing agents. The oxidising agent is itself reduced.
- Capital-letter errors. Co is cobalt; CO is carbon monoxide. The first letter of a symbol is capital, the second is not.
- Forgetting the condition — Δ, light or electricity — above the arrow in decomposition reactions.
- Giving a definition without an example. In a three-mark question the definition is usually worth one mark and the equation two.
17. Exam-style questions
Q1 [1 mark] State the colour change when silver chloride is left in sunlight.
A. White to grey.
Q2 [2 marks] Will these reactions occur? Justify.
(i) Cu + ZnSO4 (ii) Fe + CuSO4
A. (i) No — copper is less reactive than zinc, so it cannot displace it. (ii) Yes — iron is more reactive than copper.
Q3 [3 marks] Define oxidising agent and reducing agent with an example.
A. An oxidising agent oxidises another substance and is itself reduced; a reducing agent reduces another substance and is itself oxidised. In CuO + H2 → Cu + H2O, CuO is the oxidising agent and H2 the reducing agent.
Q4 [4 marks] Distinguish corrosion from rancidity and give two preventive measures for each.
A. Corrosion affects metals — slow eating away by air and moisture. Rancidity affects fatty foods — oxidation spoiling taste and smell.
Corrosion: painting, galvanising. Rancidity: antioxidants, nitrogen flushing.
18. Self-test — 15 questions
Write your answers on paper, then check. 12 or more correct means you are ready.
- Colour of the ash formed when magnesium burns — (a) grey (b) black (c) white (d) blue
- What type of reaction is CaCO3 → CaO + CO2?
- State the law of conservation of mass.
- What does (aq) mean?
- What happens to the colour when zinc is added to copper sulfate solution?
- Is respiration exothermic or endothermic?
- Which compound forms the black layer on silver?
- Which metal is used in galvanising?
- Which gas is used in food packets and why?
- Complete: 2AgBr → ? + ?
- Colour of the barium sulfate precipitate?
- In CuO + H2, which is the reducing agent?
- Name the two gases released when ferrous sulfate is heated.
- Which gas is produced in greater volume during electrolysis of water?
- Complete: 3Fe + 4H2O → Fe3O4 + ?
19. Glossary and spelling differences
| English | हिन्दी | English | हिन्दी |
|---|---|---|---|
| Reactant | अभिकारक | Oxidation | उपचयन |
| Product | उत्पाद | Reduction | अपचयन |
| Combination | संयोजन | Oxidising agent | ऑक्सीकारक |
| Decomposition | वियोजन | Reducing agent | अपचायक |
| Displacement | विस्थापन | Precipitate | अवक्षेप |
| Double displacement | द्विविस्थापन | Corrosion | संक्षारण |
| Exothermic | उष्माक्षेपी | Rancidity | विकृतगंधिता |
| Endothermic | उष्माशोषी | Galvanisation | यशदलेपन |
Both spellings below are accepted in their own regions. IUPAC, and therefore most modern international papers, prefers the second column.
| Older British / Indian | IUPAC / American |
|---|---|
| sulphate, sulphur, sulphide | sulfate, sulfur, sulfide |
| aluminium | aluminum (US only) |
| ferrous sulphate | iron(II) sulfate |
| ferric oxide | iron(III) oxide |
20. About this guide
Curriculum basis. Every fact, equation, definition and example in this guide is drawn from the NCERT Science textbook for Class 10, published by the National Council of Educational Research and Training, New Delhi, India, as prescribed by the Board of Secondary Education, Rajasthan for the 2026–27 session. NCERT is the national curriculum body of India and its science content is aligned with international standards at this level.
What has been added for international readers. Three items go beyond the NCERT Class 10 requirement and are clearly marked beyond NCERT where they appear — the electron-transfer definition of redox, the reversible-reaction symbol, and the terminology bridge between Indian, British and American syllabuses. They are included because students following IGCSE, GCSE, IB MYP or NGSS will meet them.
Diagrams. All figures are original vector drawings created for this guide and may be viewed, printed and shared freely. The interactive simulation is by PhET Interactive Simulations, University of Colorado Boulder, used under the CC-BY 4.0 licence.
Before your exam, always check your own board's specification. Syllabuses differ in which sub-topics are assessed, even where the underlying chemistry is identical.
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