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Chemical Reactions and Equations

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Chapter Notes

Overview
This chapter introduces the fundamental concept of chemical reactions, which are processes where substances change into new substances with different properties. It explains how to represent these changes using chemical equations, the importance of balancing equations according to the law of conservation of mass, and the various types of reactions such as combination, decomposition, displacement, double displacement, and redox reactions. The one big idea a student must walk away with is that a chemical reaction involves the rearrangement of atoms, and the total mass and number of atoms of each element remain constant before and after the reaction.
Key Concepts (20)
Chemical Reaction
A process in which one or more substances (reactants) are converted into one or more new substances (products) with different chemical and physical properties.
e.g. Burning of magnesium ribbon in air to form magnesium oxide.
Reactants
The substances that undergo chemical change in a reaction, written on the left-hand side (LHS) of a chemical equation.
e.g. In the reaction Mg + O₂ → MgO, magnesium and oxygen are the reactants.
Products
The new substances formed as a result of a chemical reaction, written on the right-hand side (RHS) of a chemical equation.
e.g. In the reaction Mg + O₂ → MgO, magnesium oxide is the product.
Chemical Equation
A symbolic representation of a chemical reaction using chemical formulae of reactants and products, with an arrow pointing from reactants to products.
e.g. Zn + H₂SO₄ → ZnSO₄ + H₂
Skeletal Chemical Equation
An unbalanced chemical equation that only shows the chemical formulae of reactants and products without balancing the number of atoms on both sides.
e.g. Mg + O₂ → MgO is a skeletal equation because the number of oxygen atoms is not equal on both sides.
Balanced Chemical Equation
A chemical equation in which the number of atoms of each element is the same on both the reactant and product sides, obeying the law of conservation of mass.
e.g. 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g) is a balanced equation.
Law of Conservation of Mass
Mass can neither be created nor destroyed in a chemical reaction; the total mass of the elements present in the products must equal the total mass of the elements present in the reactants.
e.g. In the reaction 2H₂ + O₂ → 2H₂O, the mass of hydrogen and oxygen before the reaction equals the mass of water formed.
Combination Reaction
A reaction in which two or more substances (elements or compounds) combine to form a single product.
e.g. CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat (Quick lime + Water → Slaked lime)
Decomposition Reaction
A reaction in which a single compound breaks down into two or more simpler products.
e.g. 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) (Ferrous sulphate decomposes on heating).
Thermal Decomposition
A decomposition reaction that is carried out by heating.
e.g. CaCO₃(s) → CaO(s) + CO₂(g) (Limestone decomposes on heating to give quick lime and carbon dioxide).
Exothermic Reaction
A chemical reaction in which heat is released along with the formation of products.
e.g. Burning of natural gas: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat
Endothermic Reaction
A chemical reaction in which energy is absorbed from the surroundings.
e.g. Decomposition of calcium carbonate on heating: CaCO₃(s) → CaO(s) + CO₂(g) (requires heat).
Displacement Reaction
A reaction in which a more reactive element displaces a less reactive element from its compound.
e.g. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s) (Iron displaces copper from copper sulphate solution).
Double Displacement Reaction
A reaction in which there is an exchange of ions between the reactants to form new products.
e.g. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Sodium sulphate reacts with barium chloride).
Precipitation Reaction
A reaction in which an insoluble solid (precipitate) is formed from the reaction of two soluble substances in solution.
e.g. Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq) (Barium sulphate is the white precipitate).
Oxidation
The gain of oxygen or loss of hydrogen by a substance during a reaction.
e.g. 2Cu + O₂ → 2CuO (Copper gains oxygen and is oxidised to copper oxide).
Reduction
The loss of oxygen or gain of hydrogen by a substance during a reaction.
e.g. CuO + H₂ → Cu + H₂O (Copper oxide loses oxygen and is reduced to copper).
Redox Reaction
A reaction in which one reactant gets oxidised while the other gets reduced simultaneously.
e.g. ZnO + C → Zn + CO (Zinc oxide is reduced to zinc, carbon is oxidised to carbon monoxide).
Corrosion
The process in which a metal is attacked by substances around it such as moisture, acids, etc., leading to its deterioration.
e.g. Rusting of iron (formation of reddish-brown coating), black coating on silver, green coating on copper.
Rancidity
The process in which fats and oils are oxidised, leading to a change in their smell and taste, making them unfit for consumption.
e.g. Stale chips or old cooking oil developing an unpleasant odour.
Formulas & Equations (24)
Mg + O₂ → MgO
Reads as: Magnesium plus oxygen yields magnesium oxide
Variables: Mg = Magnesium, O₂ = Oxygen molecule, MgO = Magnesium oxide
Use when: Writing the skeletal equation for the burning of magnesium ribbon in air.
⚠ Students forget to balance the equation, writing it as Mg + O₂ → MgO without realising oxygen atoms are unequal (2 on LHS, 1 on RHS).
Zn + H₂SO₄ → ZnSO₄ + H₂
Reads as: Zinc plus sulphuric acid yields zinc sulphate plus hydrogen
Variables: Zn = Zinc, H₂SO₄ = Sulphuric acid, ZnSO₄ = Zinc sulphate, H₂ = Hydrogen gas
Use when: Representing the reaction of zinc with dilute sulphuric acid to produce hydrogen gas.
⚠ Students may incorrectly write the product as ZnSO₄ + H₂O instead of H₂ gas.
Fe + H₂O → Fe₃O₄ + H₂
Reads as: Iron plus water yields ferrosoferric oxide plus hydrogen
Variables: Fe = Iron, H₂O = Water, Fe₃O₄ = Ferrosoferric oxide (magnetite), H₂ = Hydrogen gas
Use when: Balancing the chemical equation for the reaction of iron with steam.
⚠ Students often forget to balance the equation and may write FeO instead of Fe₃O₄.
3Fe + 4H₂O → Fe₃O₄ + 4H₂
Reads as: Three iron plus four water yields ferrosoferric oxide plus four hydrogen
Variables: 3Fe = 3 atoms of Iron, 4H₂O = 4 molecules of Water, Fe₃O₄ = Ferrosoferric oxide, 4H₂ = 4 molecules of Hydrogen gas
Use when: Writing the balanced chemical equation for the reaction of iron with steam.
⚠ Students may use incorrect coefficients, e.g., 2Fe + 3H₂O → Fe₂O₃ + 3H₂, which is a different reaction.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat
Reads as: Calcium oxide solid plus water liquid yields calcium hydroxide aqueous plus heat
Variables: CaO = Calcium oxide (quick lime), H₂O = Water, Ca(OH)₂ = Calcium hydroxide (slaked lime), Heat = Energy released
Use when: Describing the combination reaction of quick lime with water, which is exothermic.
⚠ Students forget to write the state symbols or omit 'Heat' as a product, not recognising it as an exothermic reaction.
C(s) + O₂(g) → CO₂(g)
Reads as: Carbon solid plus oxygen gas yields carbon dioxide gas
Variables: C = Carbon (coal), O₂ = Oxygen gas, CO₂ = Carbon dioxide gas
Use when: Representing the burning of coal as a combination reaction.
⚠ Students may write CO instead of CO₂, forgetting complete combustion.
2H₂(g) + O₂(g) → 2H₂O(l)
Reads as: Two hydrogen gas plus oxygen gas yields two water liquid
Variables: 2H₂ = 2 molecules of Hydrogen gas, O₂ = Oxygen gas, 2H₂O = 2 molecules of Water
Use when: Representing the formation of water from hydrogen and oxygen as a combination reaction.
⚠ Students may forget to balance the equation, writing H₂ + O₂ → H₂O.
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat
Reads as: Methane gas plus two oxygen gas yields carbon dioxide gas plus two water gas plus heat
Variables: CH₄ = Methane (natural gas), 2O₂ = 2 molecules of Oxygen, CO₂ = Carbon dioxide, 2H₂O = 2 molecules of Water, Heat = Energy released
Use when: Describing the exothermic combustion of natural gas.
⚠ Students may write H₂O(l) instead of H₂O(g) as water is produced as steam in this reaction.
C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy
Reads as: Glucose aqueous plus six oxygen aqueous yields six carbon dioxide aqueous plus six water liquid plus energy
Variables: C₆H₁₂O₆ = Glucose, 6O₂ = 6 molecules of Oxygen, 6CO₂ = 6 molecules of Carbon dioxide, 6H₂O = 6 molecules of Water, energy = Energy released
Use when: Explaining respiration as an exothermic process.
⚠ Students may forget to write 'energy' as a product or incorrectly balance the equation.
Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)
Reads as: Calcium hydroxide aqueous plus carbon dioxide gas yields calcium carbonate solid plus water liquid
Variables: Ca(OH)₂ = Calcium hydroxide (slaked lime), CO₂ = Carbon dioxide, CaCO₃ = Calcium carbonate, H₂O = Water
Use when: Explaining the whitewashing reaction where calcium hydroxide reacts with carbon dioxide to form a shiny layer of calcium carbonate.
⚠ Students may write the product as CaCO₃ + H₂ without balancing or omit water.
2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
Reads as: Two ferrous sulphate solid yields ferric oxide solid plus sulphur dioxide gas plus sulphur trioxide gas
Variables: 2FeSO₄ = 2 molecules of Ferrous sulphate, Fe₂O₃ = Ferric oxide, SO₂ = Sulphur dioxide, SO₃ = Sulphur trioxide
Use when: Describing the thermal decomposition of ferrous sulphate crystals.
⚠ Students may forget to write SO₃ as a product or write FeO instead of Fe₂O₃.
CaCO₃(s) → CaO(s) + CO₂(g)
Reads as: Calcium carbonate solid yields calcium oxide solid plus carbon dioxide gas
Variables: CaCO₃ = Calcium carbonate (limestone), CaO = Calcium oxide (quick lime), CO₂ = Carbon dioxide
Use when: Representing the thermal decomposition of limestone used in industries.
⚠ Students may write Ca(OH)₂ instead of CaO as the product.
2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g)
Reads as: Two lead nitrate solid yields two lead oxide solid plus four nitrogen dioxide gas plus oxygen gas
Variables: 2Pb(NO₃)₂ = 2 molecules of Lead nitrate, 2PbO = 2 molecules of Lead oxide, 4NO₂ = 4 molecules of Nitrogen dioxide, O₂ = Oxygen gas
Use when: Describing the thermal decomposition of lead nitrate, which produces brown fumes of nitrogen dioxide.
⚠ Students may forget to include O₂ as a product or write NO instead of NO₂.
2AgCl(s) → 2Ag(s) + Cl₂(g)
Reads as: Two silver chloride solid yields two silver solid plus chlorine gas
Variables: 2AgCl = 2 molecules of Silver chloride, 2Ag = 2 atoms of Silver, Cl₂ = Chlorine gas
Use when: Explaining the decomposition of silver chloride in sunlight, used in black and white photography.
⚠ Students may write Ag + Cl instead of 2Ag + Cl₂, forgetting to balance.
2AgBr(s) → 2Ag(s) + Br₂(g)
Reads as: Two silver bromide solid yields two silver solid plus bromine gas
Variables: 2AgBr = 2 molecules of Silver bromide, 2Ag = 2 atoms of Silver, Br₂ = Bromine gas
Use when: Explaining the decomposition of silver bromide in sunlight, also used in black and white photography.
⚠ Students may confuse this with the silver chloride reaction or forget the state symbols.
2H₂O(l) → 2H₂(g) + O₂(g)
Reads as: Two water liquid yields two hydrogen gas plus oxygen gas
Variables: 2H₂O = 2 molecules of Water, 2H₂ = 2 molecules of Hydrogen gas, O₂ = Oxygen gas
Use when: Representing the electrolysis of water, a decomposition reaction using electricity.
⚠ Students may forget that the volume of hydrogen collected is double that of oxygen.
Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Reads as: Iron solid plus copper sulphate aqueous yields iron sulphate aqueous plus copper solid
Variables: Fe = Iron, CuSO₄ = Copper sulphate, FeSO₄ = Iron sulphate (ferrous sulphate), Cu = Copper
Use when: Describing the displacement reaction where iron displaces copper from copper sulphate solution.
⚠ Students may write the product as Fe₂(SO₄)₃ instead of FeSO₄.
Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
Reads as: Zinc solid plus copper sulphate aqueous yields zinc sulphate aqueous plus copper solid
Variables: Zn = Zinc, CuSO₄ = Copper sulphate, ZnSO₄ = Zinc sulphate, Cu = Copper
Use when: Describing the displacement reaction where zinc displaces copper from copper sulphate solution.
⚠ Students may write the product as ZnSO₄ + CuO instead of Cu.
Pb(s) + CuCl₂(aq) → PbCl₂(aq) + Cu(s)
Reads as: Lead solid plus copper chloride aqueous yields lead chloride aqueous plus copper solid
Variables: Pb = Lead, CuCl₂ = Copper chloride, PbCl₂ = Lead chloride, Cu = Copper
Use when: Describing the displacement reaction where lead displaces copper from copper chloride solution.
⚠ Students may write PbCl₄ instead of PbCl₂.
Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)
Reads as: Sodium sulphate aqueous plus barium chloride aqueous yields barium sulphate solid plus two sodium chloride aqueous
Variables: Na₂SO₄ = Sodium sulphate, BaCl₂ = Barium chloride, BaSO₄ = Barium sulphate (white precipitate), 2NaCl = 2 molecules of Sodium chloride
Use when: Describing a double displacement and precipitation reaction.
⚠ Students may forget to write the state symbol (s) for BaSO₄ or write NaCl as a solid.
2Cu + O₂ → 2CuO
Reads as: Two copper plus oxygen yields two copper oxide
Variables: 2Cu = 2 atoms of Copper, O₂ = Oxygen gas, 2CuO = 2 molecules of Copper(II) oxide
Use when: Describing the oxidation of copper to copper oxide on heating.
⚠ Students may write CuO₂ instead of CuO.
CuO + H₂ → Cu + H₂O
Reads as: Copper oxide plus hydrogen yields copper plus water
Variables: CuO = Copper(II) oxide, H₂ = Hydrogen gas, Cu = Copper, H₂O = Water
Use when: Describing the reduction of copper oxide by hydrogen, where copper oxide is reduced and hydrogen is oxidised.
⚠ Students may forget to balance the equation or write CuOH instead of H₂O.
ZnO + C → Zn + CO
Reads as: Zinc oxide plus carbon yields zinc plus carbon monoxide
Variables: ZnO = Zinc oxide, C = Carbon, Zn = Zinc, CO = Carbon monoxide
Use when: Describing a redox reaction where carbon is oxidised and zinc oxide is reduced.
⚠ Students may write CO₂ instead of CO.
MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂
Reads as: Manganese dioxide plus four hydrochloric acid yields manganese chloride plus two water plus chlorine
Variables: MnO₂ = Manganese dioxide, 4HCl = 4 molecules of Hydrochloric acid, MnCl₂ = Manganese chloride, 2H₂O = 2 molecules of Water, Cl₂ = Chlorine gas
Use when: Describing a redox reaction where HCl is oxidised to Cl₂ and MnO₂ is reduced to MnCl₂.
⚠ Students may forget to balance the equation or write MnCl₄ instead of MnCl₂.
How It Works
Writing a Chemical Equation
  1. Step 1: Write the word equation, e.g., Magnesium + Oxygen → Magnesium oxide.
  2. Step 2: Replace the names with chemical formulae, e.g., Mg + O₂ → MgO.
  3. Step 3: Check if the number of atoms of each element is the same on both sides. If not, it is a skeletal equation.
  4. Step 4: Balance the equation by using coefficients (hit-and-trial method) without changing the formulae.
  5. Step 5: Add state symbols: (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous solution.
  6. Step 6: Optionally, indicate reaction conditions (temperature, pressure, catalyst) above or below the arrow.
💡 Never change the subscripts in a chemical formula to balance an equation; only add coefficients in front of the formulae.
Balancing a Chemical Equation (Hit-and-Trial Method)
  1. Step 1: Draw boxes around each formula to avoid changing them.
  2. Step 2: List the number of atoms of each element on both sides.
  3. Step 3: Start balancing with the compound that has the maximum number of atoms, and within it, the element with the maximum number of atoms.
  4. Step 4: Use coefficients to equalise the number of atoms of that element on both sides.
  5. Step 5: Repeat for other elements one by one until all atoms are balanced.
  6. Step 6: Verify by counting atoms of each element on both sides to ensure they are equal.
💡 Always balance oxygen and hydrogen atoms last if they appear in multiple compounds, as they are often the trickiest.
Identifying Types of Chemical Reactions
  1. Step 1: Check if two or more reactants form a single product → Combination reaction.
  2. Step 2: Check if a single reactant breaks down into two or more products → Decomposition reaction.
  3. Step 3: Check if an element displaces another element from its compound → Displacement reaction.
  4. Step 4: Check if two compounds exchange ions to form two new compounds → Double displacement reaction.
  5. Step 5: Check if a precipitate is formed → Precipitation reaction (a type of double displacement).
  6. Step 6: Check if there is gain of oxygen/loss of hydrogen (oxidation) and loss of oxygen/gain of hydrogen (reduction) → Redox reaction.
💡 A reaction can be more than one type; for example, a combination reaction can also be exothermic, and a double displacement reaction can also be a precipitation reaction.
Electrolysis of Water
  1. Step 1: Take water in a plastic mug and add a few drops of dilute sulphuric acid to make it conducting.
  2. Step 2: Insert two carbon electrodes connected to a 6-volt battery.
  3. Step 3: Invert two test tubes filled with water over the electrodes to collect gases.
  4. Step 4: Switch on the current; bubbles form at both electrodes, displacing water in the test tubes.
  5. Step 5: The volume of gas collected at the cathode (negative electrode) is double that at the anode (positive electrode).
  6. Step 6: Test the gases: the gas that burns with a pop sound is hydrogen (cathode), and the gas that relights a glowing splint is oxygen (anode).
💡 The volume of hydrogen collected is double the volume of oxygen because water (H₂O) has two hydrogen atoms for every oxygen atom.
Whitewashing Process
  1. Step 1: Quick lime (CaO) is mixed with water to form slaked lime (Ca(OH)₂) in an exothermic reaction.
  2. Step 2: The slaked lime solution is applied to walls.
  3. Step 3: The slaked lime reacts slowly with carbon dioxide (CO₂) in the air.
  4. Step 4: This reaction forms a thin layer of calcium carbonate (CaCO₃) on the walls.
  5. Step 5: The calcium carbonate gives a shiny finish to the walls after two to three days.
💡 The shiny finish is due to the formation of calcium carbonate, not calcium hydroxide. The chemical formula for marble is also CaCO₃.
Prevention of Rancidity
  1. Step 1: Rancidity occurs when fats and oils are oxidised, changing their smell and taste.
  2. Step 2: Antioxidants are added to foods containing fats and oils to prevent oxidation.
  3. Step 3: Food is kept in air-tight containers to slow down oxidation.
  4. Step 4: Chips manufacturers flush bags of chips with nitrogen gas to prevent oxidation.
💡 Nitrogen is an inert gas that displaces oxygen, preventing the chips from getting oxidised and becoming rancid.
Exam Traps — Don't Make These Mistakes
⚠ Confusing 'Skeletal' with 'Balanced' Equation
✗ Students write Mg + O₂ → MgO and claim it is a balanced equation.
→ The number of oxygen atoms is 2 on the LHS (in O₂) and 1 on the RHS (in MgO), violating the law of conservation of mass.
✓ The balanced equation is 2Mg + O₂ → 2MgO.
⚠ Forgetting State Symbols in Equations
✗ Students write CaO + H₂O → Ca(OH)₂ without any state symbols.
→ State symbols are required to make the equation informative, especially when specifying physical states like steam (g) or aqueous solutions (aq).
✓ Write CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat.
⚠ Misidentifying Exothermic vs Endothermic Reactions
✗ Students write that decomposition reactions are always exothermic.
→ Decomposition reactions usually require energy (heat, light, or electricity) and are therefore endothermic, not exothermic.
✓ Decomposition reactions are generally endothermic because energy is absorbed to break bonds. For example, CaCO₃(s) → CaO(s) + CO₂(g) requires heat.
⚠ Confusing Oxidation and Reduction Definitions
✗ Students write 'Oxidation is the loss of oxygen' or 'Reduction is the gain of oxygen'.
→ These definitions are reversed. Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen.
✓ Oxidation: gain of oxygen or loss of hydrogen. Reduction: loss of oxygen or gain of hydrogen.
⚠ Incorrectly Identifying the Type of Reaction
✗ Students identify Fe + CuSO₄ → FeSO₄ + Cu as a double displacement reaction.
→ This is a displacement reaction because a single element (Fe) displaces another element (Cu) from its compound. Double displacement involves exchange of ions between two compounds.
✓ This is a displacement reaction. A double displacement example is Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.
⚠ Forgetting the 'Heat' or 'Energy' in Exothermic Reactions
✗ Students write CH₄ + 2O₂ → CO₂ + 2H₂O without mentioning heat or energy.
→ Exothermic reactions release heat, and it is important to indicate this in the equation or description to show the reaction type.
✓ Write CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g) + Heat.
Compare & Contrast
Number of reactants and products
Combination Reaction
Decomposition Reaction
In a combination reaction, two or more reactants combine to form a single product. In a decomposition reaction, a single reactant breaks down into two or more products. They are opposite processes.
Mechanism of reaction
Displacement Reaction
Double Displacement Reaction
In a displacement reaction, a more reactive element displaces a less reactive element from its compound (A + BC → AC + B). In a double displacement reaction, two compounds exchange ions to form two new compounds (AB + CD → AD + CB).
Gain or loss of oxygen/hydrogen
Oxidation
Reduction
Oxidation is the gain of oxygen or loss of hydrogen. Reduction is the loss of oxygen or gain of hydrogen. They occur simultaneously in a redox reaction.
Energy change
Exothermic Reaction
Endothermic Reaction
Exothermic reactions release heat energy to the surroundings (e.g., burning of fuel). Endothermic reactions absorb heat energy from the surroundings (e.g., thermal decomposition of limestone).
Exam Focus
Marks pattern: This chapter typically carries 5-7 marks in the board exam: 1-2 MCQs (1 mark each), 1 short answer question (2-3 marks) on balancing or identifying reaction types, and 1 long answer question (3-5 marks) on writing balanced equations and explaining concepts like redox or corrosion.
Most asked: Balancing chemical equations · Identifying types of reactions (combination, decomposition, displacement, double displacement, redox) · Oxidation and reduction (redox reactions) · Exothermic and endothermic reactions · Corrosion and rancidity · Writing balanced equations with state symbols
Don't skip: Balancing a given chemical equation step-by-step (3 marks); Writing balanced equations for reactions and identifying the type of reaction (5 marks); Explaining oxidation and reduction with examples (3 marks); Differentiating between displacement and double displacement reactions (2 marks)
One-Line Revision (read aloud before the exam)
  1. A chemical reaction is a process where reactants are converted into products with different properties.
  2. A chemical equation uses symbols and formulae to represent a chemical reaction.
  3. A skeletal chemical equation is unbalanced; a balanced equation has equal atoms of each element on both sides.
  4. The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction.
  5. In a combination reaction, two or more substances combine to form a single product.
  6. In a decomposition reaction, a single compound breaks down into two or more simpler products.
  7. Exothermic reactions release heat; endothermic reactions absorb heat.
  8. In a displacement reaction, a more reactive element displaces a less reactive element from its compound.
  9. In a double displacement reaction, two compounds exchange ions to form new compounds.
  10. A precipitation reaction produces an insoluble solid called a precipitate.
  11. Oxidation is the gain of oxygen or loss of hydrogen; reduction is the loss of oxygen or gain of hydrogen.
  12. Redox reactions involve simultaneous oxidation and reduction.
  13. Corrosion is the deterioration of metals due to reaction with moisture and acids; rusting of iron is an example.
  14. Rancidity is the oxidation of fats and oils, causing a bad smell and taste; it is prevented by antioxidants and air-tight packaging.
  15. Whitewashing uses slaked lime (Ca(OH)₂) which reacts with CO₂ to form a shiny layer of calcium carbonate (CaCO₃).
  16. Electrolysis of water produces hydrogen gas at the cathode and oxygen gas at the anode in a 2:1 volume ratio.
  17. Silver chloride and silver bromide decompose in sunlight to give silver and halogen gases, used in black and white photography.
  18. The burning of magnesium ribbon is a combination and exothermic reaction producing magnesium oxide.
  19. Respiration is an exothermic process where glucose reacts with oxygen to release energy.
  20. Nitrogen gas is flushed into chip bags to prevent oxidation and rancidity.

MCQs — 10 Questions with Answers

Q1. A student performs Activity 1.1 by burning a magnesium ribbon in air and observes a dazzling white flame and white powder formation. Which observation directly indicates that a chemical reaction has occurred?

  • A. Change in colour from silvery metal to white powder ✓
  • B. The brightness of the flame produced
  • C. The speed at which the ribbon burns
  • D. The amount of ash collected

Answer: A — Change in colour is a direct sign of a chemical reaction as it indicates formation of a new substance (MgO); brightness and speed are observational details but not direct evidence of chemical change.

Q2. In Activity 1.3, zinc granules are added to dilute sulphuric acid. The student observes vigorous bubbling and the test tube becomes warm. What do these two observations together indicate?

  • A. A chemical reaction has occurred with evolution of gas and release of heat ✓
  • B. The acid is boiling due to addition of zinc
  • C. Zinc is physically dissolving in the acid
  • D. The test tube is being heated externally

Answer: A — Gas evolution (bubbling) and temperature increase are both characteristic signs of a chemical reaction; physical dissolution does not produce gas or heat, and external heating was not mentioned.

Q3. A lead nitrate solution is mixed with potassium iodide solution (Activity 1.3), and a bright yellow precipitate forms immediately. Based on the law of conservation of mass, which statement is true for the balanced chemical equation of this reaction?

  • A. The number of atoms of each element is the same on both reactant and product sides ✓
  • B. The mass of lead nitrate used must equal the mass of precipitate formed
  • C. The volume of reactants must equal the volume of products
  • D. The number of molecules is conserved during the reaction

Answer: A — The law of conservation of mass requires equal number of atoms of each element on both sides; mass is conserved overall but not necessarily distributed equally between single compounds.

Q4. A student writes the following equation for burning magnesium: Mg + O₂ → MgO. After counting atoms, they find 1 Mg on LHS but 1 Mg on RHS (balanced), yet 2 O atoms on LHS but only 1 O on RHS (unbalanced). Which step should they take next?

  • A. Add coefficient 2 before MgO to balance oxygen: 2Mg + O₂ → 2MgO ✓
  • B. Change O₂ to O to balance oxygen atoms
  • C. Add more magnesium on the RHS
  • D. Rewrite the formula of magnesium oxide as MgO₂

Answer: A — Coefficients are used to balance equations without changing chemical formulae; changing O₂ to O alters the element identity, and formulae cannot be altered during balancing.

Q5. In a laboratory, a student observes that when iron filings are added to copper sulphate solution, the blue colour gradually fades and a reddish precipitate forms. Which of the following correctly identifies what has happened?

  • A. A chemical reaction occurred; iron displaced copper because iron is more reactive ✓
  • B. Iron physically dissolved in the solution causing colour change
  • C. The blue colour faded due to dilution of the copper sulphate
  • D. Temperature decrease caused the colour change

Answer: A — Colour change from blue to colourless and appearance of reddish precipitate indicates formation of new substances (iron sulphate and copper) through a displacement reaction; physical dissolution doesn't produce a precipitate.

Q6. Assertion (A): A skeletal chemical equation for a reaction may not satisfy the law of conservation of mass. Reason (R): Skeletal equations show only the reactants and products without adjusting coefficients to balance atom numbers. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — Skeletal equations (like Mg + O₂ → MgO) are unbalanced and violate conservation of mass until coefficients are added; R correctly explains why A is true.

Q7. Assertion (A): In the balanced equation Zn + H₂SO₄ → ZnSO₄ + H₂, the number of hydrogen atoms is conserved. Reason (R): There are 2 hydrogen atoms on both the reactant and product sides of the equation. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — The equation is balanced with 2 H atoms on each side; R provides the evidence that supports assertion A perfectly.

Q8. Assertion (A): When magnesium burns in air, the product weighs more than the original magnesium ribbon. Reason (R): Oxygen from the air combines with magnesium to form magnesium oxide, increasing the total mass. Choose the correct option:

  • A. Both A and R are true and R is the correct explanation of A ✓
  • B. Both A and R are true but R is not the correct explanation of A
  • C. A is true but R is false
  • D. A is false but R is true

Answer: A — The white ash (MgO) weighs more because oxygen mass is added; R correctly explains why the product is heavier than the reactant magnesium alone.

Q9. An experiment shows: When 56 g of iron reacts with oxygen, it forms iron oxide. The total mass of the product is 80 g. What can be concluded about this reaction?

  • A. 24 g of oxygen was used; the law of conservation of mass is satisfied ✓
  • B. The reaction is incomplete and some iron remains unreacted
  • C. Some mass was lost during the experiment
  • D. The balanced equation cannot be determined from this data

Answer: A — By conservation of mass, product mass = reactant mass; 80 g = 56 g (Fe) + 24 g (O₂), confirming the law is satisfied; options B and C wrongly suggest mass loss.

Q10. A student balances the equation Fe + H₂O → Fe₃O₄ + H₂ by first writing: 3Fe + 4H₂O → Fe₃O₄ + 3H₂. They count: Fe: 3=3 ✓, H: 8≠6 ✗, O: 4=4 ✓. What does this reveal about their balancing process?

  • A. The equation is correctly balanced; all elements are equal on both sides
  • B. The equation is unbalanced because hydrogen atoms are not equal (8 on LHS from 4H₂O, but only 6 on RHS from 3H₂); the coefficient of H₂ should be 4, not 3 ✓
  • C. Oxygen is not properly balanced and requires a new coefficient
  • D. The student made an arithmetic error in counting

Answer: B — The student correctly counted 8 H atoms on the LHS (from 4H₂O) but only 6 H atoms on the RHS (from 3H₂), showing the equation is not yet balanced. Changing the coefficient of H₂ from 3 to 4 gives 3Fe + 4H₂O → Fe₃O₄ + 4H₂, which is correctly balanced: Fe: 3=3, H: 8=8, O: 4=4.

Flashcards

What is a chemical reaction?

A process in which the nature and identity of substances change, forming new substances with different properties.

Name four observations that prove a chemical reaction has occurred.

Change in state, change in colour, evolution of a gas, and change in temperature.

What is the difference between reactants and products?

Reactants are substances that undergo chemical change and appear on the left side of the equation; products are new substances formed and appear on the right side.

What is a word-equation?

A chemical reaction written in sentence form using the names of substances, showing reactants on the left and products on the right separated by an arrow.

What is a skeletal chemical equation?

A chemical equation written using chemical formulae instead of words, but which may not have equal atoms on both sides.

What is a balanced chemical equation?

A chemical equation in which the number of atoms of each element is equal on both the reactant and product sides, obeying the law of conservation of mass.

State the law of conservation of mass.

Mass can neither be created nor destroyed during a chemical reaction; total mass of reactants equals total mass of products.

Why do we use coefficients when balancing equations?

Coefficients show how many molecules of each substance react or are produced, without changing the chemical formula of any compound.

In balancing Fe + H₂O → Fe₃O₄ + H₂, why is H₂O₄ incorrect?

The chemical formula of a compound cannot be altered during balancing; only coefficients before the formula can be changed.

What does the arrow (→) in a chemical equation represent?

The direction of the reaction, showing the transformation of reactants into products.

Important Board Questions

Define a chemical reaction. Name four observations that help us determine whether a chemical reaction has taken place. [2 marks]

Chemical reaction = change in nature/identity of substances forming new substances. Four observations: colour change, state change, gas evolution, temperature change (pick any four from Activity 1.1, 1.2, 1.3 results).

Differentiate between a skeletal chemical equation and a balanced chemical equation. Why is balancing necessary? [3 marks]

Skeletal equation uses formulae but atoms are unequal on both sides; balanced equation has equal atoms of each element on LHS and RHS. Balancing is necessary because it obeys the law of conservation of mass—total mass of reactants must equal total mass of products.

Balance the following chemical equation and explain the steps you followed: Fe + H₂O → Fe₃O₄ + H₂. Why is it incorrect to write H₂O₄ instead of H₂O when balancing this equation? [5 marks]

Balanced equation: 3Fe + 4H₂O → Fe₃O₄ + 4H₂. Steps: box formulas, count atoms (Fe=1 vs 3, H=2 vs 2, O=1 vs 4), balance O first (4H₂O), then H (4H₂), then Fe (3Fe), verify all atoms equal. H₂O₄ is incorrect because chemical formulas of compounds cannot be altered—they represent the fixed composition of that substance; only coefficients before formulas can be changed to balance.

Assertion-Reason Questions for Chemical Reactions and Equations

Q1. Assertion (A):

A balanced chemical equation must have the same number of atoms of each element on both sides of the arrow.

Reason (R):

This requirement comes from the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is necessary. The law of conservation of mass is the fundamental principle that requires us to balance equations so that the number of atoms (and thus mass) remains constant before and after the reaction.

Q2. Assertion (A):

The chemical formula of a compound can be changed when balancing a chemical equation to make the atoms equal on both sides.

Reason (R):

Changing a formula would alter the identity of the substance being represented.

Answer: D — A is false but R is true — The assertion is false because we never change chemical formulae during balancing—we only adjust coefficients. The reason is true (changing formulas would alter identity), but since the assertion itself is false, the answer is D. This tests understanding that balancing uses coefficients, not formula manipulation.

Q3. Assertion (A):

When magnesium burns in oxygen to form magnesium oxide, this is a chemical reaction.

Reason (R):

The colour of the substance changes from silver (magnesium) to white (magnesium oxide), and a new substance with different properties is formed.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true. Burning magnesium is indeed a chemical reaction, and the colour change and formation of a new substance are valid observations. However, the reason is not the best explanation of why it's a chemical reaction—the fundamental reason is that a new substance with different chemical composition and properties is formed, not just because colour changes. Colour change is an indicator, not the cause.

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Case-Based Questions (CBQ) for Chemical Reactions and Equations

Case Study:

A chemistry teacher conducted an experiment where students burned magnesium ribbon in air and collected the white ash formed. The students observed that the magnesium ribbon, which was initially about 3-4 cm long and silver in colour, burned with a dazzling white flame. After the reaction, the collected ash was white in colour and appeared to have increased in mass compared to the original magnesium ribbon. When they wrote the word-equation, it was: Magnesium + Oxygen → Magnesium oxide. The teacher asked them to convert this into a chemical equation using formulae.

Q1. What is the correct chemical equation for the burning of magnesium in air?

  • A. Mg + O → MgO
  • B. 2Mg + O₂ → 2MgO ✓
  • C. Mg + O₂ → MgO₂
  • D. Mg + O₂ → MgO

Answer: B

Q2. Why did the mass of the ash appear to increase compared to the original magnesium ribbon?

  • A. Mass was created during the reaction
  • B. The magnesium combined with oxygen from the air, adding oxygen atoms to the product ✓
  • C. The heat from the reaction added mass to the substance
  • D. The white colour increases the apparent mass of the substance

Answer: B

Q3. Which observation in this experiment indicates that a chemical reaction has definitely taken place?

  • A. Only the change in colour from silver to white
  • B. Only the dazzling white flame
  • C. Both the change in colour and the formation of a new substance (magnesium oxide) ✓
  • D. The change in physical shape of the material

Answer: C

Q4. In the chemical equation 2Mg + O₂ → 2MgO, why are the coefficients 2 and 1 used?

  • A. To make the equation look more balanced visually
  • B. To indicate that two reactions occur simultaneously
  • C. Because this is how teachers write all equations
  • D. To ensure the number of Mg atoms and O atoms are equal on both sides of the equation ✓

Answer: D

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Previous Year Questions (PYQ) for Chemical Reactions and Equations

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 31/4/1 · 1 mark

The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is

  • 1 : 1
  • 2 : 1
  • 4 : 1
  • 1 : 2

CBSE 2026 · Set 31/4/1 · 1 mark

C6H12O6(aq) + 6O2(g) – 6CO2(g) + 6H2O(l). The above reaction is an example of -

  • Displacement Reaction
  • Endothermic Reaction
  • Exothermic Reaction
  • Neutralisation Reaction

CBSE 2026 · Set 31/4/1 · 3 marks

Account for the following: (a) White silver chloride turns grey when exposed to sunlight. (b) Lead (II) nitrate on heating releases brown fumes. (c) Bags of oil and fat containing food items are flushed with nitrogen gas.

CBSE 2026 · Set 31/4/2 · 1 mark

When lead nitrate is heated in a boiling tube, we observe:

  • brown fumes of O2 gas
  • white fumes of NO2 gas
  • yellow fumes of lead oxide
  • brown fumes of NO2 gas

CBSE 2026 · Set 31/4/2 · 1 mark

C6H12O6(aq) + 6O2(g) → 6CO2(g) + 6H2O(l). The above reaction is an example of -

  • Displacement Reaction
  • Endothermic Reaction
  • Exothermic Reaction
  • Neutralisation Reaction

CBSE 2026 · Set 31/4/2 · 3 marks

Answer the following: (a) Identify the substance oxidized and the substance reduced in the following reaction: 4Na + O2 → 2Na2O (b) What type of reaction occurs when Lead (II) nitrate solution is mixed with potassium iodide solution? (c) Why is silver chloride stored in dark coloured bottles?

CBSE 2026 · Set 31/4/3 · 3 marks

Answer the following: (a) Write two observations when green coloured crystals of ferrous sulphate are heated in a boiling tube. (b) Why brown coloured copper powder turns black on heating? (c) What happens when limestone is heated strongly?

CBSE 2026 · Set 31/5/1 · 1 mark

The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is:

  • 1 : 2
  • 2 : 1
  • 1 : 3
  • 4 : 1

CBSE 2026 · Set 31/5/1 · 1 mark

Assertion (A): Reaction of quick lime with water is an exothermic reaction. Reason (R): A large amount of heat is evolved on the reaction of quick lime and water.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

CBSE 2026 · Set 31/5/1 · 3 marks

What happens when (i) lead (II) nitrate is heated? (ii) iron (II) sulphate is heated? (iii) silver chloride is exposed to sunlight? Write balanced chemical equations of the reactions involved in support of your answer.

CBSE 2026 · Set 31/5/2 · 1 mark

The volume ratio of hydrogen and oxygen gases liberated during electrolysis of water is :

  • 1 : 2
  • 2 : 1
  • 1 : 3
  • 4 : 1

CBSE 2026 · Set 31/5/2 · 1 mark

Assertion (A) : Reaction of quick lime with water is an exothermic reaction. Reason (R) : A large amount of heat is evolved on the reaction of quick lime and water.

  • Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
  • Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
  • Assertion (A) is true, but Reason (R) is false.
  • Assertion (A) is false, but Reason (R) is true.

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Class 10 Science Formula Sheet

Every formula and chemical equation from the 10 chapters on one page, checked against the NCERT textbook and the CBSE 2026-27 syllabus. Free PDF download.

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Class 10 Science Previous Year Questions, Chapter-wise

All 875 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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