Welcome to the definitive study material for NCERT Class 10 Science Chapter 1. The first chapter of your science course, Chemical Reactions and Equations, paves the way for all future chemistry learning. In this chapter, you will be taught about how substances react to give rise to new chemicals and the methods by which such reactions can be written using chemical equations, apart from different kinds of reactions.
Knowledge of this chapter is important for your CBSE exam preparation as this is the basic foundation for a number of questions asked in both objective and subjective parts of the paper. You can understand the method of writing and balancing of chemical equations and identification of types of reactions and even real-life phenomenon like corrosion and rancidity with help of these solutions for Class 10 Science Chapter 1. These NCERT Class 10 Science Solutions have been made student-friendly, exam-oriented and completely in accordance with the CBSE syllabus.
Chapter Overview
In Chemical Reactions and Equations Class 10, you will explore the following major concepts:
- Identification of a chemical reaction by observation (change in state, color, production of gas, change in temperature).
- Writing chemical equations using symbols and formulae.
- Balancing of chemical equations based on Law of Conservation of Mass.
- Writing physical state and conditions (heat, catalyst, pressure) above the arrow in equations.
- Knowledge of the main categories of reactions: Combination Reaction, Decomposition Reaction, Displacement Reaction, and Double Displacement Reaction.
- Knowledge about Oxidation and Reduction Reactions (Redox Reaction).
- Effects of oxidation in everyday life: Corrosion and Rancidity.
Important Definitions
Chemical Reaction: A process where one or more substances (reactants) change into new substances (products) with different chemical properties.
Reactant: The substance that takes part in and undergoes a chemical change during a reaction.
Product: The new substance formed as a result of a chemical reaction.
Chemical Equation: A symbolic representation of a chemical reaction using chemical formulae and symbols.
Balanced Chemical Equation: An equation in which the number of atoms of each element on the reactant side is equal to the number of atoms of that element on the product side.
Combination Reaction: A reaction in which two or more reactants combine to form a single product.
Decomposition Reaction: A reaction in which a single reactant breaks down into two or more simpler products (usually requiring heat, light, or electricity).
Displacement Reaction: A reaction where a more reactive element displaces a less reactive element from its compound.
Double Displacement Reaction: A reaction in which two compounds exchange their ions to form two new compounds.
Oxidation: A process that involves the addition of oxygen or the removal of hydrogen from a substance.
Reduction: A process that involves the addition of hydrogen or the removal of oxygen from a substance.
Redox Reaction: A chemical reaction in which oxidation and reduction occur simultaneously.
Corrosion: The slow degradation of metals when they react with moisture, acids, or gases present in the atmosphere (e.g., rusting of iron).
Rancidity: The process by which fats and oils in food get oxidized, resulting in an unpleasant smell and bad taste.
Important Chemical Equations
Here are the most important equations every Class 10 student must know:
- Burning of Magnesium in Air
Equation: 2Mg(s) + O₂(g) → 2MgO(s)
Type: Combination / Oxidation
What happens: Magnesium ribbon burns with a dazzling white flame to form a white powder (magnesium oxide).
- Reaction of Zinc with Dilute Sulphuric Acid
Equation: Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑
Type: Displacement / Exothermic
What happens: Zinc displaces hydrogen from the acid, forming zinc sulphate and releasing hydrogen gas. The test tube becomes warm.
- Thermal Decomposition of Ferrous Sulphate
Equation: 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)
Type: Decomposition
What happens: Green colour of FeSO₄ changes to reddish-brown Fe₂O₃, accompanied by the smell of burning sulphur.
- Reaction of Lead Nitrate with Potassium Iodide
Equation: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) ↓ + 2KNO₃(aq)
Type: Double Displacement / Precipitation
What happens: A yellow precipitate of lead iodide is formed.
- Heating of Quicklime (Calcium Oxide)
Equation: CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Type: Combination / Exothermic
What happens: Quicklime reacts vigorously with water to form slaked lime, releasing a large amount of heat.
Note on equation formats:
Word Equation: Zinc + Sulphuric Acid → Zinc Sulphate + Hydrogen
Skeletal/Unbalanced Equation: Zn + H₂SO₄ → ZnSO₄ + H₂ (Unbalanced for H if incorrectly written, but acts as a draft before balancing atoms).
Balanced Equation: Fe + H₂O → Fe₃O₄ + H₂ balanced as: 3Fe + 4H₂O → Fe₃O₄ + 4H₂
Types of Chemical Reactions
Understanding the types of chemical reactions helps predict the products formed.
Combination Reaction
Definition: Two or more substances combine to form a single product.
General Explanation: A + B → AB
NCERT Example: Burning of coal or formation of water.
Balanced Equation: C(s) + O₂(g) → CO₂(g)
Identification Tip: Multiple reactants, single product.
Exam Point: Combination reactions can be exothermic (release heat). Example: CaO + H₂O → Ca(OH)₂ + Heat.
Decomposition Reaction
Definition: A single compound breaks down into two or more simpler substances.
General Explanation: AB → A + B
NCERT Example: Electrolysis of water, heating limestone.
Balanced Equation: 2H₂O(l) →[Electric Current] 2H₂(g) + O₂(g)
Identification Tip: Single reactant, multiple products. Requires energy input (heat, light, or electricity), making them endothermic.
Exam Point: Decomposition by heat is thermal decomposition; by sunlight is photolytic; by electricity is electrolytic. Example: 2AgCl(s) →[Sunlight] 2Ag(s) + Cl₂(g).
Displacement Reaction
Definition: A more reactive element displaces a less reactive element from its compound.
General Explanation: A + BC → AC + B
NCERT Example: Iron nail dipped in copper sulphate solution.
Balanced Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Identification Tip: Look for a free element reacting with a compound, changing colour (blue CuSO₄ turns light green FeSO₄).
Exam Point: The reactivity series of metals is crucial here. Iron is above copper, so it displaces copper.
Double Displacement Reaction
Definition: Two compounds react by exchanging their ions to form two new compounds.
General Explanation: AB + CD → AD + CB
NCERT Example: Sodium sulphate reacting with barium chloride.
Balanced Equation: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) ↓ + 2NaCl(aq)
Identification Tip: Often involves the formation of a solid called a precipitate (white BaSO₄ in this case).
Exam Point: These reactions often occur in aqueous solutions and are characterized by precipitate formation.
Oxidation and Reduction (Redox Reaction)
Definition: Oxidation is gain of oxygen or loss of hydrogen; reduction is loss of oxygen or gain of hydrogen. Both occur simultaneously.
General Explanation: A + B → A is oxidized, B is reduced.
NCERT Example: Heating of copper oxide with hydrogen.
Balanced Equation: CuO + H₂ → Cu + H₂O
Identification Tip: Track the oxygen/hydrogen. CuO loses O (reduced), H₂ gains O (oxidized).
Exam Point: The substance that gives oxygen is the oxidizing agent (CuO); the substance that takes oxygen is the reducing agent (H₂).
NCERT Exercise Questions and Solutions
Q1. Why should a magnesium ribbon be cleaned before burning in air?
Solution: The metal magnesium reacts slowly with moisture in the air to produce an oxide film on its surface. The film is stable and resists further combustion. Scrubbing off the oxide film using sandpaper allows the magnesium ribbon to burn in air readily.
Q2. Write the balanced equation for the following chemical reactions:
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Silver + Copper nitrate → (No reaction)
Solution:
(i) H₂(g) + Cl₂(g) → 2HCl(g)
(ii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(iii) Ag(s) + Cu(NO₃)₂(aq) → No reaction (Reason: Silver is less reactive than copper, so it cannot displace copper).
Q3. Write a balanced chemical equation with state symbols for the following reactions:
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.
Solution:
(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) ↓ + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
Q4. A solution of a substance ‘X’ is used for whitewashing. Name the substance ‘X’ and write its formula.
Solution:
The substance ‘X’ is Quicklime (Calcium oxide).
Formula: CaO.
Q5. Why is the amount of gas collected in one of the test tubes in the electrolysis of water double the amount collected in the other? Name this gas.
Solution: Water (H₂O) consists of hydrogen and oxygen in the proportion of 2:1 by volume. In electrolysis, hydrogen moves towards the cathode (negative electrode), while oxygen moves to the anode (positive electrode). As there are two molecules of hydrogen and one molecule of oxygen in the water molecule, the volume of hydrogen collected at the cathode would be twice the volume of oxygen collected at the anode. The twice volume gas is Hydrogen.
Q6. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
Solution: Iron is more reactive than copper. When an iron nail is dipped in copper sulphate solution, iron displaces copper from its solution, forming iron sulphate. The blue colour of copper sulphate fades and turns light green due to the formation of iron sulphate, and a brown coating of copper appears on the iron nail.
Equation: Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
Q7. Give an example of a double displacement reaction other than the one given in Activity 1.10.
Solution: When silver nitrate solution is added to sodium chloride solution, a white precipitate of silver chloride is formed along with sodium nitrate.
Equation: AgNO₃(aq) + NaCl(aq) → AgCl(s) ↓ + NaNO₃(aq)
Q8. Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(i) 4Na(s) + O₂(g) → 2Na₂O(s)
(ii) CuO(s) + H₂(g) → Cu(s) + H₂O(l)
Solution:
(i) Sodium (Na) is oxidised because it gains oxygen to form sodium oxide. Oxygen (O₂) is reduced.
(ii) Hydrogen (H₂) is oxidised to water because it gains oxygen. Copper oxide (CuO) is reduced to copper because it loses oxygen.
Q9. What is rancidity? Mention two ways to prevent it.
Solution: The oxidation of fats and oils in food when exposed to air, resulting in a foul smell and bad taste, is called rancidity.
Two ways to prevent rancidity:
- By adding antioxidants to foods containing fats and oils.
- By flushing food packaging (like potato chips) with nitrogen gas to displace oxygen.
Activity-Based Important Questions
Q1. In an experiment, the student is heating the ferrous sulphate crystals in a boiling tube. Which observations would be made?
Answer: Green crystals become reddish-brown solid (Fe₂O₃). Burning smell of sulphur is detected due to the formation of SO₂ and SO₃ gases.
Q2. If lead nitrate is added to the potassium iodide solution, which kind of reaction would be and what would be observed?
Answer: This is a double displacement (precipitation) reaction. Yellow precipitate of lead iodide (PbI₂) appears in the solution.
Q3. A student passes electricity through the acidified water. State the gases that are collected at the cathode and at the anode, respectively. Why the volume of one gas is smaller?
Answer: Gas at the cathode is hydrogen; oxygen gas is at the anode. The amount of oxygen gas is smaller because H₂O consists of two hydrogens and one oxygen.
Q4. A student put zinc granules into the dilute sulphuric acid. How does he/she realize that there is a chemical reaction?
Answer: Bubbles of hydrogen gas appear on the surface of the granules; test tube becomes warm (exothermic reaction).
Important Exam Questions
Very Short Answer Questions (VSA)
- State the Law of Conservation of Mass.
- What is the chemical formula of rust?
- Name the gas evolved when zinc reacts with dilute HCl.
- Define a balanced chemical equation.
- Name the reducing agent in the reaction: ZnO + C → Zn + CO.
- Why do we apply paint on iron articles?
- What happens when silver chloride is exposed to sunlight?
- Give one example of an exothermic combination reaction.
- What is the process called when food containing oils and fats gets spoiled?
- Write the colour of the precipitate formed when barium chloride reacts with sodium sulphate.
Short Answer Questions (SA)
- Differentiate between displacement and double displacement reactions with examples.
- Why do fireflies glow at night? (Hint: Exothermic reaction).
- Explain why respiration is considered an exothermic reaction.
- What is a precipitation reaction? Give an example with a balanced equation.
- Why should chips manufacturers flush bags with nitrogen gas?
- Translate the following statement into a balanced chemical equation: “Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.”
- Name the type of reaction represented by the equation: 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃.
- Why is it necessary to balance a chemical equation?
- What do you mean by an oxidation and reduction reaction? Give one example each.
- A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.
Long Answer Questions (LA)
- Define a chemical reaction. State the observations that help us determine whether a chemical reaction has taken place. Give one example of each observation.
- What are decomposition reactions? Explain the three types of decomposition reactions with one balanced equation each.
- What is a redox reaction? Explain with an example. Identify the substance oxidised and the substance reduced. Also, name the oxidizing and reducing agents.
- Explain the phenomenon of corrosion. Discuss the rusting of iron and write the chemical equation for the formation of rust. Mention three ways to prevent rusting.
- Describe an activity to show a displacement reaction. Include the materials required, procedure, observations, and the balanced chemical equation.
Case-Based Questions
Case 1:
A student takes water in a plastic mug, drills two holes at its base, and inserts carbon electrodes. He connects the electrodes to a 6V battery. He adds a few drops of dilute sulphuric acid to the water and places two test tubes filled with water inverted over the electrodes. He switches on the current.
Q1. Name the gases collected at the cathode and anode.
Q2. Why is dilute sulphuric acid added to the water?
Q3. What is this process called?
Answers:
- Cathode: Hydrogen gas • Anode: Oxygen gas.
- Dilute sulfuric acid is added because pure water is a poor conductor of electricity. The ions help conduct the current.
- Electrolysis of water.
Case 2:
Riya opened her lunch box after the school recess and found that the potato chips tasted bad and had a foul smell. Her teacher had explained a chemical concept the previous day regarding this.
Q1. What is this process of food spoilage called?
Q2. What is the chemical reason behind the foul smell?
Q3. Suggest two methods to prevent this from happening.
Answers:
Rancidity.
The oils and fats present in the chips get oxidized by atmospheric oxygen, changing their chemical structure and producing foul-smelling compounds.
Storing in airtight containers, adding antioxidants, or flushing with nitrogen gas.
Case 3:
During the Diwali festival, a child fills a hydrogen balloon and lights it. It bursts with a loud ‘pop’ sound, leaving behind tiny water droplets on the floor.
Q1. Write the balanced chemical equation for the reaction that occurred.
Q2. Identify the type of reaction.
Q3. Was the reaction endothermic or exothermic?
Answers:
2H₂(g) + O₂(g) → 2H₂O(l)
Combination reaction (also an oxidation reaction).
Exothermic reaction (as it produces a loud sound and releases energy).
MCQs with Answers and Explanations
Which of the following is a decomposition reaction?
(a) CaO + H₂O → Ca(OH)₂
(b) 2FeSO₄ → Fe₂O₃ + SO₂ + SO₃
(c) Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl
(d) Fe + CuSO₄ → FeSO₄ + Cu
Correct Answer: (b)
Explanation: A single reactant (FeSO₄) breaks down into multiple products.
Rusting of iron is an example of:
(a) Reduction
(b) Oxidation
(c) Decomposition
(d) Displacement
Correct Answer: (b)
Explanation: Iron gains oxygen to form iron oxide (rust), making it an oxidation process.
In the reaction ZnO + C → Zn + CO, carbon acts as:
(a) An oxidizing agent
(b) A reducing agent
(c) A catalyst
(d) A reactant that gets oxidized
Correct Answer: (b)
Explanation: Carbon removes oxygen from ZnO, reducing it to Zn. Hence, carbon is a reducing agent.
What is the colour of the precipitate formed when lead nitrate reacts with potassium iodide?
(a) White
(b) Yellow
(c) Red
(d) Green
Correct Answer: (b)
Explanation: Lead iodide (PbI₂) is a yellow precipitate.
Which gas is evolved when dilute hydrochloric acid is added to zinc granules?
(a) Oxygen
(b) Chlorine
(c) Hydrogen
(d) Carbon dioxide
Correct Answer: (c)
Explanation: Zn + 2HCl → ZnCl₂ + H₂↑
The formula of rust is:
(a) FeO
(b) Fe₂O₃
(c) Fe₂O₃·xH₂O
(d) FeCl₃
Correct Answer: (c)
Explanation: Rust is hydrated iron(III) oxide.
Which of the following reactions requires electricity to occur?
(a) Thermal decomposition
(b) Photolytic decomposition
(c) Electrolytic decomposition
(d) Combination reaction
Correct Answer: (c)
Explanation: Electrolytic decomposition uses electrical energy.
Slaking of lime (CaO + H₂O → Ca(OH)₂) is an example of a reaction that is:
(a) Endothermic and combination
(b) Exothermic and combination
(c) Exothermic and decomposition
(d) Endothermic and displacement
Correct Answer: (b)
Explanation: Two substances combine to form one product, releasing a large amount of heat.
When an iron nail is placed in copper sulphate solution, the colour of the solution changes from:
(a) Blue to green
(b) Green to blue
(c) Blue to brown
(d) White to green
Correct Answer: (a)
Explanation: Fe displaces Cu, forming pale green FeSO₄, leaving a brown copper coating.
To prevent rancidity, potato chip bags are flushed with:
(a) Oxygen
(b) Hydrogen
(c) Nitrogen
(d) Carbon dioxide
Correct Answer: (c)
Explanation: Nitrogen is an unreactive gas that prevents the oxidation of fats.
In a balanced chemical equation, the physical state of the precipitate is indicated by the symbol:
(a) (l)
(b) (g)
(c) (s)
(d) ↓
Correct Answer: (d)
Explanation: The downward arrow (↓) indicates a solid precipitate formed in an aqueous solution.
Which of the following is NOT a sign of a chemical reaction taking place?
(a) Change in state
(b) Change in colour
(c) Evolution of gas
(d) Melting of ice
Correct Answer: (d)
Explanation: Melting of ice is a physical change, not a chemical reaction.
What happens to silver chloride when exposed to sunlight?
(a) It turns black
(b) It turns grey
(c) It turns white
(d) No change
Correct Answer: (b)
Explanation: AgCl decomposes into white silver and chlorine gas, giving it a grey appearance.
The reactivity series of metals is used to predict:
(a) Corrosion
(b) Double displacement reactions
(c) Displacement reactions
(d) Decomposition reactions
Correct Answer: (c)
Explanation: A more reactive metal displaces a less reactive metal based on the reactivity series.
Which of the following is an endothermic reaction?
(a) Burning of natural gas
(b) Respiration
(c) Decomposition of calcium carbonate
(d) Slaking of lime
Correct Answer: (c)
Explanation: CaCO₃ needs to be heated to decompose, absorbing heat.
Most Important Questions for CBSE Class 10 Board Exam
- Explain the process of rusting of iron. Write the chemical equation. How can it be prevented?
- Balance the equation: Fe + H₂O → Fe₃O₄ + H₂ and state the type of reaction.
- Differentiate between exothermic and endothermic reactions with examples.
- Why is respiration considered an exothermic reaction? Explain.
- What is a redox reaction? Identify the substance oxidized and reduced in: CuO + H₂ → Cu + H₂O.
- Write one equation each for thermal, photolytic, and electrolytic decomposition reactions.
- Explain the reaction of quicklime with water. Mention the type of reaction and the observation.
- State the Law of Conservation of Mass. Why do we balance chemical equations?
- What is rancidity? Suggest three methods to prevent it.
- Identify the type of reaction: Pb(NO₃)₂ + 2KI → PbI₂ + 2KNO₃. Define the reaction type.
- Give reasons: (a) White silver chloride turns grey in sunlight. (b) Nails are coated with zinc.
- Write the balanced equation for the reaction between sodium hydroxide and hydrochloric acid.
Quick Revision Notes
Law of Conservation of Mass: Mass can neither be created nor destroyed in a chemical reaction.
Balanced Equation: Equal number of atoms of each element on both sides.
Combination: A + B → AB (Exothermic usually).
Decomposition: AB → A + B (Endothermic usually). Types: Thermal (heat), Photolytic (light), Electrolytic (electricity).
Displacement: A + BC → AC + B (Depends on reactivity series).
Double Displacement: AB + CD → AD + CB (Precipitate usually forms).
Oxidation: Gain of oxygen / Loss of hydrogen.
Reduction: Loss of oxygen / Gain of hydrogen.
Redox: Oxidation + Reduction simultaneously.
Corrosion: Rusting (Fe₂O₃.xH₂O), black coating on silver, green on copper.
Rancidity: Oxidation of fats/oils. Prevented by antioxidants, nitrogen flushing, airtight containers, refrigeration.
FAQs
Q1: What is a chemical reaction?
A: A chemical reaction is a process where reactants undergo a chemical change to form new products with different properties.
Q2: What is a balanced chemical equation?
A: A chemical equation where the number of atoms of each element on the reactant side is exactly equal to the number of atoms of that element on the product side.
Q3: What are the types of chemical reactions?
A: The main types are Combination, Decomposition, Displacement, Double Displacement, and Oxidation-Reduction (Redox) reactions.
Q4: What is oxidation?
A: Oxidation is a process that involves the addition of oxygen to a substance or the removal of hydrogen from it.
Q5: What is reduction?
A: Reduction is a process that involves the removal of oxygen from a substance or the addition of hydrogen to it.
Q6: What is a redox reaction?
A: A reaction in which oxidation and reduction occur simultaneously is called a redox reaction.
Q7: What is corrosion?
A: Corrosion is the slow process of degradation of metals when exposed to moisture and air (e.g., the rusting of iron).
Q8: What is rancidity?
A: Rancidity is the oxidation of fats and oils in food when exposed to air, resulting in a foul smell and bad taste.
Q9: How can corrosion be prevented?
A: By painting, greasing, galvanizing (coating with zinc), or alloying metals.
Q10: How can rancidity be prevented?
A: By adding antioxidants, storing food in airtight containers, refrigerating, or flushing food packets with nitrogen gas.