Class 10 Science Chapter 1 — Chemical Reactions and Equations
Welcome to HSLC Guru. Here you will find complete question-answers, summary, additional questions and important notes for ASSEB Class 10 Science (English Medium), Chapter 1 — “Chemical Reactions and Equations”. The material is organised to help students prepare confidently for the HSLC examination conducted by the Assam State School Education Board (ASSEB).
Summary
A chemical reaction is a process in which one or more substances (called reactants) are converted into one or more new substances (called products) with different chemical properties. The reactants and products may differ in colour, state, temperature, smell or evolution of gas. Common indicators that a chemical reaction has taken place are change in state, change in colour, evolution of a gas, change in temperature, and formation of a precipitate. A chemical reaction is represented in a short symbolic form called a chemical equation, where reactants are written on the left-hand side and products on the right-hand side, separated by an arrow pointing towards the products.
According to the Law of Conservation of Mass, mass can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants must equal the total mass of the products. To satisfy this law, a chemical equation must be balanced, that is, the number of atoms of each element on both sides of the equation must be the same. The method commonly used to balance simple equations is the hit-and-trial method. Additional information such as the physical state of substances [(s), (l), (g), (aq)], conditions of temperature, pressure, catalyst, and whether heat is evolved or absorbed may also be written along with the equation to make it more informative.
Chemical reactions are classified into different types based on the nature of the chemical change. The major types are: (i) Combination reaction — two or more reactants combine to form a single product (e.g., CaO + H2O → Ca(OH)2); (ii) Decomposition reaction — a single reactant breaks down into two or more simpler products by heat (thermal), light (photolytic) or electricity (electrolytic); (iii) Displacement reaction — a more reactive element displaces a less reactive element from its compound (e.g., Fe + CuSO4 → FeSO4 + Cu); (iv) Double-displacement reaction — two compounds exchange their ions to form two new compounds, often producing a precipitate (e.g., Na2SO4 + BaCl2 → BaSO4↓ + 2NaCl); and (v) Oxidation–reduction (Redox) reactions — reactions in which oxidation and reduction occur simultaneously. Reactions are also classified as exothermic (heat released) or endothermic (heat absorbed).
Oxidation is the addition of oxygen or removal of hydrogen, while reduction is the addition of hydrogen or removal of oxygen. The substance that gets oxidised is the reducing agent, and the substance that gets reduced is the oxidising agent. The slow conversion of metals into undesirable compounds (such as oxides, sulphides or carbonates) due to attack by atmospheric gases and moisture is called corrosion — the rusting of iron and the black coating on silver are familiar examples. The slow oxidation of fats and oils present in food materials, leading to a change in taste and smell, is known as rancidity. Rancidity can be prevented by adding antioxidants, packing food in air-tight containers, flushing with nitrogen, refrigeration and keeping food away from sunlight.
Textbook Question Answers — Very Short Answer (1 Mark)
Q1. What is a chemical reaction?
Answer: A chemical reaction is a process in which one or more substances (reactants) are converted into one or more new substances (products) having different chemical properties.
Q2. What is a chemical equation?
Answer: The symbolic representation of a chemical reaction with the help of symbols and formulae of substances involved is called a chemical equation.
Q3. State the Law of Conservation of Mass.
Answer: The Law of Conservation of Mass states that mass can neither be created nor destroyed in a chemical reaction; the total mass of the reactants is always equal to the total mass of the products.
Q4. Why must a chemical equation be balanced?
Answer: A chemical equation must be balanced to satisfy the Law of Conservation of Mass, which requires the number of atoms of each element to be equal on both sides of the equation.
Q5. What is a combination reaction? Give one example.
Answer: A reaction in which two or more reactants combine to form a single product is called a combination reaction. Example: CaO + H2O → Ca(OH)2.
Q6. What is a decomposition reaction?
Answer: A reaction in which a single reactant breaks down into two or more simpler products is called a decomposition reaction. Example: 2FeSO4 → Fe2O3 + SO2 + SO3.
Q7. Define oxidation and reduction.
Answer: Oxidation is the addition of oxygen or removal of hydrogen from a substance. Reduction is the addition of hydrogen or removal of oxygen from a substance.
Q8. What is corrosion?
Answer: The slow conversion of metals into undesirable compounds such as oxides, sulphides or carbonates due to the attack of moisture, air and other chemicals is called corrosion. The rusting of iron is a common example.
Q9. What is rancidity?
Answer: When fats and oils present in food materials are oxidised on exposure to air, their taste and smell change. This phenomenon is called rancidity.
Q10. Why is the colour of copper sulphate solution changed when an iron nail is dipped in it?
Answer: Because iron is more reactive than copper, it displaces copper from copper sulphate, forming light green iron sulphate. Hence, the blue colour of copper sulphate solution fades. This is a displacement reaction: Fe + CuSO4 → FeSO4 + Cu.
Textbook Question Answers — Short Answer (2-3 Marks)
Q1. Why should a magnesium ribbon be cleaned before burning in air?
Answer: A magnesium ribbon, when exposed to air, gets coated with a thin layer of magnesium oxide (MgO) due to slow oxidation. This protective layer prevents the metal from burning easily. Hence, before burning in air, the magnesium ribbon must be cleaned with sandpaper to remove this layer of magnesium oxide so that it may burn freely with a dazzling white flame to form magnesium oxide. The reaction is: 2Mg + O2 → 2MgO.
Q2. Write the balanced equations for the following reactions:
(a) Hydrogen + Chlorine → Hydrogen chloride
(b) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(c) Sodium + Water → Sodium hydroxide + Hydrogen
Answer:
(a) H2 + Cl2 → 2HCl
(b) 3BaCl2 + Al2(SO4)3 → 3BaSO4 + 2AlCl3
(c) 2Na + 2H2O → 2NaOH + H2
Q3. Why is respiration considered an exothermic reaction? Explain.
Answer: During respiration, glucose combines with oxygen in the cells of our body and produces carbon dioxide and water along with the release of energy. Since energy is released during this process, respiration is considered an exothermic reaction. The reaction is:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy
Q4. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.
Answer: In a combination reaction, two or more reactants combine to form a single product, while in a decomposition reaction, a single reactant breaks down into two or more simpler products. Therefore, decomposition reactions are the opposite of combination reactions.
Example of combination: CaO + H2O → Ca(OH)2
Example of decomposition: CaCO3 → CaO + CO2 (on heating)
Q5. Identify the substances that are oxidised and the substances that are reduced in the following reactions:
(i) 4Na + O2 → 2Na2O
(ii) CuO + H2 → Cu + H2O
Answer:
(i) Sodium (Na) is oxidised to Na2O (gain of oxygen). Oxygen is reduced (it gets combined with sodium).
(ii) Hydrogen (H2) is oxidised to H2O (gain of oxygen). Copper oxide (CuO) is reduced to copper (loss of oxygen).
Q6. What is meant by a precipitation reaction? Explain with an example.
Answer: A reaction in which an insoluble solid (called a precipitate) is formed when two solutions are mixed is called a precipitation reaction. For example, when sodium sulphate solution is added to barium chloride solution, a white precipitate of barium sulphate is formed:
Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq)
Textbook Question Answers — Long Answer (5-6 Marks)
Q1. Explain different types of chemical reactions with one example each.
Answer: Chemical reactions are classified into the following major types:
(i) Combination Reaction: Two or more reactants combine to form a single product. Example: CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat. (Quick lime + water → slaked lime)
(ii) Decomposition Reaction: A single reactant breaks down into two or more simpler products in the presence of heat, light or electricity. Example: 2H2O → 2H2 + O2 (electrolysis of water).
(iii) Displacement Reaction: A more reactive element displaces a less reactive element from its compound. Example: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s).
(iv) Double-Displacement Reaction: Two compounds exchange their ions to form two new compounds. Example: Na2SO4(aq) + BaCl2(aq) → BaSO4(s)↓ + 2NaCl(aq).
(v) Oxidation–Reduction (Redox) Reaction: A reaction in which oxidation and reduction take place simultaneously. Example: CuO + H2 → Cu + H2O. Here CuO is reduced to Cu and H2 is oxidised to H2O.
(vi) Exothermic and Endothermic Reactions: Reactions that release heat are exothermic (e.g., respiration, burning of fuels), while reactions that absorb heat are endothermic (e.g., decomposition of CaCO3 on heating).
Q2. Balance the following chemical equations and add state symbols:
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(c) NaCl + AgNO3 → AgCl + NaNO3
(d) BaCl2 + H2SO4 → BaSO4 + HCl
Answer:
(a) 2HNO3(aq) + Ca(OH)2(aq) → Ca(NO3)2(aq) + 2H2O(l)
(b) 2NaOH(aq) + H2SO4(aq) → Na2SO4(aq) + 2H2O(l)
(c) NaCl(aq) + AgNO3(aq) → AgCl(s)↓ + NaNO3(aq)
(d) BaCl2(aq) + H2SO4(aq) → BaSO4(s)↓ + 2HCl(aq)
Q3. What is corrosion? How is rusting of iron prevented? Explain.
Answer: Corrosion is the slow conversion of metals into undesirable compounds (such as oxides, sulphides or carbonates) due to the attack of moisture, air, acids and other chemicals present in the atmosphere. When iron objects are exposed to moist air for a long time, a brown flaky substance called rust (Fe2O3·xH2O) is formed on the surface. Rusting weakens the iron object and gradually destroys it.
Methods of preventing rusting of iron:
- Painting and oiling/greasing: A coat of paint, oil or grease prevents direct contact of iron with air and moisture.
- Galvanisation: Iron is coated with a thin layer of zinc, which protects it from rusting.
- Tin plating and chromium plating: Iron objects are coated with tin or chromium to prevent rusting.
- Alloying: Iron is mixed with metals like nickel and chromium to form stainless steel, which does not rust.
- Cathodic protection: Connecting the iron object to a more active metal such as magnesium or zinc which acts as a sacrificial anode.
Q4. What is rancidity? Suggest methods to prevent it.
Answer: When fats and oils present in food materials are oxidised on exposure to air, their taste and smell change and become unpleasant. This phenomenon is called rancidity. Rancid food becomes unfit for human consumption.
Methods of preventing rancidity:
- Adding antioxidants (such as BHA and BHT) to fatty foods.
- Storing food in air-tight containers to prevent contact with oxygen.
- Flushing the food packets with an inert gas like nitrogen (e.g., chips packets).
- Keeping food in the refrigerator at low temperature to slow down oxidation.
- Storing food away from direct sunlight, since light accelerates oxidation.
Q5. A solution of a substance ‘X’ is used for white-washing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.
Answer: (i) The substance ‘X’ used for white-washing is calcium oxide (quick lime). Its chemical formula is CaO.
(ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime), releasing a large amount of heat. This is a combination reaction as well as an exothermic reaction:
CaO(s) + H2O(l) → Ca(OH)2(aq) + Heat
The slaked lime so formed reacts slowly with carbon dioxide present in the air to form a thin shiny coating of calcium carbonate on the walls. Calcium carbonate gives a shiny finish, which is why slaked lime is used for white-washing the walls.
Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)
Additional Questions — MCQ
Q1. Which of the following is a combination reaction?
(a) 2KClO3 → 2KCl + 3O2
(b) MgO + H2O → Mg(OH)2
(c) Zn + CuSO4 → ZnSO4 + Cu
(d) AgNO3 + NaCl → AgCl + NaNO3
Answer: (b) MgO + H2O → Mg(OH)2
Q2. The chemical formula of rust is:
(a) FeO
(b) Fe3O4
(c) Fe2O3·xH2O
(d) FeS
Answer: (c) Fe2O3·xH2O
Q3. In the reaction Pb + CuCl2 → PbCl2 + Cu, lead is acting as:
(a) An oxidising agent
(b) A reducing agent
(c) Both oxidising and reducing agent
(d) Neither oxidising nor reducing agent
Answer: (b) A reducing agent
Q4. The reaction between zinc granules and dilute sulphuric acid is:
(a) A combination reaction
(b) A decomposition reaction
(c) A displacement reaction
(d) A double-displacement reaction
Answer: (c) A displacement reaction
Q5. Which of the following is an endothermic process?
(a) Burning of coal
(b) Respiration
(c) Decomposition of CaCO3
(d) Slaking of lime
Answer: (c) Decomposition of CaCO3
Q6. The colour of silver chloride which decomposes in sunlight is:
(a) Yellow
(b) White
(c) Black
(d) Brown
Answer: (b) White (it turns grey on exposure to sunlight)
Q7. Which of the following is a redox reaction?
(a) NaOH + HCl → NaCl + H2O
(b) BaCl2 + Na2SO4 → BaSO4 + 2NaCl
(c) ZnO + C → Zn + CO
(d) CaCO3 → CaO + CO2
Answer: (c) ZnO + C → Zn + CO
Q8. Which gas is evolved when dilute hydrochloric acid is added to zinc?
(a) Oxygen
(b) Hydrogen
(c) Carbon dioxide
(d) Chlorine
Answer: (b) Hydrogen
Q9. When ferrous sulphate crystals are heated, the residue is:
(a) Green
(b) Brown (Fe2O3)
(c) Black
(d) White
Answer: (b) Brown (Fe2O3)
Q10. The chemical reaction between calcium oxide and water is an example of:
(a) Endothermic combination reaction
(b) Exothermic combination reaction
(c) Endothermic decomposition reaction
(d) Exothermic decomposition reaction
Answer: (b) Exothermic combination reaction
Additional Questions — Fill in the Blanks
Q1. A chemical equation must be balanced to satisfy the Law of Conservation of __________.
Answer: Mass.
Q2. The reaction in which a single reactant breaks down to give two or more products is called a __________ reaction.
Answer: Decomposition.
Q3. The slow oxidation of fats and oils, leading to a change in taste and smell, is called __________.
Answer: Rancidity.
Q4. Iron is coated with a thin layer of zinc to prevent rusting. This process is called __________.
Answer: Galvanisation.
Q5. Reactions in which heat is released along with the formation of products are called __________ reactions.
Answer: Exothermic.
Additional Questions — True / False
Q1. The number of atoms of each element on both sides of a balanced chemical equation is equal.
Answer: True.
Q2. Photosynthesis is an exothermic reaction.
Answer: False (it is an endothermic reaction because plants absorb sunlight to make food).
Q3. Copper sulphate solution turns colourless when an iron nail is dipped in it for some time.
Answer: False (the blue colour fades to light green; it does not become completely colourless).
Q4. The decomposition of calcium carbonate (limestone) by heating is an endothermic reaction.
Answer: True.
Q5. Reduction is the addition of oxygen or removal of hydrogen.
Answer: False (reduction is the addition of hydrogen or removal of oxygen).
Glossary / Key Terms
| Term | Meaning |
|---|---|
| Chemical Reaction | A process in which one or more substances are converted into one or more new substances with different properties. |
| Reactants | Substances that take part in a chemical reaction (written on the left side of the equation). |
| Products | New substances formed in a chemical reaction (written on the right side of the equation). |
| Chemical Equation | The symbolic representation of a chemical reaction using formulae and symbols. |
| Balanced Equation | An equation in which the number of atoms of each element is equal on both sides. |
| 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. |
| Displacement Reaction | A reaction in which 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. |
| Precipitation Reaction | A reaction in which an insoluble solid (precipitate) is formed when two solutions are mixed. |
| Oxidation | Addition of oxygen or removal of hydrogen from a substance. |
| Reduction | Addition of hydrogen or removal of oxygen from a substance. |
| Redox Reaction | A reaction in which oxidation and reduction take place simultaneously. |
| Oxidising Agent | The substance that gives oxygen (or accepts hydrogen) to another substance during a reaction. |
| Reducing Agent | The substance that gives hydrogen (or accepts oxygen) to another substance during a reaction. |
| Exothermic Reaction | A reaction in which heat is released along with the formation of products. |
| Endothermic Reaction | A reaction in which heat is absorbed during the formation of products. |
| Corrosion | The slow conversion of metals into undesirable compounds (oxide, sulphide, carbonate) due to attack of moisture and air. |
| Rusting | Corrosion of iron in moist air to form a brown flaky substance Fe2O3·xH2O. |
| Rancidity | Oxidation of fats and oils in food, leading to change in taste and smell. |
| Galvanisation | Coating iron with a thin layer of zinc to prevent rusting. |
| Quick Lime | Calcium oxide (CaO) — used in white-washing. |
| Slaked Lime | Calcium hydroxide [Ca(OH)2] — formed when CaO reacts with water. |
| Antioxidants | Substances added to food to prevent oxidation and rancidity (e.g., BHA, BHT). |