CBSE Class 10 Science: Chemical Reactions and Equations — Important Questions with Answers 2026

Short Answer Questions (2-3 Marks)

• Q: What is a chemical reaction? List four observations that indicate a chemical reaction has taken place.
  Ans: A chemical reaction is a process in which one or more substances (reactants) are converted into new substances (products) with different properties. Observations: (1) Change in colour — e.g., CuSO₄ (blue) + Fe → FeSO₄ (green). (2) Evolution of gas — e.g., Zn + HCl → ZnCl₂ + H₂↑. (3) Change in temperature — exothermic or endothermic. (4) Formation of precipitate — e.g., BaCl₂ + Na₂SO₄ → BaSO₄↓ (white).
• Q: What are the different types of chemical reactions? Give one example of each.
  Ans: (1) Combination: A + B → AB. Example: 2Mg + O₂ → 2MgO. (2) Decomposition: AB → A + B. Example: 2H₂O → 2H₂ + O₂ (electrolysis). (3) Displacement: A + BC → AC + B. Example: Fe + CuSO₄ → FeSO₄ + Cu. (4) Double displacement: AB + CD → AD + CB. Example: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl. (5) Redox: Simultaneous oxidation and reduction.
• Q: What is a redox reaction? Identify the substance oxidised and reduced in: CuO + H₂ → Cu + H₂O.
  Ans: A redox reaction involves simultaneous oxidation (loss of electrons/gain of oxygen) and reduction (gain of electrons/loss of oxygen). In CuO + H₂ → Cu + H₂O: CuO is reduced to Cu (loses oxygen). H₂ is oxidised to H₂O (gains oxygen). CuO is the oxidising agent, H₂ is the reducing agent.

Long Answer / Application Questions (4-6 Marks)

• Q: What is corrosion? How can it be prevented?
  Ans: Corrosion is the slow destruction of metals by reaction with air, moisture, and chemicals. Example: Rusting of iron — 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃.xH₂O (brown rust). Prevention: (1) Oiling, greasing, painting. (2) Galvanisation (coating with zinc). (3) Electroplating. (4) Alloying (stainless steel). (5) Chrome plating. Silver tarnishes (Ag₂S, black) and copper develops green coating (CuCO₃, patina).
• Q: Balance the equation: Fe₂O₃ + Al → Al₂O₃ + Fe. What type of reaction is this?
  Ans: Balanced equation: Fe₂O₃ + 2Al → Al₂O₃ + 2Fe. This is a displacement reaction (aluminium displaces iron from its oxide). It is also known as the thermite reaction — highly exothermic, used for welding railway tracks. Al is more reactive than Fe.

Exam Tips for This Chapter

• Revise all definitions and laws from Chemical Reactions and Equations — they are commonly asked as 1-2 mark questions
• Practice diagrams related to Chemical Reactions and Equations — neat labelled diagrams carry 2-3 marks
• For numericals, always show formula → substitution → answer with correct units
• Previous year analysis shows Chemical Reactions and Equations carries 8-12 marks in the board exam