Chemical Reactions and Equations in Science

Updated on June 28, 2025 | By Learnzy Academy

Introduction

A chemical reaction is a process in which one or more substances (called reactants) change to form new substances (called products). These changes may involve color change, gas formation, temperature change, or formation of a solid.

1. Chemical Equation
A chemical equation is the symbolic representation of a chemical reaction using chemical formulas.

Example:
Magnesium + Oxygen → Magnesium oxide
Mg + O₂ → MgO

This is a chemical equation showing the reactants and the product.

2. Balanced Chemical Equation
The law of conservation of mass says that mass is neither created nor destroyed in a chemical reaction.
So, the number of atoms of each element must be the same on both sides of the equation.

Balanced example:
2Mg + O₂ → 2MgO

3. Types of Chemical Reactions
        Combination Reaction:--
             Two substances combine to form one compound.
              Example: CaO + H₂O → Ca(OH)₂

         Decomposition Reaction:--
              A single compound breaks down into simpler substances.
              Example: 2H₂O₂ → 2H₂O + O₂

          Displacement Reaction:--
               One element replaces another in a compound.
               Example: Zn + CuSO₄ → ZnSO₄ + Cu

         Double Displacement Reaction:
               Two compounds exchange their ions.
               Example: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl

4. Effects of a Chemical Reaction
       Chemical reactions may show:

  • Change in color
  • Evolution of gas
  • Formation of a precipitate
  • Change in temperature

 

 

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List of question on "Chemical Reactions and Equations"

  1. What do you mean by a precipitation reaction? Explain by giving examples.
  2. In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.
  3. What is the difference between displacement and double displacement reactions? Write equations for these reactions.
  4. Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.
  5. Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions
  6. Why is respiration considered an exothermic reaction? Explain
  7. What does one mean by exothermic and endothermic reactions? Give examples.
  8. What is a balanced chemical equation? Why should chemical equations be balanced?
  9. What happens when dilute hydrochloric acid is added to iron fillings? Tick the correct answer. (a) Hydrogen gas and iron chloride are produced. (b) Chlorine gas and iron hydroxide are produced. (c) No reaction takes place. (d) Iron salt and water are produced
  10. Fe₂O₃ + 2Al → Al₂O₃ + 2Fe The above reaction is an example of a (a) combination reaction. (b) double displacement reaction. (c) decomposition reaction. (d) displacement reaction.
  11. 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)
  12. Why does the colour of copper sulphate solution change when an iron nail is dipped in it?
  13. A solution of a substance ‘X’ is used for whitewashing. (i) Name the substance ‘X’ and write its formula. (ii) Write the reaction of the substance ‘X’ named in (i) above with water.
  14. 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
  15. Write the balanced equation for the following chemical reactions. (i) Hydrogen + Chlorine → Hydrogen chloride (ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride (iii) Sodium + Water → Sodium hydroxide + Hydrogen
  16. Why should a magnesium ribbon be cleaned before burning in air?
  17. A solution of substance ‘X’ is used for white washing. What is the substance ‘X’? State the chemical reaction of ‘X’ with water.
  18. Hydrogen being a highly inflammable gas and oxygen being a supporter of combustion, yet water which is a compound made up of hydrogen and oxygen is used to extinguish fire. Why?
  19. What is meant by a chemical reaction?
  20. 1 g of copper powder was taken in a China dish and heated. What change takes place on heating? When hydrogen gas is passed over this heated substance, a visible change is seen in it. Give the chemical equations of reactions, the name and the colour of the products formed in each case.
  21. Oil and fat containing food items are flushed with nitrogen. Why?
  22. A shiny brown coloured element ‘X’ on heating in air becomes black in colour. Name the element ‘X’ and the black coloured compound formed.