Question

Define a chemical reaction. State four observations which help us to determine that a chemical reaction has taken place. Write one example of each observation with a balanced chemical equation.

Answer 1

Process in which new substances with new properties are formed by the rearrangement of atoms is known as chemical reaction.
(i) Evolution of gas : The chemical reaction between zinc and dilute H₂SO₄.
Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g) ↑
(ii) Change in colour : The chemical reaction between potassium iodide solution and lead nitrate solution.
Pb (NO₃)₂(aq) + 2KI → 2 KNO₃(aq) + Pb I₂(s)
Colourless                                            Yellow
(iii) Formation of Precipitate : The chemical reaction between sulphuric acid and barium chloride solution.
BaCl₂(aq) + H₂SO₄(aq) → BaSO₄(s) ↓ + 2HCl(aq)
                                          White ppt.
(iv) Change in temperature : The chemical reaction between quick lime and water.
CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat