Define a chemical reaction. State four observations which help us to determine that a chemical reaction

Monjur Alahi

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.


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Salim Ahmed Kawsar

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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

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