How would you justify that a chemical reaction has taken place in the following cases:
(i)    Burning of magnesium ribbon in air.
(ii)    Addition of lead nitrate solution to potassium iodide solution.
(iii)    Addition of dilute hydrochloric acid to zinc granules.

(i) Burning of magnesium ribbon in air gives a powder (of MgO). So there is a change of state.

Magnesium + Oxygen --> Magnesium oxide(MgO)

(ii) Addition of colourless lead nitrate solution to potassium iodide solution gives yellow
coloured precipitate (lead iodide). So there is a change of colour.

(iii) Addition of dilute hydrochloric acid to zinc granules gives a gas (H₂) with effervescence.