# A CHEMICAL REACTION IS CHARACTERIZED BY FORMATION OF A PRECIPITATE.

When a chemical reaction occurs, following observations help us to determine whether a chemical reaction has taken place.
(1) Formation of a precipitate
(2) Change in colour
(3) Evolution of a gas
(4) Change in state
(5) Change in temperature or evolution / absorption of heat
(6) Sound, light may be produced.
(7) Change in smell or a new smell can be given off

(1) Formation of a precipitate –
Activity –
Take lead nitrate solution in test tube. Add potassium iodide solution to this.

(a) What is your inference ?
Objective – A chemical reaction characterized by formation of a precipitate.
Inference – When potassium iodide (KI) solution is added to a solution of lead nitrate [ Pb(NO3)2 ], a yellow solid compound (or precipitate) of lead iodide and a colourless potassium nitrate solution appear.
Chemical equation –
Lead nitrate + Potassium iodide —–> Lead iodide + Potassium nitrate

(Colourless Solutions)                              (Yellow solid) (Colourless Solution)
Reactants                                                                 Products

Conclusion – A precipitate is a solid insoluble product which separates out from the solution during the reaction, characterises chemical reaction.