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.

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

1.	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» Solution :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 `H_(2)SO_(4)` `Zn(S)+H_(2)SO_(4) (aq) RARR ZnSO_(4)(aq)+H_(2)(g)` (II) Change in colour : thechemical reaction between potassium iodide solution and lead nitrate solution. ` underset (("Colourless"))(Pb(NO_(3))_(2)(aq))+2Klrarr2KNO_(3)(aq)+ underset (("Yellow"))(PbI_(2)(s))` (iii) FORMATION of precipitate : The chemical reaction between sulphuric and barium chloride solution. `BaCl_(2)(aq)+H_(2)SO_(4)(aq)rarr2HCl(aq)+underset(("White precipitate"))(BaSO_(4)(S))` (iv) Change in temperature : The chemical reaction between quick lime and water . `underset (("Quicklime"))CaO(S)+H_(2)O(l)rarrCa(OH)_(2)(aq)+"Heat"`