Question

what are the observation which help you to determine wheather a chemical reaction has taken place? give an example of chimical reaction for each observation​

Answer 1

evolution of gas:

Zn+ 2HCL --> Zncl2+H2

here (H2) is hydrogen gas

formation of precipitate:

Na2SO4+BaCl2-->BaSO4+2NaCl

here (BaSO4) , barium sulphate is a precipitate

change in colour

Pb(NO3)2+2KI--> PbI2+2KNO3

here Pb(NO3)2 which is colourless turnns to PbI2 which is yellow in colour

here PbI2 is lead chloride

Answer 2

hère is your answer mate we.....

when a chemical reaction takes place a new substance with new properties are formed i. e.

1-EVOLUTION OF gas- --> 2HCl+Zn=ZnCl2+H2(gas)

2-Change in colour - CuSO4(blue)+Zn-->ZnSO4(colourless)+Cu

3-Change in temperature.. CH4+2O2->

CO2+2H2O

4-Change in state -- burning of paper to form ash.

..

hop it helps you