Question

write the 4 changes which helps us to determine whether a chemi
cal reaction takes place

Answer 1

1) Change in colour.
2) Change in smell.
3) Evolution of gas.
4) Change in shape and size.

Hope it helps :)

Answer 2

The following observations help us determine the occurrence of a chemical reaction :

  (a) Change in physical state. For e.g water when heated upto 100 degree centigrade forms water vapour.

          H2O(l) →∆100∘C H2O(g)

 (b) Change in colour. For e.g When we add aluminium rod to blue solution of copper sulpahte.colour changes from blue to colourless.
    
              2Al(s) +3 CuSO4(aq) →3Cu(s) +Al2(SO4)3                     (Blue)                              (Colourless)

(c) Evolution of a gas. For e.g When you add HCl over Zn metal.colourless H2 gas is evolved which burns with a pop sound.
         
           Zn(s) + 2HCl (aq) →ZnCl2(aq) + H2(g)

(d) Change in temperature. For e.g when sodium hydroxide is dissolved in water,there is rise in temperature as container becomes warm.
    
              NaOH(s) + H2O(l) →Na+ + H+ + 2 OH- + heat




hope it helps you!!