If 250ml of 0.5m nacl solution is diluted to 840ml what is the molarity of the resulting solution

M₁V₁ = M₂V₂ (where M₁ & V₁ are the starting molarity and volume and M₂                               & V₂ are the molarity and volume after dilution)

Substitute known values into the equation and use letters to represent unknown
(0.5M) (250ml) = (x) (840ml)

since 1 ml = 1cm³ then
(0.5M) (250cm³) = (x) (840cm³)

transpose and solve for x




        
∴  molarity when diluted to 840 ml  =  0.149 M

Yes it would be a physical change.

Answer) by studying proton collisions hope this : )