Equations E = 1 2πε0 qd z3 and E = 1 2πε0 P z3 are approximations of the magnitude of the electric field of an electric dipole,
at points along the dipole axis. Consider a point P on that axis at distance z = 4.50d from the dipole center (where d is the separation distance between the particles of the dipole). Let Eappr be the magnitude of the field at point P as approximated by E = 1 2πε0 qd z3 and E = 1 2πε0 P z3 (electric dipole). Let Eact be the actual magnitude. By how much is the ratio Eappr/Eact less than 1?
To separate a mixture of salt and water, you can try first by using filter paper hen with the extra water part set it out to the window so that the salt water evaporates and only the salt is remaining.