Amino acids are organic compounds containing amine(basic) and carboxyl (acidic) functional group with a specific side chain. Both acidic and basic group are present in the same molecule. In, aqueous solution carboxyl group can lose a proton(H⁺) and amino group can accept a proton (H⁺) giving rise to the dipolar ion called as zwitter ion. Zwitter ion is shown below:

In this zwitter ion there is the presence of both positive as well as negative charge, so there is the development of strong electrostatic force of attraction between the molecules and the water. For this reason solubility of amino acids is higher. Due to strong electrostatic interaction there is formation of strong bond between them, hence the melting point of amino acid is higher than the halo acids(as they do not exhibit such dipolar behaviour).