The solubility of a protein depends on the proportion and the distribution of polar hydrophilic group and non-polar hydrophobic groups in the molecule. Polar groups of the proteins (–NH2, COO–, OH– groups) tend to attract water molecules towards them to produce a shell of hydration. Albumin has a greater degree of hydration than globulins. These characteristics of protein molecules depend upon their molecular weights, three-dimensional structures and properties of the constituent amino acids. Precipitation reactions for protein and scheme for identification of an unknown protein are covered in this chapter.