Molecular sieves are transition metal aluminosilicates with a uniform pore crystalline structure. Molecular sieves operate on the size exclusion principle. Smaller molecules that fit into the pores are adsorbed while larger molecules pass through. Molecular sieves have regular pore openings of 3, 4, 5, or 10 Angstroms. Polarity of the molecules matter because the highly polarized molecules are adsorbed more readily into the pores than non-polar molecules. Molecules are selectively removed in this manner.