3A molecular sieve is an alkali metal aluminosilicate with a pore size of 3 angstroms (0.3 nanometers), created by potassium ion exchange in an A-type molecular sieve. Due to its internal structure containing numerous 3-angstrom crystal adsorption cavities, it can adsorb molecules with a diameter smaller than 3 angstroms while not adsorbing any molecules with a diameter greater than 3 angstroms. It has a strong selective adsorption for water molecules, making it commonly used as a selective adsorbent and drying agent for various gases and liquids.