The term 'chemical' can refer to a variety of substances, and the crossword solutions AGENT (5 letters), ESTER (5 letters), and LYE (3 letters) are all examples of chemicals. An AGENT is a substance that causes a reaction or produces a change in another substance. An ESTER is an organic compound formed by the reaction between an acid and an alcohol. Lastly, LYE is a strong alkaline solution, usually sodium hydroxide, used in making soaps and detergents.