A bleaching agent is a substance used to whiten or lighten the color of something. PEROXIDE, with 8 letters, is a chemical compound made up of hydrogen and oxygen that is often used as a bleaching agent. OZONE, with 5 letters, is a form of oxygen that can be used as a bleaching agent due to its oxidizing properties. LYE, with 3 letters, is a strong alkaline solution that is also used as a bleaching agent. Finally, SUN, with 3 letters, is a natural bleaching agent due to its ultraviolet radiation.