Bleach’s active ingredient is sodium hypochlorite, which provides broad‑spectrum disinfecting power. When freshly manufactured, a standard household concentration of 5‑6% remains stable for up to a year if stored properly. However, the chemical is highly susceptible to photolysis, oxidation, and thermal degradation, all of which accelerate over time and reduce the free‑chlorine content that is responsible for killing germs.

Beyond chemical changes, the physical environment of the bottle also matters. Containers made of PET or high‑density polyethylene help shield the solution from light, while tightly sealed caps prevent evaporation. Even under ideal storage, a small portion of the bleach will break down each month, and most manufacturers recommend using the product within 6–12 months of opening. Understanding these dynamics allows homeowners to gauge when replacement is necessary and avoid over‑reliance on weakened disinfectants.