The water that comes out of your tap has existed on Earth as a solid, liquid, or gas for about as long as the planet is old. Just about every living organism today consumes and expels water to survive, and while it can be contaminated, or the 2 hydrogen and 1 oxygen molecules (H2O) separated and put back together, at no point will anyone complain about expired water.
That’s not to say you won’t find an expiration date on your bottle of water. At one time a New Jersey law did require that companies use an expiration dates on bottled water products, despite the FDA stating there is no scientific reason to do so. Today some companies still do this to help control stock. Stores may pull unsold cases of water after a few years due to quality concerns. Water can leach plastic flavors, or if there are minerals added or naturally present, they may settle and cause unsightly granules that people won’t want to see floating in the water.
That begs the question: If water doesn’t expire, why does water that sits out in a glass overnight taste or look different?
A few different things are happening with your drinking water. If you take a glass and fill it with tap water you might find there are a few unwelcome flavors, though it is still safe to drink. The big reason for bad tastes right from the faucet is chlorine used to disinfect the water, but because water normally travels through underground pipes that are cooler than the air at ground level, the flavor pallet is diminished. The same happens when you add ice or if you keep a Brita pitcher in the fridge. As the water sits out and warms, your senses begin to pick up a more complete profile of flavors. After a day or so, due to exposure to sunlight, particles in the air, various gases, and changes to acidity, a different appearance and tastes become stronger.
Due to bottled water being sealed, even if you leave it out at room temperature for years, the tastes probably won’t change very much from the original source of water. Once you break the seal expect the water to begin a slow transformation depending on how long the cap was left off. But you probably won’t want to keep the water around too long. Even though water doesn’t expire chlorine does evaporate. This leaves room for microorganisms that are in your mouth, on the glass, your faucet, or floating around the air and will quickly multiply in your open bottle or half-empty glass. Unlike juices or sodas, or foods that contain sugars or other elements that feeds the growth of mold, water will continue to look clear despite the likelihood of potentially harmful bacteria forming after 48 hours.
When storing bottled water or other emergency supplies there are a few precautions to take. Always keep water, food stuffs, and medicine in a dark, cool place where it won’t come into contact with animals or accidentally become damaged. You probably should not bottle your own water unless you know how to do so safely. While bottled water isn’t environmentally safe and can contain many contaminants, there is a much lower risk to coming into contact with microorganisms. There are many ways to ensure your water is free of most contaminants and bacteria including Reverse Osmosis, Ultraviolet, and NSF 53-rated water filters.