Bubbles appear in standing tap water because of gases and other matter contained in the water. Some of these bubble-forming agents come from the plumbing, others from the source of the water, and still others from deliberate introduction. These bubbles should not normally cause any alarm, because they are almost always non-toxic and will disappear over time if left standing.
Air sometimes enters water pipes. This is the most common reason for bubbles in tap water. The oxygen and hydrogen in the air bonds with other water molecules, while the nitrogen and other gases form the bubbles that appear to the eye. Since this is the same air that we breathe, it has no ill health effects, and can be left to evaporate on its own. On rare occasions, other gases such as methane or radon may enter the water supply because of contamination. These, however, will often discolour the water in addition to adding bubbles. If you suspect that a poisonous gas is in your water supply, or it tests positive for such a gas, contact your local water company immediately.
As with air, sometimes small particles of dirt can enter the plumbing. When they flow with the water out of the tap and settle in a drinking glass, for example, they can cause bubbles to form because of the surface tension of water around pockets of air attached to themselves. These bubbles will evaporate, and the particles will settle away from the rest of the water over time due to gravity. If you are concerned about dirt, sand or other particles in the water, a carbon filter can help.
Over time, the metal pipes that we use to carry water into our homes can break, corrode and decay. If oxidised metal from the pipes enters the water, pieces of surface corrosion can dissolve in very small amounts in the water. When these come out of the tap, they act like other kinds of particles and capture bubbles around themselves. Again, these will settle over time, the bubbles they cause will evaporate and you can filter out these contaminants from the water supply.
Additives to Drinking Water
Some local and state governments mandate that certain chemicals, such as flouride, be added to the drinking water. On rare occasions, these chemicals may not fully dissolve, forming gases or particles that act like those described earlier. If you are concerned about additives to your drinking water, contact your local government to find out what they allow or mandate in the drinking water.