Why do air bubbles appear in a glass of water after a while? —- Abid, Karaci, Pakistan
Marshall Brain Answers:
Here is the setup. Take a clear plastic (or glass) drinking glass. Fill it with cool tap water. Let it sit on the counter for a couple of hours. In some cases you will come back to find dozens of small bubbles clinging to the inside of the glass.
What you are seeing is something like a fizzing glass of soda happening in slow motion. If you were to take Sprite or 7-Up or club soda and pour it in a glass, you see the bubbles immediately. The water in the soda was put in a container with high-pressure carbon dioxide gas, and the gas dissolved in the water. When you release the pressure, the gas comes out of solution as thousands of bubbles. They tend to appear on the sides of the glass because rough spots on the glass are where the bubbles form.
In the case of water from the tap, the bubbles form (much more slowly) because air has dissolved in the water. If the tap water is cold and it warms up to room temperature as it sits on the counter, the warmer water is able to hold less gas (the amount of gas that dissolves is temperature-dependent), so the gas comes out of solution to form bubbles. Also note that the water is pressurized (say to 50 PSI) while it is sitting in the pipes. If there is any air in the system, the air can dissolve into the water.