Why Do Bubbles Form In A Glass Of Water?

Water especially the one which flows through the pipe contains atmospheric gases, such as nitrogen and oxygen, dissolved in it. As the glass filled with water sits out for a few hours, its temperature rises slightly (water gets warmer), which causes the dissolved gases in it to come out of the water and form bubbles along the inside of the glass.

When poured water in a glass is left at room temperature for a few hours, the atmospheric pressure begins to drop as its temperature begins to rise. As a result, the dissolved gases in the water escape from the solution and form bubbles in the rough areas inside the glass.