Tap water 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.
Fill a glass with water (at or below room temperature) and leave it undisturbed for a few hours (you can do this using tap water). You will eventually notice that very small bubbles begin to appear along the side of the glass (on the inside). Why does this happen?
Solubility of gases in water
The water that comes out of taps and faucets travels through pipes before reaching large storage tanks. That’s why the water is at a higher pressure than normal (while in the pipes), and is also cooler. These two conditions are ideal for it to dissolve certain gases that are found abundantly in our atmosphere, including nitrogen and oxygen.
Gases are more soluble in water at colder temperatures
Gases, in general, are more soluble in water when it’s at a lower temperature. In other words, you can say that the solubility of gases in water decreases as water temperature increases. The graph below should help to visualize how the solubility of gases varies with changing temperature.
The downward trend of gases’ solubility with increasing temperature is quite similar to how vapor pressure increases with temperature. You see, more gas molecules are dissolved in water when it’s cold. However, when water starts getting warmer (i.e., the temperature rises), the kinetic energy of gas molecules also increases.
This allows those molecules to move more freely and break the intermolecular bonds that hold them together, thus escaping from the solution. That’s why the solubility of gases decreases as temperature rises.
Danger to aquatic life due to changing water temperatures
The temperature dependence of gases’ solubility of water has a direct implication on the lives of aquatic creatures. You see, aquatic creatures like fish need oxygen for survival, and they get it from the water they live in by absorbing the dissolved oxygen (in water) through their gills. Given that oxygen is more soluble in colder waters, it’s crucial that the temperature does not rise beyond a certain limit.
However, that’s exactly what’s happening these days, largely due to human activities. Power plants, for example, discharge huge volumes of hot water into large bodies of water, which then raise the temperature of water and drastically impact aquatic life. That’s one very undesirable outcome of such temperature dependence of gas solubility.
Gases’ solubility in water increases with increasing pressure
Although liquids and solids exhibit practically no change of solubility with a changing pressure of water, gases do. It has been observed that gases are more soluble in water at higher pressures. Carbonated drinks are excellent examples of this phenomenon.
The effects of changing the pressure of water on the solubility of gases can be explained by one of the gas laws known as Henry’s law. It states that “the amount of dissolved gas is proportional to its partial pressure in the gas phase”
The simplest way to explain this dependence is that as pressure increases, gas molecules are forced into the solution so that the applied pressure can be relieved. The following image should help to visualize this better:
Why bubbles form in a glass full of water overnight
Due to the two physical phenomenon we discussed above, tap water becomes a good candidate (if not ideal) to hold dissolved atmospheric gases. However, when that water is poured into a glass and left for a few hours at room temperature, its pressure starts to decrease, while its temperature begins to rise.
This makes the dissolved gases in the water come out of the solution and form bubbles on the rough spots on the inside of the glass. Since the temperature shift is not that rapid (meaning that the water doesn’t go from chilled to searing hot in a few minutes), it takes a few hours before the bubbles start to appear on the glass.