What are bubbles?
Bubbles, in general, are globules of one substance in another. Usually, it is a gas in a liquid. It is globular in shape since it has the most stable and lowest energy state. Bubbles collapse when the pressure of the substance inside matches that of outside.
Let us understand how the bubbles are formed in liquids. Let us consider the case of water.
Bubbles in boiling water
We know that boiling water produces bubbles; few at the beginning… then we see those rising… and more and more as the temperature is increased and the water reaches the boiling point… The boiling phenomena is accompanied with a sound.. What causes all these?
It is a known fact that water has a lot of air mixed in it. When the temperature of the water is raised, this solubility of air in water is decreased and air gets separated out in water. That is what we see as the bubbles at the bottom and sides of a pan of water over fire!
As the temperature is increased, water at the bottom of the pan (close to the source of heat) starts vaporizing into the bubbles. As these water vapours have density less than that of water around them, they rise to the surface of water and try to escape from the water.
It should be noted that the atmospheric pressure (weight of air on the surface of liquid) acts against the vapour escaping water when the vapour pressure is low. At temperatures less than the boiling point, the vapour pressure is less than the atmospheric pressure. So, at a temperature lower than the boiling point of water, the bubbles may rise, but they won’t reach the surface of water. It cools down on its way up and collapses. The sound we hear prior to the boiling water is nothing but the sound of collapsing of bubbles.
At the boiling point, when all the liquid molecules have the willingness to vaporize, they form bubbles rapidly. The vapour pressure at boiling point will be higher and equals the atmospheric pressure. So, the bubbles rise to the surface of water and burst releasing the vapour to the surrounding.
Now, a question: What causes water to boil faster at higher altitudes?
Answer: At higher altitudes, the atmospheric pressure is less than normal. So, the vapour pressure of the boiling water will be equalling the atmospheric pressure at a lower temperature and vapour escapes from water. So, boiling point will be lower at higher altitudes.