Interesting question. But first, let"s make clearthat in the process of boiling a kettle of wateryou will actually observe two types of bubbles.First, just as the water starts to get hot, a lotof bubbles will form down the walls of your watercontainer. These bubbles are AIR. Normally waterhas a lot of air dissolved on it. This is whatallows breathing to fishes and other aquaticbeings. The solubility of gases decreases whenthe temperature is raised, and that is why the dissolved air bubbles go out from the water.
Then, as the boiling point of water is reached(100 degrees Celsius or 212 degrees Fahrenheit),water vapor starts to form inside the liquid inthe form of bubbles. Remember that at boiling pointwater and its vapor are at equilibrium, that meansthat every molecule in the system has almost thesame willingness to be in the vapor phase as inthe liquid phase, so they very readily formbubbles inside the liquid.
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The pressure due to the ocean of air we live underis about 1 bar or 100 KPascals (units denotingpressure). Liquid water undergoes a phaseconversion (called boiling) at 373 K ( = 100 deg C) to steam ( water in the vapor, or gaseous stateof matter); this is because the water pressure of the steamequals the pressure of the Earth"s atmosphere(mainly containing Nitrogen and Oxygen gases) at100 deg C. When this happens a tiny gas bubble"nucleases" spontaneously within the liquid water,and the bubble grows and rises in the liquid untilit pops out at about 1 bar of water vaporpressure.
Anyway, that is what boiling is.What do you think ice is ??
When you have water (or any liquid) in a containerthere is a constant process of some of the liquidevaporating into vapor and some vapor condensinginto liquid. In equilibrium, these two processesexactly cancel and you have liquid with some vaporover it. The equilibrium pressure of that vapordepends on temperature(as well as the particularliquid).
As you heat up the liquid, thevapor pressure rises. At 100 degrees Celsius (212F) the vapor pressure of water is about equal tothe atmospheric pressure at sea level. At thatpoint, as water evaporates inside the container,the vapor pressure inside the bubbles is highenough to keep the bubbles from collapsing againfrom the pressure of the water around it. Thenthe bubbles rise (why?) and break the surface.
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Some questions for you: Whydoes it take longer to cook (by boiling) food whenin the mountains? Likewise, why might you use apressure cooker?