Everyone has boiled water at some point so everyone has a basic understanding of what is meant by boiling. We put a pot on the stove, go away for ten minutes, and come back to find the water boiling vigorously, ready for the pasta, eggs, or oatmeal to be put in.
To really watch what happens with water we must have a thermometer and some patience. When we put cool tap water into a beaker and place this on the stove and turn on the heat we may see ripples in the water as the water heats. These ripples are called Schleeren patterns and arise from light being diffracted off of the regions of water with different temperatures and hence different densities. This is exactly the same optical effect that gives rise to mirages. Since the water is being heated from below, the lighter, warm water will rise in the beaker causing cooler surface water to flow to the bottom.
At some temperature, usually around 80 oC, small bubbles will be observed on the inner surface of the beaker. Many people confuse this with boiling. These bubbles are the air that was dissolved in the water at room temperature coming out of solution. Air becomes less soluble in water as the temperature goes up.
If we continue to heat the water we will begin to observe bubbles form and collapse on the bottom of the beaker. These are the precursors to true boiling. The water on the bottom surface of the beaker is heated to the point where it converts to vapor, but this vapor cools rapidly as it expands and as it encounters slightly cooler water just above the bottom of the beaker. As the bulk water in the beaker becomes hot the bubbles begin to break free of the bottom and rise. Only when the water in the beaker is uniformly hot will the bubbles rise from the bottom and break free at the top. This is often called a rolling boil and is the point of true boiling. The phenomenological definition of boiling is the existance of sustained bubbles that break free of the surface.
Let's look more closely at this process. The bubbles that form at the boiling point contain water vapor. In order for a bubble to exist the pressure of the vapor in the bubble must be pushing against the water with exactly the same force that the water is pushing back. Assuming that we have a normal beaker and just a few cm of water, almost all of the pressure being exerted by the water is air pressure. Hence for the bubble to exist the vapor pressure must be equal to atmospheric pressure. We define a "normal" boiling point as the boiling point of the liquid at 1 atm, or another way of saying this is that the "normal" boiling point is the temperature at which the liquid's vapor pressure is equal to 1 atm.
In previous notes we have established that vapor pressure is determined by both temperature and by intermolecular forces, it should be no surprise that the "normal" boiling point will reflect the intermolecular forces of the liquid.
Does "Boiling" Mean "Hot"?
If boiling occurs when the vapor pressure inside the bubble equals atmospheric pressure, what would happen if the atmospheric pressure would drop? The simple example of this is the case of a camper who is on the top of a 10,000 ft mountain. While water boils at 100 degrees C at sea level (where the atmospheric pressure is 760mm Hg) the atmospheric pressure at 10,000 ft altitude is about 530 mm Hg. Because the boiling point of water is about 90 degrees C at this altitude., not only will our camper find that his food will cook a little slower than normal, any attempt to boil water to kill germs and pathogens may be unsuccessful since the water isn't getting hot enough.
What about boiling things at increased pressures?
Pressure cookers take advantage of just this; they are equipped with a valve that lets gas escape only when the pressure inside the pot exceeds some fixed value. This valve is often set at 15 psi, which means that the water vapor inside the pot must reach a pressure of 2 atmospheres (1520mm Hg) before it can escape. Because water doesn't reach a vapor pressure of 2 atm until the temperature is 120oC, it boils in this container at 120oC, so things cook faster because they are hotter!
Boiling Chips, what are they and why do we use them?
Liquids often boil in an uneven fashion, or "bump" as it's called in chemistry. Bumping occurs frequently when there aren't any scratches on the walls of the container to help bubbles form. Superheating then occurs- because the walls of
your boiling pot are smooth and bubbles of gas cannot easily form, the temperature
of the liquid can actually rise above it's boiling point without boiling!. This can be dangerous because when bubbles finally do form, they usually erupt violently because so much of the liquid is just itching to boil but hasn't been able to, that it all goes at once!
Bumping is easily prevented by adding a few boiling chips to the liquid, which provide a rough surface upon which bubbles can form. When boiling chips are used, essentially all of the bubbles that rise through the solution will form on the surface of these chips.
Boiling chips are small, insoluble, stones made of calcium carbonate, silicon carbide, or carbon (crushed coal) just to name a few. These stones have pores inside or sharp points outside which provide a place for bubbles to easily form.
Always use a few boiling chips when boiling liquids, especially solvents.
Never add a boiling chip to a hot liquid, because it can cause immediate boiling over of the solution. If you forget to add a boiling chip before you begin, you must cool the solution before adding one to prevent product loss. "Porous" boiling chips cannot be re-used since the pores inside these stones become filled with liquid on cooling. "Sharp" boiling chips like silicon carbide or coal can often be reused until they become coated with guk and become innefective.
