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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 other studies it has been established that vapor pressure is determined by both temperature and by intermolecular forces, and 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 120 degrees C, it boils in this container at 120 degrees C, 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 you are distilling solvents or mixtures and there aren't any scratches or irregularities on the walls of the boiling pot that would help boiling bubbles to form. If this happens, a thing called superheating occurs- because the walls of your boiling pot are smooth and bubbles of vaporized liquid cannot easily form, the temperature of the liquid can actually rise above it's boiling point without boiling- the solution is now a superheated liquid. This can be dangerous because bubbles eventually will form, and when finally do, 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 bloooop and releases all of it's superheated boiling energy at once!
The result is that the entire solution turns into a mass of gas and bubbles, and with a resounding "thump" it can send a shockwave of undistilled boiling solution shooting up and through the entire distillation system- at the very least contaminating everything you've just distilled, and at worst it can actually cause the glassware to come apart or even break due to the sudden pressure wave!!
An example of this happening at home- there are many reports of people heating coffee in a microwave- and when the oven goes "ding" and the door is opened, there sits the nice cup of coffee... until they touch it and it unexpectedly goes "SPLAT"- half the cup of scalding coffee is suddenly all over the place inside the oven, on their hand and on the floor in a big scalding mess- what happened? the coffee was too still to boil and it just sat there superheated until they moved it, and at that point it bumped!!
Bumping is easily prevented by adding a few things called "boiling chips" to the liquid. Boiling chips are small, insoluble, stones (the size of grains of rice or sand) made of calcium carbonate, silicon carbide, or carbon (crushed coal) just to name a few. These stones contain many microscopic pores and/or or sharp points which provide a rough surface upon which boiling bubbles can form. When boiling chips are used, essentially all of the bubbles that rise through the solution will have formed on the surface of these chips. Always use a few boiling chips when boiling liquids, especially solvents. A frequent exception to this rule is when boiling water- and for our water-based steam distllers we've found that boiling chips are not necessary.Never add boiling chips to a hot liquid, because it can cause immediate boiling over of the solution. If you forget to add boiling chips before you begin heating a solution, 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.
So what does all of this have to do with herbs?
Tinctures are made with solvents, and some herbs and fragrent flowers are better extracted with solvents than by steam distillation for various reasons, (ie being destroyed by steam temperature). Often times, you'll want to remove some or all of the solvent from the extracted plant material, or concentrate your tincture that you've just made, and boiling it off is a good way to do it, as long as the boiling temperature of the solvent won't hurt your product. This is at least one procedure in which you'd want to use boiling chips.
Much of the above information was gleaned from, amongst others, http://www.chem.uidaho.edu/~honors/