Water droplets above the mercury column boiled at temperatures well below 100°C, because the pressure was much lower than normal. Evaporation from the surface of a liquid is a slow, stable process that occurs at all temperatures. Boiling is a phenomenon that occurs when the vapor pressure of the liquid exceeds the pressure inside the liquid and bubbles form. In boiling, evaporation happens within the liquid and not just from its surface. The bubbles create new surfaces throughout the body of the liquid.
Since the pressure within the liquid is slightly higher than that at the surface, because of the weight of the liquid itself, the bubbles expand as they rise, which makes boiling more dramatic than evaporation,
though it is really same process. In both evaporation and boiling the same amount of energy is required to overcome surface tension, wherever that surface may be - on top of the liquid or surrounding a bubble. Boiling speeds up evaporation by producing a more surfaces, but the same amount of energy is needed for each molecule to escape from the liquid and enter the vapor phase.
The scientific measure of heat energy is the calorie, which is the amount of heat required to raise the temperature of one gram of pure water by 1°C. A thousand calories is called a Calorie (with a capital C) and this is what we use to measure the energy contained in food or expended in exercise. It takes 80 calories to heat one gram of water from 20°C (68°F) to the boiling point of 100°C (212°F), but it takes 540 calories to turn that same gram of water into vapor without raising the temperature! The surface tension of water may not seem like much to us, but for molecules, its a huge hurdle to jump!
Physical constants like the boiling point of a liquid are measured or defined under standard conditions. When a liquid is heated, it is possible to superheat it, or raise its temperature above the boiling point, without boiling occurring. This is because it takes more energy to create a bubble than to simply evaporate into one. Once the liquid is boiling, the bubbles that are already formed trigger the formation of others.
Superheated liquids can bump or suddenly erupt into frothy boiling, also known as surge boiling. Placing materials with sharp corners and edges into the liquid before heating it can prevent this by promoting the formation of bubbles, which allows boiling to begin in a gentle and controlled fashion. Boiling chips are used in laboratories for this purpose, but we have found that ordinary plain metal pot scrubbers do an excellent job as well. Surge boiling is discussed in more detail in Chapter 4, as a factor in boiler control.
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