Posted on 08 April 2011.
Why does water boil? The answer to the question “Why does water boil” is important knowledge for everyone, as water is part of the basis of human life and provides many health benefits. Boiling water has a number of uses, includingboiling to purify water, sterilizing medical tools, cooking, and as a starting material or solvent in chemical reactions. The boiling point of water and the process of boiling itself is determined by various chemical properties of the substance.
Why Does Water Boil: Boiling Water for Food Preparation
One place to begin investigation of the question “why does water boil” is to survey the most familiar usage of boiling water: food preparation. A variety of simple foods including spaghetti and hard boiled eggs can be prepared simply by placing them in a pot of boiling water. One Food Network show even has the tongue-in-cheek title “How to Boil Water.” With a subject this familiar, a few myths and sayings have naturally arisen. “A watched pot never boils,” says one popular expression. Of course, this is not technically true, an intense gaze having absolutely no effect on the boiling point of water.
Does adding salt to water change anything besides taste?
Something that does affect the boiling point of water, however, is the old tradition of throwing a pinch of salt in the pot. Adding salt to water will raise the boiling point of the water due to the reduction in vapor pressure. This means that the water will boil more slowly, which can be an advantage when trying to hard boil the perfect egg. For an in depth analysis on these effects, referring to Volker Thomsen’s article “The Boiling Point of Water” written for The Physics Teacher may be helpful, or simply just continue reading.
Why Does Water Boil: The Chemistry
When asking why does water boil, people may come across such phrases as “vapor pressure,” “atmospheric pressure,” and “heat of vaporization.” While the boiling point of water is listed as 100 degrees Celsius, the truth is that the boiling point given in the literature is that for standard pressure, approximated as 1 atm.
Why Does Water Boil: More Information on Pressure
But why does water boil at different temperatures depending on the pressure? The short answer is that in order for water to boil, the vapor pressure must be equal to the atmospheric pressure, as molecules are inclined to move from a high pressure to a lower pressure. Though the transition of water from a liquid phase to a gas phase is constantly occurring (think of evaporation), when the vapor pressure of the liquid is lower than the atmospheric pressure, the equilibrium of the process is in favor of the return of the molecule to the liquid phase, or condensation. Boiling, then, occurs when equilibrium favors transition to the gas phase over the vapor phase.
Why Does Water Boil: Boiling Points
When looking at the question of “why does water boil,” it may be interesting to note that water has a higher boiling point and thus takes a longer time to boil than many other liquids such as ethers. Water is made up of a bond of two hydrogen bonds to an oxygen molecule, and as a relatively highly electronegative molecule, oxygen attracts electrons more than hydrogen, creating a partial negative charge on the oxygen molecule and a partial positive on each of the hydrogen molecules. These partial charges mean that the molecule is polar. When this polar molecule is close to other molecules of its kind, hydrogen-hydrogen bonding occurs. These weak bonds may break easily, but breaking these bonds in order to bring all of the molecules into vapor phase requires some additional energy.
Why Does Water Boil: Hydrogen Bonding
The phenomenon of hydrogen bonding is a large contributing factor to the high specific heat of water, 4.18 J/gK. The specific heat is related to how rapidly water can be brought to a boil because it is a measure of how much energy it takes to raise one gram of the substance by one degree Celsius. In more familiar units, it takes one calorie of energy to raise one gram of water by one degree Celsius. Of course, this is the scientific calorie, which is 1/1000 of the food calorie, so just one food calorie could raise one gram (or mL, density of water being 1 g/mL) of water by 1,000 degrees Celsius: good news for cells, which need the energy for metabolism.