If water is poured in a pan and put it on a stove over heat, the water will eventually start boiling. The phrase “water is boiling” essentially means that the water is transitioning from a liquid to a gaseous state very rapidly and this brings images of energetic bubbles and steam to mind. This is usually imagined happening at a very high temperature and boiling water is considered to be very hot. Most people have reported being severely burned by “boiling hot water.” However, it is interesting to note that water does not always boil at such high temperatures. At high altitudes, water can be brought to a boil at temperatures that the human body can easily endure. Essentially, the boiling point of water depends on two factors: atmospheric pressure and the amount of impurities present in the water.
Boiling Point of Water at 1 Atmosphere
Pressure can be measured in atmospheres; 1 atmosphere is the standard atmospheric pressure at sea level. At 1 atmosphere, the boiling point of pure water (that is, water with no added minerals or salts) is 100 degrees Celsius (or 212 degrees Fahrenheit). This is a very high temperature that can scald the human skin if it comes into contact with the hot water.
Effect of Pressure on Water’s Boiling Point
At higher altitudes, the atmospheric pressure decreases. For instance, atmospheric pressure up on a high mountain might be somewhere around 0.8 atmosphere. As this pressure decreases, the boiling point of the water also decreases. Water boils when it receives enough energy from the heat to increase the vapor pressure within the water enough so that it overcomes the atmospheric pressure, causing the water to transition from liquid to gas. The higher the external pressure exerted on the water, the more energy (heat) is required to raise the vapor pressure required to overcome the atmospheric pressure. This is why when the atmospheric pressure is lowered, water requires less heat to boil. It is noted that at high enough altitudes, water can be boiled at quite low temperatures (e.g. as low as 69.4 degrees Celsius (157 degrees Fahrenheit)), which is low enough to be bearable if someone puts his or her hand in it. It has been determined that for every 285 meters rise in altitude, the boiling point of water decreases by 1 degree Celsius (or by 1 degree Fahrenheit every 500 feet). Although it may look like that height is a factor that affects the boiling point of water, it is actually the decrease in atmospheric pressure due to increase in altitude.
Effect of Impurities on Water’s Boiling Point
Another thing that affects the boiling point of water is the amount of impurities that are present in it. Pure water has a fixed boiling point at standard atmospheric pressure; however, if some impurities are added in the water, e.g. common table salt (sodium chloride), the boiling point of the water changes; if 100 grams of salt is added to 1 kilogram of water, the boiling point of the impure water would increase by 1 degree Celsius. Similarly, adding other impurities also affects the boiling point of water. It is for this reason that boiling points are often used to determine the purity of a substance, as pure substances have certain fixed boiling points. For instance, if water comes to a boil at exactly 100 degrees Celsius at 1 atmosphere, it means that is pure and does not contain any impurities. If the boiling point differs, it must mean that the water is impure.
Thus, the boiling point of water depends on how much heat is required to vaporize the water; the amount of heat required is affected by atmospheric pressure and impurities in water. The higher the altitude, the lower the pressure, the lower the heat required, i.e. water will boil at a lower temperature. Similarly, impurities also affect the amount of heat required for boiling water; the higher the impurities, the higher temperature needed to bring the water to boil.