Boiling points
The boiling point is the specific temperature at which a liquid transitions to gas, occurring when its vapor pressure matches the external atmospheric pressure. This process, known as vaporization, is influenced by several factors, including the nature of the liquid's molecules, their intermolecular forces, and the surrounding environmental pressure. For instance, water, a common reference point, boils at 212°F (100°C) at sea level, but this temperature decreases with increasing altitude due to lower atmospheric pressure.
Different liquids have varying boiling points based on their molecular structure and vapor pressures. For example, ethanol has a boiling point of 173°F (78.4°C), while sulfuric acid reaches boiling at 590°F (310°C). Additionally, the phenomenon of boiling-point elevation occurs when a nonvolatile solute is added to a liquid, raising its boiling point, as seen when salt is dissolved in water. Understanding boiling points is essential in fields like chemistry and cooking, as it influences both scientific experiments and everyday practices.
Boiling points
The boiling point is the temperature at which a liquid boils. At this temperature, the liquid’s vapor pressure is equal to the external pressure around the liquid. The liquid's molecules gain energy and move far enough apart to become a gas. When a liquid becomes a gas, it is called vaporization.
Several factors determine a liquid’s boiling point, including the molecules that comprise the liquid; the liquid’s vapor pressure; the atmospheric pressure and elevation; a phenomenon known as boiling-point elevation; and the external pressure. The best-known boiling point is that of water, which has a boiling point of 212°F (100°C). The boiling points of other common liquids include chloroform at 142°F (62.2°C), ethanol at 173°F (78.4°C), turpentine at 320°F (160°C), petroleum at 410°F (210°C), and sulfuric acid at 590°F (310°C).
Boiling Point Fundamentals
Several factors determine a liquid’s boiling point, including the molecules within the liquid. The molecules in a solid are packed tightly together. The molecules in a liquid are father apart, and those in a gas are even farther apart. Intermolecular forces hold molecules together. Liquids with strong intermolecular forces have a high boiling point. Liquids with weak intermolecular forces have a low boiling point.
A liquid’s vapor pressure also determines its boiling point. Vapor pressure increases when the liquid heats up. To reach its boiling point, a liquid's vapor pressure must equal the atmospheric pressure. Therefore, liquids with a low vapor pressure have a high boiling point, and those with a high vapor pressure have a low boiling point.
Atmospheric pressure varies with elevation. A liquid boils at a higher temperature at a low altitude and at a lower temperature at a high altitude. In other words, it takes a higher temperature for a liquid to boil if it is below sea level and a lower temperature if the liquid is at a high elevation.
When a liquid reaches the temperature at which its vapor pressure equals one atmosphere, it has reached its normal boiling point. An atmosphere is a unit of pressure that is equivalent to the air pressure at sea level. An atmosphere equals about 14.7 pounds per square inch. A liquid’s boiling point will be lower than its normal boiling point if the external pressure is less than one atmosphere. However, the liquid’s boiling point will be higher than its normal boiling point if the external pressure is greater than one atmosphere.
A liquid’s boiling point also changes with boiling-point elevation, a phenomenon in which adding a nonvolatile solute to a liquid will increase the boiling point of the liquid. This means that impurities in the liquid increase its boiling point. For example, salted water has a higher boiling point than plain water. The boiling-point elevation is dependent on the number of particles that are in the liquid. The Clausius-Clapeyron equation, as well as Raoult’s law, can be used to compute the boiling-point elevation.
Once a liquid is boiling, its temperature remains the same. Adding more heat will not increase the liquid’s temperature.
Boiling Point of Water
At sea level, water boils at 212 degrees Fahrenheit(212°F) or 100 degrees Celsius (100°C). Water’s boiling point changes as altitude is increased or decreased. For example, at 1,000 feet below sea level, water boils at 213.9°F (101.1°C). At 1,000 feet above sea level, water’s boiling point is 210.1°F (98.9°C). At 10,000 feet above sea level, water boils at 193.2°F (89.6°C).
The boiling point of water played an important role in the development of the Celsius and Fahrenheit temperature scales. In 1742, Swedish astronomer Anders Celsius based his scale on the boiling and freezing points of water. However, on his scale, 0° was the boiling point and 100° was the freezing point. A similar scale developed by French physicist Jean-Pierre Christin reversed the boiling and freezing points. By 1747, Celsius’s scale was inverted on some thermometers so that 100° was the boiling point and 0° was the freezing point of water. Today, the Celsius scale is the most common temperature scale in the world. Around the same time Celsius developed his scale, German-born Daniel Gabriel Fahrenheit developed his own scale, with 212° as the boiling point and 32° as the freezing point of water. The United States is one of the only countries in the world that uses the Fahrenheit scale.
Boiling Points of Other Liquids
The boiling points of some other liquids are as follows:
- Diethyl ether: 94.4°F (34.7°C)
- Carbon disulfide: 115°F (46.2°C)
- Methyl acetate: 135°F (57.2°C)
- Chloroform: 142°F (62.2°C)
- Methanol: 151°F (66°C)
- Ethanol: 173°F (78.4°C)
- Formic acid: 214°F (101°C)
- Toluene: 231°F (110.6°C)
- Nitric acid: 248°F (120°C)
- Turpentine: 320°F (160°C)
- Furfural: 323°F (161.7°C)
- Aniline: 363°F (184.4°C)
- Propylene glycol: 368°F (187°C)
- Petroleum: 410°F (210°C)
- Glycerin: 554°F (290°C)
- Sulfuric acid: 590°F (310°C)
- Mercury: 675°F (356.9°C)
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