Adding a solute to a solvent raises the boiling point of the resulting mixture due to a reduction in vapor pressure. This effect is measured by the molar boiling point elevation constant and is used in applications such as antifreeze and cooking. The boiling point will continue to increase as more solute is added, but there are practical limits. Pure ethylene glycol has a high boiling point but is too viscous for practical use.
Boiling point elevation is the effect that occurs when a material is dissolved in a pure solution, causing the boiling point of the mixture to rise. The solute, the material to be dissolved, is added to the pure solution, called a solvent, which reduces the vapor pressure of the mixture. Reducing the vapor pressure of the mixture means that it takes more energy to boil the mixture, which results in a higher boiling point of the mixture.
Every chemical has a measurable change in boiling point for mixtures with different solvents. This measurable quantity is known as the molar boiling point elevation constant or molar elevation constant. If the concentration of the chemical in the mixture is known or measured, this concentration can be multiplied by the molar elevation constant and the resulting boiling point elevation can be calculated and compared with the measured values. The molar elevation constant can also be used to determine the concentration of solute in a mixture by measuring the boiling point of the mixture and dividing the boiling point elevation of the solvent by the molar elevation constant.
A common and useful application of boiling point raising is the addition of antifreeze, typically ethylene glycol, to automobile cooling systems. Ethylene glycol is added in a concentration of 50% by volume to the water in the automobile radiator to prevent freezing, but the resulting solution’s increased boiling point is an advantage. Water boils at 212° Fahrenheit (100° Celsius); the mixture of ethylene glycol and water boils at 225° Fahrenheit (107.2° Celsius) and even higher when the cooling system is pressurized, which is normal for automotive cooling systems.
Cooks have exploited the boiling point elevation for centuries. Adding salt to water raises the boiling point of the mixture, which results in faster cooking times. Ocean seawater, which contains about 3.5% total salts, boils at 216.5° Fahrenheit (102.5° Celsius). This maybe not a big difference from plain water, but usually the faster cooking is preferred by cooks.
The resulting boiling point elevation of a mixture is a factor of the molar boiling point constant, so the boiling point of a mixture will continue to increase as more solute is added to the mixture. This results from the reduction in the vapor pressure of the solvent as its molecules are trapped by the solute. There are practical limits to boiling point elevation in consumer and industrial applications. In automotive cooling, for example, the boiling point of pure ethylene glycol is 386° Fahrenheit (197° Celsius) which could be seen as an advantage. The viscosity, or thickness, of pure ethylene glycol at colder temperatures makes its use impractical, however, because at 40° Fahrenheit (4.4° Celsius) pure ethylene glycol has a measured thickness that is seven times greater than 50 percent ethylene glycol. e-aqueous solution.
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