Boiling Point Elevation

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A liquid’s boiling point is the temperature at which the vapour pressure of a liquid exceeds the surrounding atmospheric pressure and transforms to a gas phase. This can be determined using the capillary method, where an inverted capillary is placed into a liquid of interest and heated until the liquid begins to fill the capillary.

The boiling point of a solution is higher than that of a pure solvent and the freezing point of a solution is lower than that of a pure solvent. The amount to which these changes occur depends on the amount of solute that is added to the solvent.

Boiling point elevation

When you add salt, a nonvolatile solute, to water, you raise the water’s boiling point. For every 58 grams of salt dissolved in a kilogram of water, the temperature needed to boil increases by about 0.5 degrees Celsius.

However, this doesn’t always mean that the salt is the cause of the increase in boiling point. Often times, the change is more of a result of water’s overall structure than its chemical makeup. For example, ethanol has stronger intramolecular forces than acetone because of its OH structure, which causes hydrogen bonding between molecules.

In addition, the solute’s molality will also play an important role in the change in boiling point. This molality is a property of the solute’s molecular structure and can be calculated by comparing its boiling temperature change with its solute (w/w) concentration.

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