What Does Salt Do to the Boiling Point of Water?

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Dissolving salt in water, or any solute in any solvent, raises the boiling temperature. This results from the thermodynamics of the dissolving process and the boiling process. The change in boiling temperature is calculated by the equation ΔT = iKm, where ΔT is the change in temperature, i is a constant for a particular solute, K is a constant for a particular solvent and m is the molal concentration of the solution measured in moles of solute per kilogram of solvent.

Get out of My Way

  • Water at any temperature will evaporate. We call the tendency for water molecules to escape from the surface into the gas phase the vapor pressure.This tendency becomes suddenly much stronger when water reaches its boiling temperature. Dissolved salt gets in the way of water molecules leaving the surface. This lowers the vapor pressure.

Let's Get Disorganized

  • In a gas, water is more free to move around and find different molecular states, meaning it has a higher entropy. Systems always try to evolve in ways that increase entropy. Dissolving salt in water also increases the entropy. Now, the entropy gain from boiling is less.

In the Kitchen

  • People often say to add salt to water when cooking food to make the boiling temperature higher, causing the food to cook faster. Dissolving one teaspoon of table salt in one liter of water gives a molal concentration of 0.097 moles per kilogram. For table salt, i = 2, and for water, K = 0.52 degrees Celsius multiplied by kilograms per mole. Using this calculation, adding 1 teaspoon of salt per liter of water raises the boiling temperature by only 0.10 degrees Celsius (0.18 degrees Fahrenheit).

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