Expert: Amy - 11/24/2004

Question
Salt added to ice, melts the ice.  I think this is because it lowers the temperature water needs to get to before it freezes.  How does adding salt to water affect the boiling point of water?

Answer
Hi Dave,

As soon as any of the salt dissolves in the water, the boiling point of the water will begin to rise -- by about one half degree Celsius for every 58 grams
of salt dissolved per kilogram of water. In fact, any non-volatile soluble substance will
raise the boiling point of water.

So why does the salt affect the boiling point?
Here's how boiling works

The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure. For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100°C. Since the vapor pressure increases with temperature, it follows that for pressure greater than 760 mmHg (e.g., in a pressure cooker), the boiling point is above 100°C and for pressure less than 760 mmHg (e.g., at altitudes above sea level), the boiling point will be lower than 100°C. As long as a vessel of water is boiling at 760 mmHg, it will remain at 100°C until the phase change is complete.

Here's what salt does

Part of the pressure in the liquid comes from the salt, not the water. So the pressure due to the water alone is reduced compared to that of pure water at the same temperature. The vapour pressure, that is, the pressure of water vapour that would stay in equilibrium with the liquid, is reduced by the same amount because of the salt.

I hope this helps,
PLease let me know if you have any more questions

Amy