Boiling and Freezing Points of Pure and Salty Water
Most recent answer: 10/22/2007
- maria (age 13)
congress middle, lake worth , F.L., Florida
For pure water, the boiling point is 100 degrees Celsius (212 Fahrenheit) at one atmosphere of pressure, and the melting point is 0 degrees Celsius (32 degrees Fahrenheit) at one atmosphere of pressure. At at high altitudes the lower pressure makes the boiling point several degrees lower. For example, in Denver, Colorado, the boiling point is about 95°C or 203°F.
For saltwater, the boiling point is raised, and the melting point is lowered. By how much depends on how much salt there is. I’ll assume the salt is sodium chloride, NaCl (table salt). The melting point is lowered by 1.85 degrees Celsius if 29.2 grams of salt are dissolved in each Kg of water (called a "0.5 molal solution" of salt. The Na and Cl dissociate right away when dissolved, and so for a 0.5 molal solution of salt, there is a 1.0 molal concentration of ions). The boiling point is raised by 0.5 degrees Celsius for water with 29.2 grams of salt dissolved in each kg of water.
If your concentrations of salt are different, then you can scale the boiling point elevation and melting point depression predictions directly with the concentration.
These numbers come from the CRC Handbook of Chemistry and Physics.
(published on 10/22/2007)
Follow-Up #1: pressure and freezing
muswellbrook nsw australia
That’s rather unusual for freezing points, since the solid occupies less space than the liquid for most substances.
(published on 10/22/2007)
Follow-Up #2: efficiently salting water
- jacob hansen (age 39)
A little energy is lost to evaporation on the way, however. If you wait until the water is boiling, then add salt, then boil again, the water is spending a little more time near boiling than it would if you add the salt first. So you lose a little more energy. It's marginally more efficient to add the salt first.
(published on 06/18/2009)
Follow-Up #3: salt water boiling
(published on 04/21/2011)
Follow-Up #4: salt nucleating steam
- Luke (age 30)
(published on 01/17/2012)
Follow-Up #5: Using salt in humidifiers
- John (age 45)
Overland Park KS, USA
I use that sort of humidifier. Adding salt does work. The heating comes from the electrical current flowing through the water. Tap water has a pretty low conductivity, so not much current flows. Adding salt raises the conductivity, since the ions are electrically charged. You actually have to be a bit careful not to add too much salt, since you don't want to blow a fuse.
The effects on the boiling point are very minor compared to the effects on the conductivity.
BTW, although these humidifiers are cheap they do have the nice advantage that since they output water vapor, not drops, you don's have to worry about bacteria etc. getting sprayed into the air.
(published on 02/24/2014)
Follow-Up #6: altitude and boiling point
- Barry Carter (age 65)
Albuquerque, NM, USA
You're absolutely right. I've repaired the old error.
(published on 12/23/2017)