# Why is the melting point of a solution lower than one of both of its components?

The melting point of ice (H2O) is at 273K, and one of salt (NaCl) is at 1074K. However, if one dissolves salt in water, the melting point of the solution will be at ~250K. Why is it so low if the contained ice must melt at 273K and salt at 1074K?

First, this only happens for dilute solutions.

And in this case, it does not matter what you put in the solvent, the melting/freezing point will always decrease. The solute particles displace some solvent molecules in the liquid phase and therefore reduce the concentration of solvent. The reduced the concentration of solvent will reduce the melting/freezing point because the molecules are farther away from each other and more energy must be removed to bring them to a solid state.

Homework: what if you dissolved a small amount of H2O into solid NaCl? Would the melting point increase or decrease?

One can explain this phenomenon with entropy change. I am moving from liquid state to solid state

As we reach the melting point of a pure solvent the entropy goes on decreasing. As entropy is defined as 'randomness' it can be said that when we add a solute in a solvent the entropy increases. So, to again attain the entropy achieved earlier we need to take more heat away from the solution, than we did earlier from the pure solvent. This further decreases the temperature at which solidification is attained.

• It should be noted that this only applies to dilute solutions. And the second paragraph is completely wrong. Feb 2, 2017 at 20:32
• Why does this only apply to dilute solutions? Feb 24, 2017 at 13:55
• Because for dilute solutions, the solute only interacts with the solvent. With non-dilute solutions, the solute molecules/particles also interact with each other and the phase transition becomes more complex. Feb 27, 2017 at 22:08
• Hey I have a slightly related question. How exactly does a salt gets back to solid while the solution is completely evaporated? Mar 1, 2017 at 20:08
• As the solvent evaporates, the solution eventually becomes saturated and the salt precipitates out of the solution and forms a crystalline solid due to electrostatic attraction of the positive and negative ions. Read your chemistry book about saturated solutions. Mar 6, 2017 at 15:28