Why does adding salt to water lower the freezing point?

In summary, when salt dissociates into Na and Cl as it is dissolved in water, it forms ion-dipole attractions which break some H-Bonds and lower the melting point of ice. This is due to the decrease in temperature caused by the van't Hoff equation, which is dependent on the mole fraction of the solute. This phenomenon of freezing point depression occurs for any solute and cannot be explained by the properties of a specific substance like salt. For a more detailed explanation, one can refer to the links provided.
  • #1
erjkism
54
0
i was just wondering why salt, when it dissociates into Na an Cl as it is dissolved into water, decreases the freezing point of water. Why does more energy have to be taken out of the system? does it have to do with entropy or what?
 
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  • #2
this might be a silly answer but,

maybe it is because when NaCl dissolves, it breaks some H-Bonds, to form ion-dipole attractions.

this then lowers the melting point of ice.
 
  • #3
If you add some solute to an ice/water mixture in equilibrium in a thermostat, the ice will melt. If you do the same thing in a thermally insulated vessel, the temperature will decrease until a new equilibrium between ice and the solution is attained. In dilute solutions, the change in temperature is given by the van't Hoff equation:
dT = -x RT2/(Heat of fusion of ice),
where x is the mole fraction of the solute. The point here is that there will be depression of the freezing point for ANY solute, so the explanation cannot depend on the properties of a particular substance such as salt. What happens for large values of x will depend on the properties of the solvent and solute. For a pictorial "explanation" go to
http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml

For a more advanced treatment, click on the link "How can freezing point depression be explained in terms of free energies":

http://antoine.frostburg.edu/chem/s...explanation-of-freezingpoint-depression.shtml
 

What is freezing point depression?

Freezing point depression is a phenomenon in which the freezing point of a solvent is lowered when a non-volatile solute is added.

What causes freezing point depression?

Freezing point depression is caused by the disruption of the formation of solvent's crystal lattice by the presence of a solute.

How is freezing point depression measured?

Freezing point depression is measured by comparing the freezing point of the pure solvent with the freezing point of the solution containing a solute.

What are the practical applications of freezing point depression?

Freezing point depression is used in various industries, such as food preservation, antifreeze production, and medical testing.

What factors affect the magnitude of freezing point depression?

The magnitude of freezing point depression is affected by the number of particles of the solute, the nature of the solute, and the temperature of the solution.

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