Problem 2 Figure 2 shows a portion of the H20NaCI phase diag

Problem 2 Figure 2 shows a portion of the H20-NaCI phase diagram. Using the diagram, briefly explain how spreading salt on ice causes the ice to melt. Show numerical examples in your discussion. At a salt composition of 10%, what is the temperature at which ice will start melting? 40 30 Salt 20 liquid 10 Liquid (brine 0 1 10Ice liquid brine brine) 10 Ice + salt 30 NaCI 0 10 2030 H2O 100 90 8070 Composition, weight 9% Fig. 2

Solution

a)

The actual reason that the application of salt causes ice to melt is that a solution of water and dissolved salt has a lower freezing point than pure water. When added to ice, salt first dissolves in the film of liquid water that is always present on the surface, thereby lowering its freezing point below the ices temperature. Ice in contact with salty water therefore melts, creating more liquid water, which dissolves more salt, thereby causing more ice to melt, and so on. The higher the concentration of dissolved salt, the lower its overall freezing point. There is a limit, however, to the amount of salt that can be dissolved in water. Water containing a maximum amount of dissolved salt has a freezing point of about zero degrees Fahrenheit. Therefore, the application of salt will not melt the ice on a sidewalk if the temperature is below zero degrees F.

To this reason in places where lots of ice snow is more salt is used to clear roads.

Spreading salt on ice will lower the melting temperature, since the liquidusline decreases from 0°C (at 100% H20) to the eutectic temperature at about -21°C (23 wt%NaCl).Thus, ice at a temperature below 0°C (and above -21°C) can be made to form a liquidphase by the addition of salt.

However salt becomes less effective when the temperature is less than/ decreases below -6.5 degree centigrate.to 9.5 degrees. The lower the temperature, the more the salt you need.