Referencing the figure below of the H_2O-NaCI (water-salt) phase diagram: a. Exp
ID: 1024801 • Letter: R
Question
Referencing the figure below of the H_2O-NaCI (water-salt) phase diagram: a. Explain why placing salt on ice can cause it to melt (give an example 1or a specific temperature and discuss the phase changes that occur as salt is added). If the solution is 15% salt by weight, determine the temperature at which ice will begin to melt (this can be solved graphically on the diagram). c. Does this system have a eutectic point? If so, what is the eutectic temperature and composition (weight % of H_2O and NaCI)?Explanation / Answer
(A) Placing salt on ice results in melting of ice. This can be explained by considering a specific temperature and studying the effect of addition of salt. Let us consider ice at -10oC, as we keep adding salt to ice at this temperature, the system moves along line AB. Any point on line AB (such as point C) represents a mixture of Ice and salt solution. The actual composition of the mixture can be determined by considering the Horizontal tie-line passing through this point (i.e. line AB in this case) and looking at what points it touches at both ends.
On the left it hits the vertical axis showing 100% water - i.e. pure ice crystals.
On the other end, it hits the sloping line - this gives us the composition of the remaining salt solution. Which for the given case is around 12% (as evident from the graph).
As we keep adding more salt to the system at point C, it keeps moving to the right along line AB. The composition of the phases in equilibrium at at any point on line AB will be the same, only the relative amounts will vary. As we keep moving away from the water axis ( i.e. vertical axis representing 100% water), we will have less ice and more brine solution in the system
(B) In order to determine the temperature at which ice starts melting in a system with 15% salt, we need to draw a vertical line at 15% NaCl and see where it intersects the curve representing the ice - brine equilibrium and then determine the corresponding temperature by drawing a horizontal line (as shown in the graph). This temperature lies between -10 and -15 oC (~ -12 oC)
(C) The eutectic point is represented by point O in the graph, where three phases are in equilibrium (ice, solid salt and brine). The temperature and composition at this point can be determined by looking at the corresponding values at x and y axes.
The corresponding temperature is ~ -21 oC and composition is ~ 23% NaCl in water ( the literature values are -21.1 0C and 23.3 % salt in water )
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