A. Sugar is gradually added to a glass of water. At first, the sugar fully disso

Question

A. Sugar is gradually added to a glass of water. At first, the sugar fully dissolves in the water, and after a few teaspoons it starts to precipitate. How many degrees of freedom does the system have before and how many after precipitation? Suggest values that can be chosen as degrees of freedom. B. NaCl is gradually added to a glass of water. At first, the salt fully dissolves in the water, and after a few teaspoons it starts to precipitate. How many degrees of freedom does the system have before and how many after precipitation? Suggest parameters that can be chosen as degrees of freedom. In the field of solid materials, 'dissolving' a small amount of one solid in another is called dosing. SrTiO_3, for example, can be dosed with Lu_2 O_3. How many degrees of freedom are there if an excess of Lu_2 O_3 is added to SrTiO_3? Suggest parameters that can be chosen as degrees of freedom.

Explanation / Answer

3) A.
F = C - P + 2
   where components = 2 = C [ because it involves physical no products are formed]
   P= Phases = 1 (before precipitation)
   degrees of freedom, F = 2-1+2 = 3 (pressure,volume,temperature can be degrees of freedom)
Phases = 2 (after precipitation), has 2 phases because it forms solid after precipitation
F = 2-2+2 = 2
B. Salt when dissolved in water splits into ions as it involves a chemical change
NaCl ----H2O------------> Na+ + Cl-
   C = 4 ; P = 1 (before precipitation)
  F = 4-1+ 2 = 5 ( temperature, pressure, volume, solubility, concentration)
  After precipitation P = 2
F = 4 - 2 +2 = 4

4) Since the state of the crystal does not vary after dosing, we can that involves physical change
   If the concentration is under solubility limit,
   F = 1 - 1 + 2 = 2 ( solubility, concentration)
If the concentration of Lu2O3 exceeds then (over-saturated)
F = 1-2 + 2 = 1

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