A vertical glass column contains 500 mL of water at 60 °C that is saturated with KNO3. The solubility of KNO3 is 106 g per 100 g H2O at 60 °C and 32 g per 100 g H2O at 20 °C. The column is sealed and allowed to cool to 20 °C while standing undisturbed. After cooling is complete, the system is left untouched for one additional hour so that any solid KNO3 can settle.
(a) Calculate the mass of KNO3 that is expected to precipitate as the solution cools from 60 °C to 20 °C. Assume the density of the solution is approximately that of water and remains close to 1.00 g mL−1 over the temperature range.
(b) Immediately after the temperature reaches 20 °C—but before any settling has occurred—predict how the concentration of dissolved KNO3 is expected to vary with height in the column. Explain your reasoning.
(c) After one hour, two 10.0 mL liquid samples are withdrawn simultaneously—one from 1 cm below the surface and one from 1 cm above the settled solid layer. Predict the relative KNO3 concentrations in these two samples and indicate whether visible solid is expected in either sample. Justify each prediction.
(d) Classify the contents of the column as homogeneous or heterogeneous at three distinct times: (i) at 60 °C before cooling begins, (ii) immediately after reaching 20 °C, and (iii) after the additional one-hour settling period. Provide reasoning for each classification.