A KCl solution containing 42 g of KCl per 100.0 g of water is cooled from 60 °C to 0 °C. What happens during cooling? (Use Figure 14.11.)

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Identify the solubility of KCl at 60 °C using Figure 14.11. This will tell you how much KCl can dissolve in 100 g of water at this temperature.

Determine the solubility of KCl at 0 °C using the same figure. This will indicate how much KCl can remain dissolved in 100 g of water at the lower temperature.

Compare the initial concentration of KCl (42 g per 100 g of water) with the solubility at 60 °C to confirm if the solution is saturated, unsaturated, or supersaturated at the starting temperature.

Compare the initial concentration of KCl with the solubility at 0 °C to determine if the solution will remain saturated, become unsaturated, or if excess KCl will precipitate out as the solution cools.

Conclude what happens to the KCl as the solution cools from 60 °C to 0 °C based on the solubility changes and the initial concentration.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Solubility

Solubility refers to the maximum amount of a solute that can dissolve in a solvent at a given temperature and pressure. In the case of KCl in water, solubility decreases as temperature decreases, meaning that less KCl can remain dissolved in water at lower temperatures. Understanding solubility is crucial for predicting whether a solution will remain homogeneous or if precipitation will occur upon cooling.

Saturation Point

The saturation point is the stage at which a solution can no longer dissolve additional solute at a specific temperature. When a KCl solution is cooled, it may reach a saturation point where the dissolved KCl exceeds the solubility limit, leading to the formation of solid KCl crystals. Recognizing the saturation point helps in understanding the behavior of solutions as they undergo temperature changes.

Crystallization

Crystallization is the process by which dissolved solute comes out of solution and forms solid crystals. As the KCl solution cools, the decrease in temperature can cause KCl to crystallize out of the solution if the concentration exceeds the solubility limit. This process is essential in many applications, including purification and the formation of solid salts from solutions.