Video Transcript
In an experiment, a student added sodium acetate to a fixed volume of water. Eventually, the student reached a point at which no more sodium acetate dissolved. The student then warmed the solution until no crystals of sodium acetate were visible. The solution was then allowed to cool slightly. The diagram below shows the student’s observations. Part (a) which of the following terms describes solution B? (A) Colloidal, (B) saturated, (C) supernate, (D) supersaturated, (E) unsaturated.
In part (a) of the provided diagram, solid sodium acetate is being added to a fixed volume of liquid water. We know from the question that the solid sodium acetate dissolves in the water. A substance that is dissolved into another substance is called a solute. In this case, sodium acetate is the solute. The substance that does the dissolving is called the solvent. In this problem, liquid water is the solvent. When the solid sodium acetate dissolves in water, a solution is formed. A solution is a type of mixture called a homogeneous mixture. As solid sodium acetate dissolves, it looks as if it’s disappearing, giving the solution a uniform appearance.
Sodium acetate is an ionic compound. It’s composed of an ionic lattice of alternating positive and negative ions. If we had a way of zooming in on the solution, we’d see that each positive and negative ion is completely surrounded by water molecules. Because individual ions are much too small to see, the solution has a uniform and transparent appearance. Initially, when the student adds sodium acetate to the water in the beaker, all of the sodium acetate dissolves. The student continues to add sodium acetate, and more of it dissolves. Eventually, a point is reached at which no more sodium acetate will dissolve into the water.
Because the amount of water has not changed during the process, the solution becomes more and more concentrated with solute particles. Initially, when more sodium acetate can still be dissolved in the water, the solution is said to be unsaturated. When the point is reached at which no more solute can be dissolved at a given temperature, the solution is said to be saturated. Solution B in the diagram depicts a saturated solution with excess sodium acetate accumulated on the bottom of the beaker. The term that describes solution B in the diagram is answer choice (B) saturated.
Part (b) which of the following terms describes solution D? (A) Colloidal, (B) nonsaturated, (C) saturated, (D) supernate, (E) supersaturated.
When adding a solute to a solvent to make a solution, the solvent can only dissolve a certain amount of solute at a given temperature. We know that solution B is a saturated solution. If a student added more sodium acetate to solution B, it would just accumulate along the bottom of the beaker. So how is it possible that solutions C and D have no sodium acetate crystals along the bottom of the beaker?
First of all, in many cases, heating a solvent to a higher temperature allows more solute to dissolve. As the student warms solution B, the sodium acetate at the bottom of the beaker begins to dissolve. As more heat is added, eventually, all of the sodium acetate dissolves and no more crystals are visible at the bottom of the beaker. The student then allows solution C to cool slightly, producing solution D, which has a lower temperature. Cooling a solution containing dissolved ions of a salt like sodium acetate typically results in a decrease in the amount of solute that can be dissolved.
We would expect that less sodium acetate would be dissolved in the cooler solvent in solution D, and crystals would appear at the bottom of the beaker. However, according to the diagram, no sodium acetate crystals appear at the bottom of the beaker. When solution C was cooled slowly, the excess sodium acetate remained dissolved in the solvent. The temperature of the solvent changed, but the amount of solvent and solute particles did not. So solutions C and D actually have the same concentration. When a concentrated solution is cooled slowly so that the solvent holds more solute than it normally could, the solution is called supersaturated. The term that describes solution D is answer choice (E) supersaturated.
