Glass of iced tea

Source: Iced Tea, Renee Cornet, Wikimedia Commons

The next time you add sugar to unsweetened tea, take notice of the changes to the solution in the following scenarios.

  1. You add the first packet of sugar, and stir until it all dissolves with no sugar settled on the bottom.
  2. You add a second packet of sugar and it takes a little more time for the sugar to dissolve. After stirring for a minute, you realize that there are still some sugar crystals at the bottom of the glass.
  3. After tasting your tea, you decide it is still not sweet enough and add more sugar. However, it all settles at the bottom, and regardless of how much you stir, the crystals stay at the bottom of the cup.

What can you do to help the sugar crystals dissolve?

Glass of iced tea

Source: Kang Kim, Real Simple

Did you think about heating up the tea? You are right! Heating up the tea (or most any solution) will help the sugar dissolve more. However, it is Texas, and most of us like to drink cold tea in the summer, not hot. Add ice to make your now sweetened tea cold again. Will the sugar stay dissolved? Yes!

In each of the three steps above, you created one of the following types of solutions: saturated, unsaturated, and supersaturated.

Interactive exercise. Assistance may be required. Identify which type of solution was created in each step.

A supersaturated solution is one that has more solute than it can hold at a certain temperature. Typically when the temperature of a solution is increased, more particles can be dissolved, thus increasing the amount of solute. A supersaturated solution goes through all of the steps listed above for the iced tea.

Interactive exercise. Assistance may be required. Drag and drop the steps below to create a supersaturated solution (use the picture below to help guide you).

Remember, a supersaturated solution will look just like an unsaturated solution with no solute settled at the bottom. However, there is more solute than the solvent can actually hold. Any slight change to the solution will cause all of the solute to come out.

Now you know how to make a supersaturated solution, but why, outside of chemistry class or making really sweet iced tea, would you want to do this?

Video segment. Assistance may be required. Watch the video below and use your notes to answer the following questions.

Source: Crystal Growing - Cool Science Experiment, SteveSpanglerScience, YouTube

  1. What two commercial applications of supersaturated solutions are sold at stores like Wal-Mart? Interactive popup. Assistance may be required.

    Check Your Answer

    Rock candy and reusable hand warmers are two commercial applications of supersaturated solutions. Rock candy is a supersaturated solution of sugar and water, and reusable hand warmers can be heated in the microwave to be reused.Close Pop Up
  2. The liquid inside reusable hand warmers will crystalize over time. However, they can be reheated in the microwave and go back to the liquid form. Explain why the crystals form and why reheating the hand warmers “removes” the crystals. Interactive popup. Assistance may be required.

    Check Your Answer

    The hand warmers are a supersaturated solution. As the solution transfers heat to your hands, the temperature of the solution decreases. The solution can dissolve fewer particles at the lower temperature, so crystallization begins. When the solution is heated in the microwave, the temperature increases allowing it to create more solute and thus the crystals dissolve. Close Pop Up

It is important to know that the terms saturated, unsaturated, and supersaturated are relative terms. As the temperature of the solution changes, so does the amount of particles that can be dissolved in the solvent. Solubility curves show how changing the temperature changes the solubility of particles in a solvent.