Rock Cycle - Chemistry (5)

  • Illustrating how molecules move.
  • Exploring physical changes.
  • dissolve
  • molecule
  • saturated
  • solute
  • solution
  • solvent
  • supersaturated
  • coffee filter
  • sugar
  • salt
  • beakers or clear cup
  • hot and cold water
  • spoon

Students experiment with supersaturation.

The coffee is the solvent, the sugar is the solute, and the sweeten coffee would be the solution. 


A solution consists of a solute (the dissolved material) and a solvent (the substance in which the solute is dissolved). The solute is present in a smaller quantity than the solvent. As the amount of solute in a solution increases, the solution becomes more concentrated. At the point of maximum concentration, the solution is saturated. Any additional solute added to a saturated solution will precipitate and drop to the bottom of the container. In general, increasing the temperature of a solution will increase how much solute can be dissolved in it (solubility). Once solute temperature has been increased, more solute has been added, but no more will dissolve, the solution is termed supersaturated.

Supersaturation is important when crystals form. For example, when making rock candy from sugar you have to boiling the water to a high temperature so you can dissolve more sugar in the solution. Only then can you precipitate crystals of sugar on a string.


Exercise 1.

  1. Instruct the students to fill a 250 ml container with 200 ml of water. Have them put 4 ml of salt in a filter and fold it as diagramed on the lab sheet.
  2. Place the filter on the surface of the water. Have the students record what they see on their lab sheets. They should see very thin waves, called schlieren, flowing from the filter into the water.
  3. Explain what is happening to the salt. The water can pass through the filter. It then dissolves the salt crystals into individual salt molecules, which can move through the holes in the filter. The salt + water forms a solution.
    Exercise 2.
  4. In this exercise the students will see that more solute will dissolve when the temperature of the solvent is increased. Depending on the beakers you are using, you may want to predetermine the amount of hot and cold water for each trial.
  5. Have the students measure cold water into a beaker. Then have them add salt (start with 1 ml increments) and allow it to dissolve. Have the students record how much salt the solvent will hold in solution.
  6. Repeat the experiment, using the same volume of hot water. Again add salt, and record how much salt can be held in solution.
  7. Repeat steps 5 and 6, but using sugar instead of salt. Record the data on the sheet. For both solvents, the cold water forms a saturated solution, and the hot water forms a supersaturated solution.
  8. The students have observed a physical change in this activity. The properties of the solute are maintained in their respective solutions. They also observed how solutes (salt and sugar) are dissolved in different temperatures of water.

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