How to Make Saline & Sugar Crystals in the Classroom

Sugar can be dissolved and reformed as crystals on sticks or string.

There are several middle school-aged science experiments that become indelibly marked in a youth's memory. Among them is the lesson on crystals. If your class is studying crystals, solvents and solutions, what better way to illustrate the concept than to have students create a crystal and have it grow before their eyes. Some teachers use sugar, some use salt, still others use both to compare the results. For this experiment, the solvent is hot water, the substance is the salt or sugar and the crystal is formed as the hot water solution cools.

Things You'll Need

• Salt
• Sugar
• String
• Lead weight
• Pencil
• Mason jar
• Wooden coffee stir sticks

Instructions

1. 

   Boil the water to aid the dissolving process.

   Boil two cups of water and add as much salt or sugar into the solution as possible. Stir the salt/sugar solution and keep adding until it stops dissolving in the water. When the salt/sugar begins collecting on the bottom of the container you are boiling it in, the solution is super-saturated.

2. 

   A natural fiber string works best to form crystals.

   Pinch the end of the lead weight and attach to the bottom of the string. The string you use is important as the crystals will cling to and form more easily on a natural fiber than on a fishing line. Cotton or yarn will work best.

3. 

   The pencil acts as a holder for the string to keep it upright.

   Tie the top of the string to a pencil and submerse the weight and the string into the solution. The formation of the crystal will occur as the water evaporates leaving the salt or sugar behind. The students should be able to make observations about the growth of the crystal as this process may take several days.

4. Vary the experiment. Create multiple jars with the same solution, either salt or sugar, or both. Set some out to cool at room temperature. Set another in a refrigerator to slow the process of evaporation. Set another in an area with a slightly elevated temperature. Draw a comparison between the results to determine in which area the crystals will grow the best. Compare the growth of sugar versus salt crystals to determine which grows faster, and which grows bigger.

5. Tell the students to record the exact amount of salt or sugar that is used before the water becomes saturated. Alter the experiment by not boiling the water first and just dissolving the salt or sugar in room temperature water. Compare the results with the boiled water sample.

6. Form rock candy by dissolving sugar in the boiling water and instead of using a string, use a wooden coffee stirring stick. To form colored rock candy add some food dye to the sugar before melting it in the water solution. If you form more than one piece of rock candy, use separate jars for each one.

References

• Ms. Nucleus: Comparing Salt and Sugar Crystals
• Ms. Nucleus: Rock Cycle - Chemistry (5) Post Lab