Take Home Lab #7

Take Home Lab #7: Growing Crystals

Question: How are crystals of different chemicals similar and different?

Safety: Use only hot tap water; do not heat water in a microwave or on the stove. Do not eat any of the crystals or other materials during this activity, as mold quickly grows on some of the crystals. Mark the cups clearly and keep them out of the reach of children. Clean up spills immediately. Cover the sugar crystals to avoid ants seeking it out.

Materials: Cups, water, sugar, salt Epsom salt, thread, pencil, paperclip

Procedure: In this lab, you will grow three different types of crystals: Sugar, salt, and Epsom salts. Crystals are grown by either dissolving more material than the solvent can normally hold by heating it, or by allowing the solvent to evaporate over time. In this procedure, you will do a little of both. Your will put a lot of solute in a little solvent and heat it to dissolve; then you will let it sit for a few days to evaporate.

1. For each of the solutes, dissolve a large spoonful in about 30 mL of hot tap water. Stir until all of the solute is gone.

2. Once all of the solute has dissolved, transfer the solution to a disposable cup, leaving any undissolved solid behind. Hang a short thread in the solution. If the string will not sink, you can tie a paper clip to the end. Allow it to sit on a windowsill for several days while the water evaporates. Pull the string out and let the crystals dry overnight. Record observations of the crystals every day and include the number of crystals, size, shape and other characterisitcs.

3. Find a good example of each crystal and sketch its shape. Try to find a single crystal without other crystals growing out of it. If your crystals are small, you can look at them under a magnifying glass. Just place in a bag and bring to school.

Post-Lab Questions:

1. Every day make a quick sketch of your different crystals and label Day 1, Day 2 and so forth.

2. Make sure to sketch your crystals after you take them out of the solvent.

3. Which substance grew the largest crystals?

4. How would you describe the shape of each crystal?

5. If equal masses of each substance were used, then the one with the lowest molar mass would have contained the largest number of molecules of that substance and would have the highest concentration. The highest concentration should grow the fastest but will produce the smallest crystals. Use the formulas to determine if your crystals followed this patter, NaCl (salt), C₁₂H₂₂O₁₁ (sugar), and MgSO₄ (Epsom salt). Did the substance with the highest molar mass grow the largest crystals?