MnSTEP Teaching Activity Collection > MnSTEP Activities > Overnight Crystals to illustrate solubility concepts

Overnight Crystals to illustrate solubility concepts

Char Bezanson
Faribault High School
Faribault, Minnesota

Summary

In this "quick lab", students evaporate a saturated salt solution to produce a variety of salt crystals (table salt, epsom salt, alum) over a night or weekend. The following day, they draw, describe, and compare their crystals with those of their classmates.

This activity is intended to augment the study of solubility and the interpretation of solubility curves, which are often very abstract to students. It is also appropriate when studying ionic compounds. It can function as a bridge to related sciences such as geology and meteorology. Related quiz questions are provided.

Learning Goals

Key concepts:
1. Saturated salt solutions can become more concentrated when the solvent (water) evaporates, which causes the salts to precipitate.
2. When salts precipitate, they form characteristic crystals, depending on their composition.
3. Crystals self-assemble according to the shapes, sizes, and charges of their components.
4. The shapes of crystals provide insight into the structure of molecules.

Vocabulary:
saturated solution, unsaturated solution, solvent, solute, precipitate, crystal, ionic compound, ion, salt

Context for Use

This is a very quick "illustration" lab that can be set up in five to ten minutes on one day, and is then observed for five to fifteen minutes on a subsequent day. It can be done by individuals, by groups, or as a demonstration.
Materials are flexible, but require several salts: table salt, epsom salts, and alum are suggested but others could be used. Hand lenses would be helpful but are not essential.

Subject: , Chemistry:General Chemistry:Mixtures, Solutions, & Compounds
Resource Type: Activities:Lab Activity, Classroom Activity:Short Activity:Demonstration
Grade Level: Middle (6-8), High School (9-12)

Description and Teaching Materials

I use this activity during a unit on solutions, in which students interpret solubility curves and work on solubility problems. Although the problems students work are generally somewhat dry and abstract, evidence of the dynamic nature of solutions are everywhere in their lives: solar salt is produced from ocean water, sugar crystallizes from honey, calcium deposits clog water heaters, coffee pots, and pipes, frost appears on windows, snow falls from the sky, caves produce stalactites and stalagmites, seeping water produces geodes and agates.

Materials: dry petri plates, plastic weigh-boats, or similar items
Salts: Table salt(NaCl), epsom salts (MgSO4), alum (AlSO4), and/or other. NaCl makes cubic crystals, Epsom salts make needle-like crystals, and Alum makes polygonal crystals.

Prep: I make saturated solutions of each salt ahead of time by stirring the salts into warm water until no more will dissolve. 500 ml of each solution would easily supply multiple sections; less would work if students work in groups. Supply each solution with a disposable pipet; I color-code each solution with a bit of food coloring.

Students label a petri plate or weigh-boat, then just cover the bottom of their container with solution, using the pipet. Collect the containers on a tray and set out overnight or longer to evaporate. You can "seed" a few of the containers with the appropriate crystals if you like; this will tend to encourage a few larger crystals.

The next day, students observe and draw their crystals, which will likely be covering the bottom of their plates like "frost". They should describe the shapes. Find a way to project good specimens (document camera or overhead?) and have students compare their crystals with others. Discuss the differences; discuss what the shapes could tell us about the ions or molecules that make up the crystal.

I use some readings about crystals from the Snowflake book (see attachment) and demonstrate mineral crystals such as quartz, amethyst, pyrite, and others. We discuss the formation of geodes and agates. I sometimes show the "CAVES" segment of the Planet Earth (BBC) series, and ask students to look for examples of crystal formation and dissolving (good example of gypsum formations in the video!)

I find that incorporating crystals and related ideas makes the solubility problems much more tolerable for my students and ties solubility concepts to their reality. Overnight Crystals notes (Microsoft Word 155kB Dec22 09) Quiz problem (Microsoft Word 29kB Dec22 09)

Teaching Notes and Tips

Assessment

I use a quiz problem related to the lab, which allows students to use the "lab" to predict what they would see in a related situation.

Standards

9C2.1.2.6: Describe the dynamic process by which solutes dissove in solvents and calculate concentrations...
9C2.1.2.7: Explain the role of solubility of solids, liquids, and gases in natural and designed systems.

References and Resources

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