Overnight Crystals to illustrate solubility concepts
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.
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.
saturated solution, unsaturated solution, solvent, solute, precipitate, crystal, ionic compound, ion, salt
Context for Use
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.
Resource Type: Activities:Lab Activity, Classroom Activity:Short Activity:Demonstration
Grade Level: Middle (6-8), High School (9-12)
Description and Teaching Materials
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
9C18.104.22.168: Explain the role of solubility of solids, liquids, and gases in natural and designed systems.