Background: Many rocks on Earth form from salts precipitated from seawater. Salts precipitate when a salt solution becomes saturated. Solubility is the ability of a substance to dissolve in a solution. When a solution is saturated, no more of that substance can be dissolved.
Question: Under what conditions do salt solutions become saturated, and under what conditions does salt precipitate out of solution?
Materials halite (sodium chloride) 250-mL glass beakers (2) distilled water plastic wrap laboratory scale hot plate shallow glass baking dish refrigerator glass stirring rod
Safety Precautions SEIDil^Q Procedure
1. Read and complete the lab safety form.
3. Pour 150 mL of distilled water into a 250-mL glass beaker. Add 54 g of sodium chloride and stir until only a few grains remain on the bottom of the beaker.
4. Place the beaker on the hot plate, and turn on the hot plate. Stir the solution until the last few grains of sodium chloride dissolve. The salt solution will then be saturated.
5. Pour 50 mL of the warm, saturated solution into the second 250-mL glass beaker, and cover it with plastic wrap so that it forms a seal. Put this beaker in the refrigerator.
6. Pour 50 mL of the saturated solution into the glass baking dish. Place the dish on the hot plate and heat the salt solution until all the liquid evaporates. WARNING: The baking dish will be hot. Handle with care.
7. Place the original beaker with 50 mL of the remaining solution on a shelf or windowsill. Do not cover the beaker.
8. Observe both beakers one day later. If crystals have not formed, wait another day.
9. Once crystals have formed in all three containers, observe the size and shape of the crystals. Write your observations in your data table.
Analyze and Conclude
1. Describe the shape of the precipitated crystals in the three containers. Does the shape of the crystals alone identify them as sodium chloride?
2. Infer how heating the salt solution affected the solubility of the sodium chloride.
3. Interpret what effect cooling has on the solubility of salt. What effect does evaporation have on the solubility of salt?
4. Evaluate the relationship between rate of cooling and crystal size.
Was this article helpful?