Create a giant “snowflake crystal” scene in a jar while learning about solubility principles. This lab had a serendipitous beginning when a solution of benzoic acid was forgotten on a lab bench and allowed to cool! Voila! “Snow”!
Benzoic acid, 20 g*
Bottle, plastic, 3-L*
Holiday plastic figurine*
Hot glue gun
*Materials included in kit.
Benzoic acid is moderately toxic by ingestion. Avoid contact with skin, eyes, clothing and respiratory tract as it is a severe irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Use exhaust ventilation to keep airborne concentrations low. Keep the bottle capped to avoid breathing the vapors.
Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The benzoic acid solution should be neutralized, then flushed down the drain with excess water according to Flinn Suggested Disposal Method #24a.
- Slowly heat about 1.8 L of tap water in a 2-L beaker using a hot plate. Do not allow the water to boil.
- As the water is heating, add the 20 g of benzoic acid supplied with the kit. Continue to heat and stir the mixture until the benzoic acid completely dissolves. You will not have to boil the solution.
- Remove the beaker from the hot plate and allow the solution to begin cooling.
- Thoroughly clean and dry the 3-L bottle.
- Glue the holiday figurine on the inside of the lid of the 3-liter plastic bottle using a glue gun. Put some hot glue on the bottom of the figurine and immediately place the figurine in the center of the lid. Hold it in place for a few seconds. Allow the glue to cool for several minutes. Apply an extra line of glue around each foot for more support.
- Observe the benzoic acid solution as it begins to cool. Snowy-looking crystals will appear.
- After the solution has completely cooled to room temperature and the figurine is secure to the lid, stir the snow mixture, and then quickly pour it into the 3-liter bottle.
- Slowly fill the bottle with tap water. Leave a 1 to 2 inch gap of air in the bottle.
- Cap the bottle tightly. You should now be able to turn the snow scene upside down and watch the holiday figurine become buried in the snowstorm.
- If desired, place some electrical tape around the bottle lid to seal it completely.
- If the holiday figurine comes loose, it can be repaired by opening the bottle, thoroughly drying the lid and the figurine and regluing. Replace the lid, adding a bit more tap water, if necessary.
- A snowman or other holiday figurine may be substituted for Santa depending on availability.
- Encourage students to bring in their own figurines (whatever they find interesting) to add alongside (or in place of) the included figurine. Solid plastic figurines work the best. You may consider purchasing inexpensive figurines at a local discount or craft store.
- Student-sized models may be made following the same procedure. “Microscale” the demonstration down to 1 gram of benzoic acid and 75 mL of water. Use a 4-oz ointment bottle (Flinn Catalog No. AP8445) or a baby food jar. Fill the bottle to the brim with tap water.
- Do not try to speed up the cooling process of the solution by using ice; beautiful crystals form when allowed to form slowly.
- If the solution is still hot when added to the jar, the figurine will have a good chance of falling off. Be patient.
- If your tap water is very hard, distilled water may be used.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter
HS-PS1.A: Structure and Properties of Matter
Stability and change
MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-1: Use the periodic table as a model to predict the relative properties of elements based on the patterns of electrons in the outermost energy level of atoms.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
Answers to Questions
- Describe the appearance of the solution in the bottle.
The solution in the bottle, while clear, contained a lot of white precipitate that resembled snow crystals. These crystals did not dissolve in the solution.
- As temperature increases, solubility increases. Explain how the benzoic acid precipitate appeared in the solution.
Benzoic acid was added to hot water. Since the temperature was higher than at room temperature, it was all able to dissolve. But when the solution then cooled, the solubility of the solution decreased and it could no longer contain all the benzoic acid present, so some precipitated out.
A solution is formed by dissolving a solute such as benzoic acid in a solvent such as water. The process of a solid solute dissolving in a solvent is a surface phenomenon. Dissolving is a surface phenomenon because it is those molecules or ions at the surface of the solid, not those in the interior, or bulk, of the solid, that interact with and dissolve in the surrounding solvent. As the surface ions dissolve, the next layer of ions now becomes the surface layer. This new surface layer interacts with the ions already in solution. This interaction at the surface of a crystal continues until the crystal is completely dissolved, or until the solution can accept no more solute.
A solution is said to be unsaturated when it contains a lower concentration of solute than it can at a given temperature and pressure. A saturated solution contains as much solute as it can at a given temperature and pressure. When the temperature is increased, the solubility usually increases and more solute will dissolve. If this solution is then cooled, the “extra” solute that dissolved with heat will once again precipitate. This is the chemical phenomenon occurring when the benzoic acid precipitates out as “snow” in this demonstration.
Special thanks to Richard Chapleau, Lancaster High School, Lancaster, CA, for providing Flinn Scientific with this idea.