Mesmerize your students with the fascinating properties of a supersaturated solution. Create crystallized flasks with this fun exothermic activity!
Sodium acetate trihydrate, CH3CO2Na•3H2O, 210 g*
Water, distilled or deionized
Beaker, borosilicate glass, 1000-mL
Florence flask, borosilicate glass, 250-mL*
Graduated cylinder, 50- or 100-mL
Heat-resistant gloves or tongs
Rubber stopper to fit flask*
Stirring rod, glass
*Materials included in kit.
Sodium acetate is slightly toxic by ingestion, inhalation and skin absorption. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Follow all laboratory safety guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.
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 sodium acetate solution may be re-used or flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.
- Fill a 1000-mL beaker approximately a third full with water and place on the hot plate. Heat the water. This will be used as the warm water bath. Note: The water should not be boiling.
- Measure out 210 g of sodium acetate trihydrate and add it to the 250-mL Florence flask.
- Add 63 mL of distilled or deionized water to the flask.
- Place the flask in the warm water bath and allow the sodium acetate trihydrate to dissolve.
- After heating for a few minutes, stir the solution with a glass stirring rod.
- Continue heating in the water bath until all the sodium acetate trihydrate has dissolved and the solution is clear. If any sodium acetate trihydrate gets stuck on the side of the glassware, squirt a very small amount of distilled or deionized water to wash it down into the solution
- Cover the flask with the rubber stopper and allow the solution to cool to room temperature. Take care not to disturb the solution as even slight movement may cause crystallization. Note: It may be helpful to have one to two backup flasks prepared, in case the solution crystallizes from a disturbance.
- Place a single sodium acetate trihydrate crystal into the flask. The single crystal will start a chain reaction of crystallization.
- The flask will also be warm since this is an exothermic process.
- The solution may be used again by reheating it to dissolve the sodium acetate.
- This kit contains enough chemicals to perform the reusable demonstration seven times: 750 g of sodium acetate trihydrate, one 250-mL borosilicate glass Florence flask, and one rubber stopper.
- Enough sodium acetate trihydrate is provided to make three reusable 250-mL flasks.
- The prepared flask(s) can be stored and reused for several years.
- The addition of too much water will result in leftover liquid after recrystallization.
- Use only borosilicate glassware for this demonstration. The temperature difference may crack glass that is not borosilicate.
- Another way to present this demonstration is to show it to students in small groups. A small test tube or Erlenmeyer flask of supersaturated solution can be placed at each lab station. The instructor can then add a small seed crystal and students can see the demonstration closer in more detail.
- For instructions on how to make a sodium acetate tower, please request Supersaturation Tower, Flinn publication number 11308.
- Use a document camera or other video microscopy unit to provide a close-up view of the crystallization process.
- The Heat Solution Instant Handwarmer (Catalog No. AP1933) is another way of demonstrating supersaturation.
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Asking questions and defining problems
Developing and using models
Planning and carrying out investigations
Obtaining, evaluation, and communicating information
Engaging in argument from evidence
Using mathematics and computational thinking
Disciplinary Core Ideas
MS-PS1.B: Chemical Reactions
MS-PS3.D: Energy in Chemical Processes and Everyday Life
HS-PS1.B: Chemical Reactions
Energy and 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.
MS-PS3-4. Plan an investigation to determine the relationships among the energy transferred, the type of matter, the mass, and the change in the average kinetic energy of the particles as measured by the temperature of the sample.
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
- Draw the before and after images of the demonstration. What did the flask look like before the seed crystal was added? What did it look like after?
The flask started with a clear solution. After the seed crystal was added, the clear solution began to crystallize into a white solid.
- Describe what you observed in this demonstration.
When the clear liquid turned into a white solid, the flask gave off heat/energy.
- Was the crystallization process endothermic or exothermic? Use evidence from the demonstration to support your answer.
This demonstration was exothermic because the flask became warm when the solution crystallized. This exothermic demonstration gave off energy in the form of heat.
- Define saturated, unsaturated and supersaturated.
Saturated solutions hold the maximum amount of solute (solid/crystal) at a given temperature. If more solute is added, it does not dissolve into the saturated solution. Unsaturated solutions are solutions with less solute than a saturated solution and more solid can be dissolved in the solution. Supersaturated solutions are solutions with more solute dissolved than a saturated solution at the same termperature. For example, with the sodium acetate supersaturated solution, the solution is supersaturated and supercooled—this means that it contains more dissolved sodium acetate than a saturated solution and has been cooled to below its freezing point without crystallization occurring.
- For the flask demonstration, 210 g of sodium acetate trihydrate is used. Write the chemical formula of sodium acetate trihydrate and calculate how many moles of sodium acetate trihydrate were used in the flask demonstration.
A supersaturated solution of sodium acetate trihydrate (the clear solution) recrystallizes with the addition of a single crystal. When making a solution, either a supersaturated, saturated or unsaturated solution is possible. A saturated solution contains the maximum amount of a dissolved solid (or solute) at a given temperature. A solute is a substance (such as a solid or crystal) that is dissolved in the solvent.
A supersaturated solution contains a greater amount of dissolved substance than is present in a saturated solution at the same temperature. A supersaturated sodium acetate solution can be made by gradually cooling a heated saturated solution without agitation so that crystals do not form. At this point, the solution is supersaturated and supercooled—this means that it contains more dissolved sodium acetate than a saturated solution and has been cooled to below its freezing point without crystallization occurring. Supersaturated solutions are extremely unstable and will precipitate, or crystallize, upon addition of just one crystal of the solute. Even slight shaking or agitation may be enough to cause crystallization to begin.
In this demonstration, the seed crystal is the start of a chain reaction that causes the entire solution, all of the sodium acetate trihydrate molecules, to crystallize. Crystallize means a liquid is becoming a solid, or “freezing.” When the crystallization is activated, the solution climbs from room temperature to its freezing point, which is 58 °C (136 °F). At this temperature, the sodium acetate solution changes from a liquid to a solid.
When the solidified sodium acetate trihydrate crystals are then heated to a temperature greater than 58 °C (136 °F), the crystals will melt. This process is endothermic since heat is being added from an external source. The sodium acetate will to some extent actually dissolve in its own water of hydration. The liquefying (melting) and solidifying (freezing) of the sodium acetate trihydrate is a reversible reaction represented by the following equation:
The forward reaction represents the crystallization process. Notice that heat is a product that is given off by the reaction. The reaction is exothermic (ΔH
= –19.7 kJ/mol), which was easily observed by feeling the very warm glassware.
The reverse reaction represents the melting process. Notice, this time, that heat is a reactant that is put into the reaction. The reaction is endothermic (ΔH
= +19.7 kJ/mol), which was evident by the need to heat the solution.