Unexpected Precipitation

Introduction

A precipitate forms when a saturated calcium acetate solution is heated and dissolves when the solution cools. This demonstration is great for introducing the concept of solubility and the many exceptions to rules in chemistry.

Concepts

• Exceptions to chemical rules
• Saturation
• Precipitation
• Solubility

Materials

Safety Precautions

Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

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 solution may be reused many times. When it is no longer useful, the saturated calcium acetate solution may be disposed of according to Flinn Suggested Disposal Method #26b.

Procedure

1. Pour 150 mL of saturated calcium acetate solution into an Erlenmeyer flask. Note: It is at the teacher’s discretion how much calcium acetate solution to heat. Typically 150 mL should be adequate for student viewing purposes and requires approximately 10 minutes of heating on a medium setting in order to precipitate. Heating additional saturated calcium acetate solution will require additional time.
2. Heat the solution on a hot plate, stirring often. A precipitate will begin to form around 65 °C and will continue to form as the solution heats to 90 °C. Note: Do not allow the solution to boil.
3. Remove the flask from the hot plate using a heat-resistant glove or pad, and run it under cool water. Note: Do not allow any water to get inside the flask.
4. Stopper the flask and cool the solution in an ice bath, swirling often. As the temperature decreases, the precipitate will go back into solution. Alternatively, the solution may be allowed to gradually cool to room temperature.

Student Worksheet PDF

13980_Student1.pdf

Teacher Tips

• The solid may precipitate if kept in a hot chemical storage room. If this occurs, simply cool the mixture to dissolve the precipitate.
• Some water will evaporate as the solution is heated. After the demonstration has been performed many times, the solution may become supersaturated due to the loss of water, making it necessary to add 1–2 mL increments of deionized water per 100 mL of solution. Test the solution after each addition of water by heating. If the solution begins to precipitate before the solution reaches 60 °C, add another small increment of water. The calcium acetate solution needs to be saturated for the “discrepant event” to occur, so it is important not to add too much water.
• You may choose to preheat the solution and start the demonstration by showing students the thick white mixture containing the precipitate and ask for hypotheses of what will happen as it chills in an ice bath.
• Only use borosilicate glass (e.g., Pyrex^®) flasks since the heating and immediate cooling may cause other glass flasks and chipped/cracked flasks to break. Check the Erlenmeyer flask for chips and cracks before use. If chips or cracks are found in glassware, do not use in this demonstration.

Answers to Questions

1. Describe what happened in this demonstration.

   A saturated calcium acetate solution was heated on a hot plate. At around 65 °C a precipitate began to form. More formed as the solution grew hotter. When the solution was removed from the heat and placed, swirling, in an ice water bath, the precipitate disappears and goes back into solution.

2. Why is this demonstration an example of a discrepant event? Make sure to explain what you expected to happen, and why.

   A discrepant event is an occurrence that is the opposite of what is expected, that is the exception to the rule. Generally, solids are more soluble in higher temperatures, but in this case, calcium acetate was less soluble—forming a precipitate when it was heated, dissolving when it was cooled. I expected the opposite to happen—the calcium acetate forming a precipitate when it was cooled, dissolving when it was heated.

3. The solubility value of calcium acetate is about 37.4 g per 100 mL at 0 °C and about 29.7 g per 100 mL at 100 °C. How much solid do you think had precipitated out at 90 °C?

   Approximately 5 g probably had precipitated out when the calcium acetate solution reached 90 °C.

4. Why does calcium tend to leave hard water deposits on faucets and shower heads?

   Since calcium acetate precipitates out at high temperatures, hot water that contains the substance will leave the precipitate as a residue on the faucet.

Discussion

A discrepant event is an incident involving inconsistency between what is logically expected to happen and what is actually observed. For example, solubility normally increases with temperature. Typically solids are more soluble in hot solutions than cold solutions. This is the result of molecules moving faster and colliding more frequently. For many, although not all, ionic compounds the heat of solution is endothermic and the solubility increases as the temperature goes up. The effect temperature has on solubility is solely dependent on the “differential heat of solution” at the saturation point. This is different than the physical temperature of solution. Calcium acetate precipitates out of solution as it is heated—opposite of what is generally observed, classifying this occurrence as a discrepant event. The solubility values for calcium acetate ranges from approximately 37.4 g per 100 mL of water at 0 °C, to about 29.7 g per 100 mL of water at 100 °C. As the saturated solution is heated from room temperature, 25 °C, to 90 °C, approximately 4–5 g of calcium acetate per 100 mL of solution will precipitate out of solution.

Calcium is the major component of hard water deposits found on faucets. Faucets that typically run warm water, such as a shower head, tend to have a higher accumulation of deposit than, for example, a hose nozzle which only unheated water runs through. Calcium acetate is commercially used in medications, baking, pet food and as a preservative.