Hazards
- Chromium salts are considered carcinogenic. Use care.
- Potassium dichromate is a strong oxidizer.
Chemicals and Solutions
- Calcium acetate at 40g per 100mL water (see hint following the procedure)
- Two florence flasks containing saturated solution of potassium dichromate
Materials
- Hotplate
- Thermometer (optional)
Procedure
- Heat one of the florence flasks of potassium dichromate on the hotplate. As it is heated more of the solid dissolves and the solution becomes a darker red-orange color. Compare it to the second flask of potassium dichromate that remained at room temperature.
- Heat the beaker of calcium acetate solution on the hotplate. As it is heated a precipitate forms (the precipitate starts to form at about 80ºC). Remove the flask from heat and cool it. The solid goes back into solution. Calcium acetate is less soluble at higher temperatures. At 100ºC the solubility is 36-45g/100g water. At 0ºC the solubility is 44-52 g/100 g water.
Disposal
Solutions can be reused.
Hint: This demo is tricky. The process below can take a few hours. The idea is to make a solution that will precipitate ~80ºC.
Prepare a beaker of ice water (~250mL). For the solution, weigh 50g calcium acetate hydrate into a 250mL erlenmeyer flask. Add ~100g of ice water. Use a wash bottle and a minimal amount of water to rinse any solid calcium acetate accumulated on the flask walls into the solution. Stir the solution in an ice bath until the slurry turns thick and just slightly opaque, about 15 minutes. A few fine, undissolved chunks remaining in solution are OK.
Next, heat the solution to 80±5ºC. If the precipitate forms immediately upon heating (below 40ºC) add a bit more water to the solution, return to the ice bath for ~5min. before heating. By ~80ºC a precipitate should form. If it doesn’t, heat at ~80ºC until a precipitate forms.
Once the precipitate has formed, cool the solution slightly then return to the stir plate/ice bath. Your may observe small amounts of white precipitate (calcium carbonate), that’s OK. When the solution is between 0-10ºC, it should once again be thick and slightly opaque. Gravity filter into a clean 250mL erlenmeyer flask.