Pink Color Change Chem Demo
Here’s a fun chemistry demonstration that’s perfect for Valentine’s Day or to illustrate an oxidation-reduction reaction. The Vanishing Valentine involves shaking a solution, causing it to turn pink. If the pink Valentine solution is left undisturbed, it will become colorless. The color change cycle can be repeated several times. It is caused by the oxidation and reduction of resazurin. an indicator that is pink or colorless depending on its oxidation state.
Vanishing Valentine Materials
- 100 ml of a 0.133 M dextrose solution (C6H12O6)
- 100 ml of a 1.0 M sodium hydroxide solution (NaOH)
- 1 ml of a 0.1% resazurin solution
- a 250-ml or 500 ml Erlenmeyer flask or separatory funnel (resembles a heart)
- stopper for the flask
- dropper or pipette
Prepare the Solutions
Dextrose Solution: Dissolve 2.4 g of dextrose in distilled or deionized water to make 100 ml of solution.
Sodium Hydroxide Solution: Prepare the 1.0 M sodium hydroxide solution by dissolving 4.0 g of sodium hydroxide in enough distilled or deionized water to make 100 ml of solution. Add the sodium hydroxide a little at time, stirring constantly. Heat is evolved from this reaction.
Resazurin Solution: Dissolve 0.1 g of resazurin in distilled or deionized water to make 100 ml of solution. The shelf life of resazurin solution is 6-12 months. This solution should be a deep blue color.
Perform the Vanishing Valentine Demonstration
- Pour 100 ml of dextrose solution and 100 ml of sodium hydroxide solution into the Erlenmeyer flask or separatory funnel.
- Add 8 drops of resazurin indicator solution to the flask or funnel.
- Stopper the solution and swirl the flask to mix the contents. Initially the solution will be blue.
- Allow the solution to sit undisturbed. Once the resazurin is fully reduced the solution will become clear or colorless. This may take up to 10 minutes.
- Swirl or shake the solution to turn it a pink Valentine color.
- The clear-pink cycle may be repeated by allowing the solution to sit and then shaking it again. Once prepared, the solution lasts approximately an hour (depending on temperature and available oxygen in the flask). The pink color will become less vivid over time.
Vanishing Valentine Chemical Reactions
Dextrose irreversibly reduces resazurin to resorufin. The red resorufin molecule is further reduced (reversibly) to colorless dihydroresorufin. Dihydroresorufin (clear) may be oxidized back to resorufin (pink) by swirling or shaking the flask to introduce oxygen from the air into the solution. The reaction is an example of a color-change redox reaction, like the classic blue bottle chemistry demonstration.
Vanishing Valentine Demo Safety
Wear appropriate chemistry lab safety gear when performing this demonstration, such as a lab apron, gloves and safety goggles. While the resazurin and dextrose solutions are not hazardous, sodium hydroxide solutions are caustic and could produce a chemical burn if spilled on the skin or splashed into the eyes.