Innovative Uses

The Chemistry of Bathtub Tints

A cute bath product made by Crayola®, called Bathtub Tints, offered us an interesting opportunity to conduct a few simple chemistry activities. The ads that we have seen for this product describe it as effervescent water color tablets. The first three ingredients listed on the label of a container of Bathtub Tints are sodium carbonate, sodium bicarbonate, and citric acid. It may not be bath time, but it’s time to do some investigations with these little bundles of fun.

(NOTE: If you search the web using “fizzies” as the key word, you will find several recipes for homemade versions of Bathtub Tints.)

Activity #1: Measuring the pH of a Dissolving Bathtub Tint Tablet

Measure out 100 mL of tap water into a 250 mL beaker, or similar sized container. Set up a pH Sensor and immerse the tip of the pH Sensor in the container of tap water. Start the Vernier data-collection program. Add one Bathtub Tint tablet to the beaker of water and stir the water gently. After about 20 seconds, start the data collection. The graph below shows typical results.

Tub Tint Tablet Test: pH vs Time

Activity #2: Measuring the Conductivity of a Dissolving Bathtub Tint Tablet

Measure out 100 mL of tap water into a 250 mL beaker or similar sized container. Set up a Conductivity Probe and immerse it in the container of tap water. Start the Vernier data-collection program. Place the toggle switch on the probe in the 0-20,000 position. Add one Bathtub Tint tablet to the beaker of water, stir the water gently, and start the data collection. The graph below shows typical results.

Tub Tint Test Two: Conductivity vs Time

Activity #3: Tub Tints Beer’s Law

Prepare a stock solution by dissolving 3 blue Tub Tint tablets in 50 mL of distilled water. Prepare four standard solutions using the following chart.

Standard Volume of Distilled Water Drops of Stock Solution
1 20 mL 20
2 20 mL 15
3 20 mL 10
4 20 mL 5

Set up a Colorimeter. Set the wavelength at 635 nm, place a blank in the Colorimeter, and press CAL on the Colorimeter to calibrate it. Start the Vernier data-collection program. Set the data collection for events with entry. Record the absorbance of each of the four standard solutions. A sample graph is shown below.

Tub Tints Beer’s Law: Absorbance vs Drops of Tub Tint Solution

There are several variations you can use for this experiment. One common activity with Beer’s law is to have the students determine the concentration of an unknown that you prepare from the stock solution.

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