March 2016

# Chemical rainbow

##

Kenneth Lyle*, PhD, Duke University, Durham NC

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## Concepts

Acids, bases, universal indicator, pH, neutralization.

## Materials

- Two 250 mL beakers
- Universal indicator
- Glass stirring rod
- Disposable pipet
- 0.1 M HCl in a dropper bottle
- 0.1 M NaOH in dropper bottle
- Water
- Gas tube with stoppers for each end — we use a 24” long by 14 mm ID tube purchased from Flinn Scientific, GP9146,
  $US 15.45. The tube came with two #2 rubber stoppers.
- Plastic bowl, needed to catch spills.

## Procedure

- Partially fill one of the two beakers with water.
  (~175 – 200 mL, enough to fill glass tube within 1 inch from top).
- Add universal indicator while stirring until the mixture is dark green — assuming the water is neutral from the source.
- Securely stopper one end of the glass tube and stand it vertically in the plastic tub pressing stoppered end against the bottom of the tub.
- Fill the tube to within 1 inch from the top.
- Add exactly 10 drops of the 0.1 M HCl. Hold the dropping bottle vertically, upside down, in order to deliver uniformed size drops.
  The universal indicator turns red where acid was added.
- Stopper the top end of the glass tube with the remaining stopper.
- Carefully tilt the tube while keeping the bottom stopper against the bottom of the tub to prevent the stopper from coming out of the tube, releasing the liquid inside. Slip one finger underneath the bottom stopper and another against the top stopper. Invert the tube one time, while holding both stoppers in place, allowing the air bubble inside to rise to the top.
- The acidic portion of the spectrum is now visible — red, orange, yellow, green (neutral).
- Similarly, add 10 drops of 0.1 M NaOH to the other end of the tube — turns purple — and then mix by inverting one time. The base portion of the spectrum is now visible — purple, blue, green — along with the acidic portion. You can invert the tube several times, allowing the air bubble to rise to the top after each inversion; the complete spectrum is still visible.
- Finally ask the audience to predict the color of the solution after the content of the tube is emptied into a beaker. This can be done by carefully removing one stopper of tube and pouring the solutions into the beaker. (answer: green).

## Safety and disposal

Wear protective gloves and goggles/safety glasses.

The mixture may be poured down the drain. Rinse tube, stoppers and tub.

Flinn Scientific, Inc. has a ChemFax #10079 that describes this demonstration. Penny Sconzo, Westminster Schools and Kennesaw State University, Kennesaw GA has this demonstration along with another rainbow test tube demo as a Flinn Demonstration video. This video is part of the Flinn Scientific Best Practices for Teaching Chemistry Video Series, a collection of over 125 hours of free professional development training for chemistry teachers.

Danielle Holdner** involved in the Outreach, had these impressions of the activity room at Nasher Museum.

> The children gathered around each activity table excited to learn about the chemistry of color. We tailored the activities depending on age — the children ranged from 4-13 years of age. No matter what the age, the children still displayed the same amount of interest and excitement in learning how colors play a key role in chemistry.

*Kenneth Lyle is a lecturing-fellow in the Department of Chemistry at Duke University NC. **Danielle Holdner attends East Carolina University majoring in Family Community Services. Upon graduation she plans to attend medical school, to practice pediatric medicine.

(Powell Family Trust, the Duke-Durham Neighborhood Partnership, and Biogen Idec – Research Triangle Park, fund the Duke Chemistry Outreach Program.)

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## More about March 2016

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  Activities Sharing chemistry with the community: The chemistry of color

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