The Rainbow Reaction

Science Concept:

Acid and Bases, Indicators, Diffusion of Liquids

Materials:

Hydrochloric acid, 0.1 M HCl (100 mL)

Sodium carbonate, saturated solution, Na₂CO₃ (100 mL)

Universal indicator

Graduated Cylinder (100 mL)

Beaker (250 mL)

Introduction:

Who loves to see a rainbow after it rains? Well, I love rainbows but do not get to see them very often so I am going to make my own rainbow without any rain.

Preparation:

Add 3 mL of Universal indicator to 100 mL of 0.1 M hydrochloric acid.
Make a saturated sodium carbonate solution by adding 13 g of Na₂CO₃ to 100 mL of water.

Procedure:

1. Fill the graduated cylinder almost full of HCl and the indicator solution.

2. Tilt the graduated cylinder slightly and pour the saturated sodium carbonate solution down the inside wall.

3. Hold the graduated cylinder vertically and the colors will develop with the red at the top and deep blue at the bottom.

Introductory Commentary:

Explanation:

The universal indicator is an acid-base indicator that turns blue in the presence of strong base and red in the presence of strong acid. Actually a variety of other colors are displayed that correspond to different acid/base pH values.

The liquid in the graduated cylinder at the beginning contained hydrochloric acid, which is an acid that causes the indicator to turn red. The sodium carbonate solution that I added into the cylinder is a base, so it causes the solution at the bottom to turn blue. The sodium carbonate is also more dense than the HCl solution so it sinks to the bottom. That is why the bottom of the cylinder is a deep blue showing the presence of the denser sodium carbonate.The rest of the cylinder has a gradient or continuous range of colors in between and red at the top.

Diffusion means to blend and become all one color. If the tube is left undisturbed, the rainbow will last as long as a couple of weeks, before becoming all one color. This demonstrates the very slow rate of diffusion in liquids.

Safety:

Wear goggles and gloves. Do not ingest or make contact with skin. Take usual precautions.

Disposal:

Solutions can be flushed down drain with excess water.

Reference: Unknown