The Aloha Chemical Sunset

Demonstration Kit

Introduction

A simple chemical reaction produces a colloidal solution, resulting in a chemical “sunset.”

Concepts

  • Colloids and precipitates
  • Tyndall effect/light scattering

Materials

Hydrochloric acid solution, 1 M, HCl, 60 mL*
Sodium thiosulfate solution, 0.2 M, Na2S2O3, 100 mL*
Chemical sunset cutouts (printed palm tree and circular cutout to fit Petri dish)*
Overhead projector
Petri dish or glass culture dish, 100 mm x 15 mm*
Tape of Hawaiian music (optional)
*Materials included in kit.

Safety Precautions

Hydrochloric acid solutions are highly toxic by ingestion or inhalation; severely corrosive to skin and eyes. The sulfur produced in this reaction has low toxicity and may be a skin irritant. Sulfur dioxide is an irritant to eyes and other tissues. Wear chemical-resistant gloves, splash goggles and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The resulting solution/mixture may be fil¬tered and thrown in the trash according to Flinn Suggested Disposal Method #26a. The filtrate solution should be diluted with water, then neutralized with sodium carbonate, and then flushed down the drain with excess water, according to Flinn Suggested Disposal Method #24b.

Procedure

  1. Place Cutout 1 on the overhead.
  2. Place Cutout 2 inside the hole in Cutout 1.
  3. Place the Petri dish inside the hole in Cutout 1, on top of Cutout 2 (see Figure 1).
    {13057_Procedure_Figure_1}
  4. Turn on the overhead projector. Focus the image on a projector screen or wall.
  5. Measure out 14 mL of Na2S2O3 solution; pour into the Petri dish.
  6. Measure out 8 mL of 1 M HCl solution; pour into the Petri dish.
  7. If desired, the tape of Hawaiian music can be played.
  8. Observe the color of the projected light on the screen or wall.

Student Worksheet PDF

13057_Teacher1.pdf

13057_Student1.pdf

Correlation to Next Generation Science Standards (NGSS)†

Science & Engineering Practices

Developing and using models
Obtaining, evaluation, and communicating information

Disciplinary Core Ideas

MS-PS1.B: Chemical Reactions
MS-PS1.A: Structure and Properties of Matter
MS-PS4.A: Wave Properties
HS-PS1.A: Structure and Properties of Matter
HS-PS4.A: Wave Properties

Crosscutting Concepts

Cause and effect
Structure and function

Performance Expectations

MS-PS1-2: Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
MS-PS1-5: Develop and use a model to describe how the total number of atoms does not change in a chemical reaction and thus mass is conserved.
MS-PS1-3: Gather and make sense of information to describe that synthetic materials come from natural resources and impact society.
MS-PS4-2: Develop and use a model to describe that waves are reflected, absorbed, or transmitted through various materials.
HS-PS1-2: Construct and revise an explanation for the outcome of a simple chemical reaction based on the outermost electron states of atoms, trends in the periodic table, and knowledge of the patterns of chemical properties.
HS-PS4-1: Use mathematical representations to support a claim regarding relationships among the frequency, wavelength, and speed of waves traveling in various media.

Answers to Questions

  1. Describe what happened in this demonstration.

    Sodium thiosulfate solution was poured into a Petri dish. A black-and-white picture of a sunset on a Hawaiian beach was underneath the dish. When hydrochloric acid was added to the sodium thiosulfate, the image from the projector beam began to turn red. The image got darker and darker until you could no longer see it.

  2. Write a balanced chemical equation for the decomposition of the thiosulface ion in an acid, producing colloidal sulfur.

    S2O32–(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l)

  3. The solid colloidal sulfur scattered the light from the overhead. Propose an explanation for the slow color changes the projected light underwent.

    As the amount of sulfur increased, it began to scatter shorter wavelengths of light. Since only longer wavelengths were being let through, the image in the projector light turns red. Then, as the sulfur concentration continues to increase, only very long wavelengths of light are let through. Eventually all the light was scattered, and the image could not be seen, since no projector light was being let through the solution.

Discussion

The thiosulfate ion decomposes in acid solution, producing colloidal sulfur according to the reaction

S2O32-(aq) + 2H+(aq) → S(s) + SO2(aq) + H2O(l)

As the colloidal sulfur particles grow, the light from the overhead projector is scattered. As the concentration of colloidal sulfur and its particle size increases, the shorter wavelengths of light are scattered while the longer wavelengths pass through, producing a red color in the projected beam. Eventually, the concentration and particle size of the colloid becomes so great that no light can be transmitted through the solution.

References

Special thanks to David A. Katz, retired, Wilmington, DE, for bringing this demonstration to our attention.

†Next Generation Science Standards and NGSS are registered trademarks of Achieve. Neither Achieve nor the lead states and partners that developed the Next Generation Science Standards were involved in the production of this product, and do not endorse it.