Spectroscopy at Home

Hello again from the secret ACS demonstration laboratory! Today, we have a demo video for you to help explain one of the mainstays of analytical chemistry: spectroscopy. Use this demonstration to show how we scientists know if a substance is present in a sample, and how much of that substance is present.

From: metryq.deviantart.com/art/fluorescing-tonic-water-416559283

From: metryq.deviantart.com/art/fluorescing-tonic-water-416559283

To perform this demonstration, you will need:

  • a highlighter (we used the bright yellow Sharpie brand)
  • A bottle of tonic water
  • a purple laser (do NOT point this at anyone’s face!) (We bought ours here)
  • a small glass vial (plastic will do)
  • 3 clear plastic cups
  • water (tap water will do)
  • goggles

Safety Notes:

Although you’re only using household items, you’ll be performing this as a part of a demo show, so you should be wearing your safety goggles. Even if you’re performing this demo by itself, wear your goggles anyway- its a science thing, and we only do science things if safety is addressed.

Also, don’t shine the laser in anyone’s face.

Procedure:

spectroscopy from ACS Undergraduate Programs on Vimeo.

To perform this demo, start by explaining that scientists often need to know what is present in our samples. One of the ways we do this is through the interaction of light and and radiated energy, through a process called spectroscopy.

Experiment 1: Light can show us if a substance is present in a material.

Demonstrate that a laser, shined through a vial of tap water, leaves no trail.  This is because nothing in the tap water fluoresces when hit by the laser.

Take your highlighter, and dip the tip of the highlighter into the water. Demonstrate that now the laser now fluoresces the liquid due to the addition of material from the highlighter.  Explain how, just as the laser reveals the presence of the highlighter, scientists can use other forms of radiant energy to detect certain chemical compounds.

Experiment 2: Light can show us how much of a substance is present in a material.

Start by filling one of the plastic cups with water, one with tonic water, and one with a 1:4 ratio of tonic water to water. Ask your audience to predict which glass will glow the brightest when the laser is shined through it. Show the laser shining in each glass, and explain that the more tonic water is present, the brighter the laser will fluoresce the liquid.  Explain that, using similar techniques, analytical chemists can determine the amount of substances present, such as the amount of chlorine in tap water or the amount of sugar in cola.

Take a look at our video for our take on this. Feel free to comment about how you think it could be done better!

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