Balloon Flame Tests

Everybody loves this demo.

I actually learned how to do this because my senior year chemistry teacher for better or for worse trusted us to do demos. Naturally, we all picked one that involved exothermic reactions, if you catch my drift.

Invariably once you get into astronomy you’re going to get asked how we know what the cosmos is made out of. We simply took a look at the cosmos through spectroscopic telescopes and observe the lines of color that appear. Each element has its own spectral “fingerprint” we can observe, and you can show this in a more exciting way with exploding hydrogen balloons mixed with a small amount of powdered material. The explanation is pretty physics-y, but I’ll try to explain it simply.

Molecules are collections of atoms which are bonded by electrons. The electrons exist at a basic energy level relating to their distance from the center of the atom. When a molecule absorbs the appropriate amount of energy (in the form of a packet of light energy, or photon), the electron becomes “excited” and raises itself to a higher energy level. This level is usually unstable, and the electron “falls” back to its previous level. In doing so, it releases an amount of energy equal to the gap between that high energy level and the lower energy level. Since the energy of that photon is determined by its frequency, and frequency is related to wavelength, each molecule will release a photon with a unique wavelength of light. In our case the energy used to excite the molecules will be heat from the combustion of hydrogen, but the combustion of methanol in a spray bottle also works. Look at the difference in the color of the flames between a solution of sodium ions and strontium ions.

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Special thanks to Gordon Gore for this glamour shot.

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There are a number of ways to collect hydrogen, a couple of which you can incorporate into science lessons. You can either create hydrogen and oxygen through the electrolysis of pure water (the ‘ole car battery and the two pipes trick, or you can get a kit online), or by dissolving a pure metal in hydrochloric acid. I usually use aluminum because its dirt cheap, but zinc dissolves more readily. I’ll walk you through the process.

1. Weigh out about 7 grams of zinc pieces or finely torn aluminum foil, and place this into a pre-stretched balloon.

2. Pour 100mL of 6M hydrochloric acid into a 150-mL Erlenmeyer flask. Set this up on a lab stand at a tilt so you can stop the metal salt from falling into the acid.

3. Put a pinch of metal salt into the balloon. Strontium, lithium, sodium, potassium, copper, and iron produce colorful results.

4. place the metal pieces into the flask and quickly stretch the balloon over the mouth of the flask while holding the bulb of the balloon in place so we don’t get premature reactions.

5. When ready, pour the dry metal out of the balloon and into the flask. When bubbles stop forming, pinch and tie off the balloon. If you’ve done this my way, you should have a collection of hydrogen in the balloon with a small amount of metal salt.

6. In late High School, you may be able to ask them to calculate the wavelength of light that would be emitted and make an educated guess on the colour. For other grades, provide the dominant wavelengths and give students a chart that maps out wavelengths on the visible light spectrum and ask them to make a guess.

Did the colours line up with what they expected? Always ask students to reflect on what they have done. Here are the balloon explosions for plain hydrogen, strontium nitrate, and sodium carbonate. The photo doesn’t do it justice, but you can plainly see the red light of strontium and the brilliant orange of sodium. Copper chloride produces a green flame, potassium compounds are more purple, and powdered iron produces yellow sparks.

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SAFETY CONCERNS: Acid and explosions are bad for you; always wear gloves, labcoats, and safety goggles when performing this procedure. Hydrogen production can be doen in a fume hood but the explosions may damage it. If possible, perform activity behind a safety screen, make sure all students are sitting at the back of the class, far from the demonstration. Metal salts may react unfavorably with the hydrochloric acid, so be careful that none ends up in the flask.

 

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