Active Time: 20-30 minutes
Total Project Time: 45-60 minutes
Key Concepts: chemicals, metal salts, fire, atoms, energy
- Chemicals to burn, specifically table salt and copper sulfate:
- Table salt is technically called sodium chloride.
- Copper sulfate is available through pet or aquarium stores to combat algae, or through home improvement stores as a root killer. Make sure the product is pure copper sulfate and that it is in powder or small crystals form.
- Alternatively, if you would like these chemicals—plus two other colorful, hard-to-find metal salts, as well as some safety equipment, all in one convenient package—try the Rainbow Fire kit from the Science Buddies Store.
- Small plastic bag
- Bamboo skewers (at least 6)
- White glue
- Matches or lighter
- Container of water
- An outdoor surface you can safely burn a candle on when it is dark outside (or twilight). Be sure it is in an open area to allow good air flow.
- Adult helper
- Recommended: Disposable gloves. Dishwashing gloves are a fine alternative. These are for handling the copper sulfate.
- Safety goggles
- Optional: Flashlight
- Optional: Masking tape and pen or marker to label the skewers with the chemical names
- When you burn the skewers, be sure to do it in an open, outdoor area and be careful not to breathe the fumes or smoke from the skewers.
- Adult supervision is needed when using fire, burning the skewers, and handling the chemicals.
- This activity uses a small candle flame to maximize safety, but you should have a container of water handy for dousing any unintended flames.
- Pour a small amount of table salt (roughly one tablespoon) into a small plastic bag.
- Apply a thin layer of glue to the last inch of the tip of a skewer. Just a little bit of glue is enough.
- Dip the glue-coated tip of the skewer into the salt in the bag. Twist the skewer back and forth a bit to coat the skewer’s tip with the salt. Set the skewer aside to dry.
- Repeat this process with two more skewers so you have a total of three skewers that have their tips coated with salt.
- Next, have an adult coat a skewer with copper sulfate—but first read and follow all safety precautions on the packaging that the copper sulfate came in. Be careful not to let anybody breathe in any copper sulfate dust or get any on their skin or face. If desired, use disposable gloves and safety goggles. If not using gloves, be sure not to touch the copper sulfate. The adult should coat the last inch of the tip of a skewer in a thin layer of glue and dip the glue-coated tip into the bag of blue-colored copper sulfate, twisting it back and forth to coat the tip in copper sulfate.
- The adult should repeat this with two more skewers, so that you have three total that are coated in copper sulfate.
- Allow all of your skewers to dry. This will take about half an hour for each.
- When it is dark (or twilight) outside, take your candle, matches, and prepared skewers to an open area outside where you can safely burn a candle. Be sure there is good air flow. Also have a container of water with you as a safety precaution.
- Have an adult light the candle.
- Once the candle is burning well, carefully take out one of the prepared skewers and hold the chemical-coated end in the flame. Be sure not to breathe the fumes or smoke from the burning skewer! What color does the chemical burn? How does it compare to the normal color of the candle’s flame?
- Tip: You may be able to see the chemical’s color in the candle’s flame, or by holding the skewer away from the candle (once the chemical has caught on fire) and looking at the flame on the chemical-coated end of the skewer.
- The skewer may also catch fire after a few moments. Simply remove it from the flame and blow it out, or extinguish it in the container of water.
- Repeat this process for the other chemical-coated skewers. (To help you distinguish which chemical you are burning, remember that the skewers coated in table salt will have white-coated tips, while the skewers coated in copper sulfate will have blue-coated tips.) Which color does the table salt (sodium chloride) burn? Which color does the copper sulfate burn? If you burn other chemicals, what color(s) do they burn?
Be sure to extinguish the candle and the skewers when you are done with the activity. When you are sure they have been extinguished, you can dispose of the used chemical-coated skewers in the garbage.
A typical flame will burn yellow/orange with a little bit of blue near the base of the wick. When you burned the skewer tip coated with sodium chloride, you should have seen that the flame on the sodium chloride was pure yellow/orange (without any blue). This is because when the metal sodium is heated, it makes intense yellow/orange light. When you burned the skewer tip coated with copper sulfate, you should have seen that the flame gained blue-green traces. This is because when the copper is heated, it makes bluish-green light. If you tested additional chemicals from the Rainbow Fire kit, you should have seen that the boric acid burned a vivid green and the strontium chloride burned red.
Today when we watch fireworks displays, there are many colors represented, but this was not always the case. From the time fireworks were invented (the earliest documentation is from around the 7th century C.E. China) until the 1830s, all fireworks were either white or orange. Then, in the 1830s, the Italians discovered that adding metal salts to the fireworks mixture resulted in interesting colors, just like in the flame test in this activity.
Colored flames are not just for fireworks displays; the same science that shows us a pretty colored flame allows astronomers to figure out what distant stars are made of (in other words, their atomic composition) by seeing what type of light is produced by the star. How does it work exactly? It comes down to atoms and energy.
All matter is made of atoms. Atoms have a nucleus that is surrounded by electrons. Electrons moving around the nucleus have certain amounts of energy, called energy levels. If electrons gain energy, they move from one energy level up to a higher level through distinct steps. Likewise, if electrons lose energy, they drop down to a lower energy level. The lost energy can be carried away in the form of heat or in the form of light (as a photon). Exactly how much energy is lost affects what kind of light is made, which we may perceive as a difference in the color of the light. Different types of atoms, or elements, have different gaps between their energy levels, which causes them to make light of different colors when they are burned and lose that energy.
You could try this activity using other metal salts known to produce certain flame colors when burned. Check out the Science Buddies Rainbow Fire science project for some ideas. Be sure to always look into and follow all proper safety precautions when handling different chemicals and burning them. What colors do other metal salts make when burned?
- For a printable version of this activity, visit Science Buddies.
- Watch Science Friday's video "Celebrating Explosive Chemistry" for another explanation of the salts in fireworks.