A page from the "Causes of Color" exhibit...
When you heat metallic elements in a gas, their photons exhibit bright characteristic colors. With a few household materials, you can create your own tiny sparks of color.
- Sodium chloride (table salt or sea salt)
- Potassium chloride (salt substitute)
- Calcium chloride (“Ice melt”)
- Strontium chloride (a road flare)
- Copper sulfate, an algaecide (available at pet stores that specialize in aquaria, and at pool supply stores)
- A wine cork
- A sewing needle
- Sand paper or steel wool
- A gas flame: methane (home stove), propane (gas grill), or butane (camp stove)
How To Do It
- Insert the sharp end of the needle into the cork.
- Wet the eye of the needle with water, and dip into table salt.
- Hold the cork so that the end of the needle with the salt is in the top part of a gas flame. You should be able to see a bright, predominantly yellow color. (Note: The top third of the gas flame is the hottest part of the flame.)
- If the salt has not burned off, remove the salt from the needle with sand paper or steel wool. Wet it with water again, and place it into salt substitute. The potassium chloride can produce a beautiful lavender flame.
- Repeat the above procedure with calcium chloride (a red-orange flame); strontium chloride (a scarlet flame); and copper sulfate (a green flame). (Note: Carefully cut open the road flare to access the powder that contains the strontium.)
- It is not necessary to use all of these substances; one or two will give you the opportunity to see the bright colors.
How It Works
Metallic elements in compounds liberate a characteristic color when heated in a gas flame. Photons of light are emitted when electrons that are “excited” fall back to the ground state. When the atoms of a gas or vapor are excited, for instance by heating or by applying an electrical field, their electrons are able to move from their ground state to higher energy levels. As they return to their ground state, following clearly defined paths according to quantum probabilities, they emit photons of very specific energy. This energy corresponds to particular wavelengths of light, and so produces particular colors.
Flame tests are useful in lab situations when it is necessary to identify a compound that is an unknown. This happens frequently when labels fall off of containers. Many fireworks get their color because salts burn brightly. Copper salts give fireworks a green color and lithium and strontium salts produce red colors. Magnesium is bright and white.
You can buy Crystal Color Fire Sticks for $4.99 from Amazon that produce bright greens, blues, and reds in a fireplace or campfire.
You can test sodium carbonate and calcium carbonate to prove to yourself that it is the metallic part (the cation) of the compound producing the color. Hence, sodium carbonate will produce a yellow flame and calcium carbonate (chalk or limestone) will produce a red-orange flame.