two salts that burn with decrepitation
Answers
Materials
Watch glasses (6)
Matches
Scoopula
Ethanol
Salts:
Potassium Chloride (purple)
Copper(II) Sulfate (Blue)
Copper(II) Chloride (Green)
Sodium Chloride (Orange-Yellow)
Calcium Chloride (Orange)
Lithium Chloride (Red)
Safety Precautions
Please read the Fire Safety section and the Dry Chemical section of the Demonstration Safety page before performing this demonstration.
This demonstration requires: goggles, rubber or latex gloves.
Demonstration
Prior to the performance, set out the six watch glasses and pour a small amount of each salt onto its own watch glass. Try to keep the salts in color order to improve the effect.
At the start of the performance, fill the watch glasses 3/4 of the way with ethanol. Let the salts soak for a few minutes in the ethanol. During this time, ask the audience about ways to identify elements from the periodic table.
Light the ethanol. As the ethanol burns, the salts will start to give color to the flames. As the flames start turning colors, ask the audience to try to identify the metals you used to get the colors before revealing the answers.
Why This Works
Wave Chromatography is when you identify an element or several elements in a material by looking at what wavelengths of light it gives off. When you excite an atom of any element, or give extra energy to an atom, it may release the extra energy as light. the colors of light that an atom will give off are unique to the atom, meaning that atoms of Lithium will always give a different set of colors than atoms of Potassium. We get to see this effect by burning the metal salts in ethanol, and can see as each salt gives out a different color.
This effect is something that you may have already known, but never thought about. The majority of paints get their colors simply by adding a dye, which are often complex molecules. However, some dyes we use have very simple names, like "Cobalt Blue" and "Titanium White". This is because these element salts reflect these very specific colors, and if you were to heat up a titanium salt or a cobalt salt you would see these colors!
It is worth noting that elements do give off more than one color. For this experiment, we use two Copper salts, which one gives off a blue color and the other a green color. This is because Copper, like most metals, can have different numbers of electrons when it bonds with other elements, and this can affect the color that you see it give off. In this case, Copper Sulfate will give a blue flame, because the Copper has (and holds onto) two additional electrons. For the Copper(II) Chloride, it gives a green color because there is actually a reaction taking place. Copper(II) Chloride, when heated, will give off a blue flame, but it will also lose a single chlorine, becoming Copper(I) Chloride, which gives off a yellow flame. Since the salt is giving off a yellow and blue flame mix, we see a green flame!