Just one tiny crystal turns this liquid into a solid
Did you ever sneak an extra spoonful of sugar into your Kool-Aid as a kid but got caught when Mom saw the undissolved sugar at the bottom of the glass? If you had only known how to make a supersaturated solution, Mom would have never been the wiser.
This popular demonstration performed by chemistry teachers demonstrates how to make a supersaturated solution. It also illustrates an exothermic reaction or the heat of crystallization.
- Sodium acetate trihydrate
- Hot plate
- Put 160 grams of sodium acetate in a flask and add 30 mL of water.
- Put the flask on a hot plate, heat it gently and stir until the crystals of sodium acetate dissolve. Use a small amount of water to rinse down the inside of the flask.
- Remove the flask from the heat and let it cool slowly without disturbing it.
- Add one or two crystals (that’s right, it only takes a single crystal) to the liquid in the flask. Don’t take your eyes off of the liquid as beautiful crystals begin to form inside the flask.
- Feel the flask… it’s warm!
How Does It Work?
If you attempt to dissolve sugar in water, you reach a point where you cannot dissolve any more sugar. This is called a saturated solution. However, if you heat this solution, more sugar will dissolve. When the solution is cooled, the sugar will remain in solution. This is called a supersaturated solution, which is very unstable and will crystallize easily. If you had really wanted to fool your mom when you added more sugar to the Kool-Aid, you should have heated the liquid to dissolve the extra sugar and then allowed it to cool. Your supersaturated Kool-Aid would have been “super sweet!”
The process of crystallization gives off heat. It’s said to be exothermic. That’s why the solution is used in hand warmers (the old-style liquid-type of hand warmers).
How do Hand Warmers Work?
Commercially available hand warmers use a supersaturated solution of sodium acetate.These products consist of a concentrated aqueous salt solution together with a flexible metallic activator strip (usually stainless steel) in a sealed, flexible container. Sodium acetate and calcium nitrate are examples of suitable salts. These salts are much more soluble in hot water than in cold water.
The flexible metal strip is bent back and forth a few times, whereupon a white cloud of crystals begins to precipitate. Within seconds, the entire pack is filled up with solid crystalline needles of sodium acetate without any solution left, and the temperature rises to 130°F for about 30 minutes. Because heat is released upon this precipitation, it is called an exothermic reaction (the opposite is called an endothermic reaction).
Supercooled liquids can be cooled below their normal freezing point without turning solid. Then, at the flick of button, the supercooled liquid is triggered to solidify (crystallize) and at the same time release large amounts of heat. Salt solutions that have been processed in such a way that their temperature can be lowered well below their solidification (or melting) temperature and still remain in liquid are defined as supercooled or metastable liquids.
The triggering device initiates the rapid solidification of the solution. In the case of salt solutions that release or absorb large amounts of energy during phase changes (common table salt sodium chloride does not do this), the solidification process is a rapid crystallization that releases a large amount of heat at the salt solution's normal melting temperature.
The activator is a thin metal piece with ridges and a specially roughened surface. The flexing causes metal-to-metal contact that releases one or more very tiny particles of metal from the roughened surface. This acts as a nesting site for one crystal deposited from the solution and (voila!) all of the crystals fall out instantly. These heat packs are reusable because, by reheating the pack in boiling water for a few minutes, the salt re-dissolves and the pack again contains a clear solution. Best of all, the activator strip can be reused dozens of times!