Super Saturated Solutions (Mark Yaney)

Title: The Pillar of Salt: Supersaturated Solutions

Principles Investigated: How heat and/or pressure can cause stability in an otherwise unstable solution

California Content Standards

Grades 9-12 Chemistry: Solutions

Solutions are homogeneous mixtures of two or more substances. As a basis for understanding this concept:

a. Students know the definitions of solute and solvent.

b. Students know how to describe the dissolving process at the molecular level by using the concept of random

molecular motion.

c. Students know temperature, pressure, and surface area affect the dissolving process.

Materials:

• Sodium acetate trihydrate
• 2 Flasks
• small measuring flask
• Hot plate
• Water
• Clear 2 Liter Soda bottle

Procedure

In this demonstration, 30ml of water is added to 160g of sodium acetate trihydrate, and the mixture is slowly heated. Once the solute is completely dissolved, the solution is removed from heat and allowed to cool. One cooled, a tiny amount (barely a pinch) of the solute is added to the now unstable solution. Upon making contact with this super saturated solution, the solute precipitates out of solution, undergoes a phase change, and rapidly crystallizes, and releases heat.

As an additional demonstration, a small amount of the sodium acetate trihydrate is placed on a plate, and the solution is poured over it. If done slowly and carefully, this will build a tower of crystalline sodium.

Student prior knowledge

Students must know that solutes dissolve in solvents until saturation is achieved.

Explanation:

A supersaturated solution is one that contains a greater concentration of solute than is expected from the solubility.

This demonstration shows how to make a supersaturated solution. It also illustrates an exothermic reaction or the heat of crystallization. If you attempt to dissolve a solute in a solvent at room temperature, a point is reached where it is no longer possible dissolve any more solute. This is called a saturated solution. However, if you heat this solution, more solute will dissolve. When the solution is cooled, the solute will remain in solution. This is called a supersaturated solution, which is very unstable and will crystallize easily if jostled, or if even a small amount of solute is added.

The triggering device, in this case, the adding of more crystalline sodium acetate, initiates the rapid solidification of the solution. The sodium precipitates crystals. 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.

Questions & Answers:

Student predictions: will the powder we are adding dissolve like the rest?

What do you think will happen?

Where did that heat come from?

Was there a chemical reaction here or phase change? Why?

Applications to Everyday Life

In order for a beverage to be fizzy when opened, the solvent must be supersaturated with CO2. For this to be sustainable over the shelf life of the product, the solution must be stored under pressure greater than atmospheric pressure.

The supersaturation of sugar in water allows for rock candy to form.

Honey is an everyday example of a supersaturated solution. It is a supersaturated solution of glucose and fructose (common sugars). If it is seeded with crystals of glucose or fructose, as often inadvertently happens if some honey around the mouth of the jar dries out to form crystals, the excess solid crystallizes out and forms candied honey. It can be clarified again by heating in hot water. At the high temperature, the amounts of glucose and fructose present can all dissolve without supersaturating the hot solution. Once all the solids have dissolved the honey can be slowly cooled. Provided care is taken to remove all traces of solids (seed crystals) from around the mouth of the jar, the honey will remain clear, that is supersaturated.

Scuba divers' tissues become saturated with breathing gases during a dive. Supersaturation is a theoretical term describing a state in which the tension of a dissolved gas is greater than its inspired partial pressure when the diver ascends, in contrast to Henry's law.[1] If the diver ascends too fast, these gases form bubbles, resulting in decompression sickness.

Photographs: Include a photograph of you or students performing the experiment/demonstration, and a close-up, easy to interpret photograph of the activity --these can be included later.

Videos: http://www.stevespanglerscience.com/experiment/instant-hot-ice

References:

Conquering Chemistry: http://www.cci.net.au/conqchem/PCmod3text06.htm

Wikipedia: http://en.wikipedia.org/wiki/Supersaturation