Home Activities

Make Stalagmites and Stalactites
· Epsom Salts (magnesium sulfate)
· 2 small jars (baby food jars work well)
· cotton string
· scissors
· 2 washers
· spoon
· ruler
· paper

1. Fill each jar with Epsom Salts and add water to the height of the Epsom Salts.
2. Stir the contents of the jars.
3. Cut a 24 inch piece of string.
4. Tie a washer to both ends of the string.
5. Place one washer in each jar.
6. Position the jars so that the string hangs between them with the lowest part of the string being about 1 inch above the paper.
7. Allow the jars to stand undisturbed and *out of any draft for one week*.

1. Describe what happened.
2. Why was there some undissolved crystals under the washers?
3. Describe how the stalagmites and stalactites formed in your model and relate this to the formation of real stalagmites and stalactites in caves.

Make Edible Sugar Crystals
You can eat these. But don' try to eat the others. Bad news!
· Sugar
· Quart jar(very clean)
· Heat source

1.) Get a VERY CLEAN jar (this is important, or you will end up with a jar of sugar sludge).
2.) Bring a quart or so of water to a boil and dissolve as much sugar as you can. Pour this into the clean jar. Make absolutely sure you don't have any undisolved crystals of sugar in the jar!
3.) Tie a string to a pencil and hang the string into the jar with the pencil resting on the rim. You might want to put a piece of wax paper or something over the top of the jar to prevent evaporation.

What Will Happen?
As the sugar solution cools, it become super-saturated. The excess sugar will crystalize out on the string. Note that if the jar is scratched on the inside, or there are stray sugar crystals, they will form the nuclei forcrystal formation and your crystals won't end up on the string where you want them.

How do crystals form and how do they grow?

Crystals start growing by a process called "nucleation". Nucleation can either start with the molecules themselves, or with the help of some solid matter already in the solution.

Just Molecules:
When molecules of the "solute" (the stuff of which you want to grow crystals) are in solution, most of the time they see only solvent molecules around them. However, occasionally they see other solute molecules. If the compound is a solid when it is pure, there will be some attractive force between these solute molecules. Most of the time when these solute molecules meet they will stay together for a little while, but then other forces eventually pull them apart. Sometimes though, the two molecules stay together long enough to meet up with a third, and then a fourth (and fifth, etc.) solute molecule.

Most of the time when there are just a few molecules joined together, they break apart. However, once there becomes a certain number of solute molecules, a so-called "critical size" where the combined attractive forces between the solute molecules become stronger than the other forces in the solution which tend to disrupt the formation of these "aggregates". This when this "protocrystal" (a sort of pre-crystal) becomes a nucleation site. As this protocrystal floats around in solution, it encounters other solute molecules. These solute molecules feel the attractive force of the protocrystal and join in. That's how the crystal begins to grow.

It continues growing until eventually, it can no longer remain "dissolved" in the solution and it falls out (as chemists like to say) of solution. Now other solute molecules begin growing on the surface of the crystal and it keeps on getting bigger until there is an equilibrium reached between the solute molecules in the crystal and those still dissolved in the solvent.

Something to help the crystals grow:
Pretty much the same thing happens as in unassisted nucleation, except that a solid surface (like a stone, or brick) acts as a place for solute molecules to meet. A solute molecule encounters the surface of a stone, it adsorbs to this surface, and stays on it for a certain time before other randomizing forces of the solution knock it off. Solute molecules will tend to adsorb and aggregate on the surface. This is where the protocrystal forms, and the same process as described above happens.

You can probably see why, from what I wrote above, crystals grow fastest in a solution in which the concentration is near saturation. If there are more solute molecules in a given volume, then there is more of a chance they will meet one another. You also don't want to heat up the solution because that acts as the major randomizing force in solution which causes the aggregates of molecules to break up.

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