Growing Crystals, Week 2
Rather than giving a lecture to the whole group at the start of class, this week I’d like to
have the individual tutors talk with their groups of students about why each crystal
experiment turned out as it did. Below, I’ve provided a brief description of what each
experiment demonstrates, and why certain phenomena occur.
The major objective is to relate the experimental results to what we learned last week:
1) A crystal is a solid made of molecules that fit together in a regular, orderly
pattern.
2) Crystallization occurs when too much solid is dissolved in a liquid. The solid
molecules stick to each other, forming crystals.
Charcoal Briquettes
We dissolved different chemicals in water and poured the solution over charcoal, which
soaked it up like a sponge. As the water evaporated from the charcoal, the molecules
from the chemicals stuck to each other, forming a layer of crystals. However, there were
spaces left between the crystals, and more solution moved up through the spaces between
the crystals. When the water evaporated from this solution, a new layer of crystals was
formed on top of the old layer. This process happened over and over, producing many
layers of fluffy crystals.
Table Salt Crystals
We dissolved as much solid (table salt) as possible in a liquid (water). When the water
started evaporating, the molecules of salt began to stick together, forming tiny crystals.
As more and more water evaporated, new salt molecules stuck to the tiny crystals. Over
time, the crystals became large enough to see with your eyes. Because salt crystals grow
equally quickly in all three directions, they have a cube shape.
You may have many small crystals, or a few larger crystals. If a solution has dust, food
coloring, or other “junk” in it, you will tend to see many small crystals. This is because
molecules of the dissolved solid like to stick to the junk, forming lots of new crystals.
Epsom Salt Crystals
The Epsom salt experiment is conceptually just like the table salt experiment. In fact,
Epsom salt even looks a lot like table salt. However, the crystals do not look the same.
Instead of cube crystals, Epsom salts produce long, skinny, needle-like crystals. This is
because the crystals do not grow equally quickly in all three directions. Instead, they
grow faster in one direction than in the other two. This produces a needle-like shape.
A good point to reinforce: Epsom salt and table salt produce different-looking crystals
because they are made of different molecules. Epsom salt stick molecules to each other
(and to water) differently than table salt molecules, giving rise to a different shape.
Table Salt Crystal (on a String)
To start this experiment, we tied a small salt crystal onto the end of a piece of thread.
This crystal is called a seed crystal. When a seed crystal is placed in a salt solution, it
provides a place for new salt molecules to stick. In fact, salt molecules in the solution will
tend to stick to the big seed crystal rather than sticking together to form brand-new, tiny
crystals. Over time, the solution evaporates, and more and more salt molecules stick to
the surface of the seed crystal. This makes the seed crystal grow bigger and bigger.
You may see crystals in the bottom of the dish or along the length of the thread as well.
Crystals form in the bottom of the dish if there is dust or other “junk” down there. In a
similar way, crystals form on the string because salt molecules stick to the fibers.