There are lots of ways to grow crystals in the classroom. (See here for example.) The question is: why should we do it? It's fun, it's "sciency," it's mysterious...but how can we ensure that students learn some fundamental principles by doing it?
Students have difficulty understanding that matter is composed of tiny pieces--atoms, most of which are bonded together to form molecules. It is particularly difficult to visualize molecules that have shapes unlike the macroscopic things we can see. It's counter-intuitive that a piece of wood is made of strings of cellulose molecules, between which is absolutely nothing. Therefore, atoms and molecules are often introduced into the curriculum no sooner than 7th grade. Yet, to understand much of the biology that enters the curriculum in 3rd or 4th grade, it is essential to recognize the existence of atoms, molecules, and chemicals in general.
The goal of this set of investigations is to provide a foundation for the concept of matter being made from tiny parts.
Help students dissolve sugar in water, and observe the formation of sugar crystals over time. Allow them to think about what they observed...and realize that the sugar they put into the water did not "disappear." Rather, the visible sugar dissolved in the water, and was able to form into new visible sugar as the water evaporated.
Repeat the above using salt. The basic principle is the same: material that dissolves in water is still present--but in a form that blends with the water itself. The dissolved material can be recovered by evaporating the water.
Hmmmmm....if things like sugar and salt (that we can see) can dissolve in water and not be visible but still be present, how can we explain this? Challenge your students to think of possible explanations.footnote
Perhaps these materials are able to come apart into very tiny pieces that are simply too small for us to see--pieces that (for lack of a better term right now) can "swim" in the water. "Breaking into tiny pieces that can swim in water" is what we call dissolving in water.
Now the fun part: as students compare the appearance of their sugar crystals to the appearance of their salt crystals, they should see that the crystals are not the same shape. Challenge them to come up with ideas footnote about why this might be so.
Perhaps tiny pieces of salt are a different shape than tiny pieces of sugar, so they pack together differently when we take away the water.
Quite simply, because science is not being told what to learn. It's making observations and building understanding from the evidence. When we give students this challenge, it is essential to break it into two parts:
1. Students kick around ideas with each other, in groups no larger than three.
2. The teacher acts as facilitator of a classroom-wide discussion in which students describe their ideas and how the evidence leads them to favor those ideas. During this discussion, the teacher guides students to the scientific principle that the investigation is designed to address. A particularly fun way to do this is for the teacher to "think out loud" about how she or he might think about the evidence; this brings the teacher into the investigation as a collaborator with the student-investigators, working with them to understand what they have observed.
The Rationale Behind the Strategy
As noted in the footnote above, "science" is building understanding from the evidence. Here, we give students a number of observations -- evidence -- that enable them to discover for themselves that there is something going on at a microscopic level, undetectable to the naked eye. When things dissolve in water, they don't just disappear. If the material can be recovered, the material must still be present, but apparently in pieces too small to see. If different materials form crystals of different shapes, then the tiny pieces must somehow be different; they probably have different shapes. Students can work through this logic, particularly if their teachers can guide them in this direction. Once students recognize that some kinds of matter are composed of tiny particles, they will have little difficulty applying the same fundamental principle to other kinds of matter.
Note that we ask students to construct their own understanding from the evidence. This is essential to students' long-term learning, and to their development of skills in problem-solving and scientific reasoning. (If we simply tell them the answers, they may be able to answer test questions in a few days, but they are unlikely to retain the learning for a significant length of time. Nor will they have the opportunity to practice the thinking skills that they will eventually need in the world they will enter as adults.) However, we do not leave their knowledge-construction to chance. We designed the learning experience to reach a goal; we need to ensure that it gets there.
A good way to both reinforce students' understanding and to assess their understanding is to ask them to model what's going on in the dissolve-recrystalize process. A two-dimensional model is probably easiest, if smallish plastic or cardboard squares and hexagons are available.
1. Build a model of each starting material. [salt: many squares assembled into a cluster; sugar: many hexagons assembled into a cluster]
2. Illustrate "dissolving." [move individual squares or hexagons away from the cluster]
3. Illustrate salt water or sugar water. [the squares or hexagons are scattered around not touching; they are "swimming" in the water]
4. Illustrate crystallization. [the squares assemble one-by-one into a large cluster; the hexagons assemble one-by-one into a large cluster]
5. Explain, using the model, why salt crystals are a different shape than sugar crystals. [the pieces are different shapes, so the clusters are different shapes.]
Can we somehow watch the material dissolving? We know we can see the sugar or salt pieces become smaller in water, but can we see the tiny pieces moving into the water and away from the visible chunks?
Yes. Dissolve sugar or salt crystals in a small amount of water on a reflective surface, or in a clear glass on an overhead projector. This enables us to see light that passes through the fresh water and through the water near the crystals. The latter, with its locally-high concentration of dissolved material, is visible as "little wiggly lines" (called schlieren) around the crystals.
(Schlieren is visible whenever there is a difference in refractive index of substances that light goes through. Water's refractive index changes with dissolved material. Both water and air show refractive index changes with temperature--sometimes as wavering images when we look across a hot surface, or shadowy swirls in light that shines on a wall or floor after passing over a hot surface.)
Can we show that water with dissolved sugar or salt is different from water that we did not dissolve anything in?
Yes. On the same principle of schlieren, it's quite easy to put a few drops or pour a thin stream of salt- or sugar-containing water into a beaker or clear glass of tap water. The thicker and heavier salt- or sugar-water will be visible as "little wiggly lines" going down through the glass.
What about other chemicals?
Anything that we can dissolve in water can be used in the same way as sugar or salt--washing soda, alum, tartaric acid, Epsom salts, etc--there are quite a number of chemicals in the kitchen that can be used. With other chemicals, the "learning opportunity" will primarily be that crystals can be formed in many different shapes.
Sugar Crystals: it is most efficient to dissolve roughly two volumes of sugar in one volume of hot water, to create an almost-saturated solution. Then, stand up a pipecleaner in the solution, or dangle a string into it. Let it cool, and let it sit (without touching it!) for several days until the crystals are to your liking.
Salt Crystals: do as for sugar, but use one volume of salt and 4 volumes of water.
Heating vs cooling: you can dissolve more salt or sugar if you boil the water. Crystals will start to form more quickly if you use a "supersaturated" solution prepared at high temperature--they should begin to form as the solution cools. If you put the solution into the refrigerator, it will cool very nicely indeed. But, rapid crystallization tends to result in smaller more numerous crystals. Larger crystals may be easier to obtain with more patience--a solution less close to saturation, and allowing crystals to form as the water slowly evaporates. Use whatever method best fits your time frame and your students' ability to see the crystal shapes.