Have you ever seen that commercial by the Girl Scouts of America, where the little girl asks her father why the sky is blue and when he gives her a lame answer ("to match the color of your eyes") she tells him the scientifically correct one? Which by the way is not so lame.
Well, that's me. I love that commercial and I hope that it does inspire girls to be more involved in the sciences. I always try to give the scientifically correct answer to any question my little friends or family members ask. It's the scientist in me. I have quite a few little cousins around, so any opportunity I get I turn something into a science project.
Awhile ago when my little cousin was in to visit we made 'rock candy' on popsicle sticks. This is incredibly easy to turn into a science project discussing evaporation, saturation of a solution and crystal formation.
The big question that always gets asked is WHY or HOW do the crystals form? You can answer that question in the 'lame' way or you could tell them something along the lines of this.
First, there are a few things we need to define before we can answer why the crystals actually form. For instance, what are solutions or what the heck is a crystal anyway? What is evaporation? All of those things are important in understanding the making of 'rock candy.'
A solution by definition is a homogeneous mixture that remains in one phase (solid, liquid or gas). It is a mixture of a solute and a solvent. The solute will be a substance that will dissolve in another substance. The substance that does the dissolving is the solvent. So, in the case of rock candy the solution is a mixture of sugar and water, where the sugar is the solute and the water is the solvent, and the phase of the solution is liquid. Quite frankly you can discuss solvents and solutes in a number of ways, with all different phases. You could use oxygen in water. Here oxygen is a gas acting as a solute in water a liquid solvent. It however can get complicated when we talk about gases as solvents. Really only gases dissolve in gases and generally they are referred to as mixtures (which by the way is very different than a solution), not solutions.
The solubility (the property of a solute to dissolve in a solvent) of a solute depends on a ton of things. Solubility will be greatly affected by physical and chemical properties of a solute and solvent, as well as the temperature, pH and pressure of the solution. At some point the solvent can no longer take on anymore solute and the solution becomes saturated, meaning adding more solute does not increase the concentration (amount of solute in solvent) of the solution. Honestly, I think this is the most important part of being able to make crystallization to occur. You have to be sure that your sugar water solution is supersaturated. However, you don't want to heavily saturate it either, because that can cause spontaneous crystallization, and is a lot harder to control the process. And to allow crystals to form on just the stick or string. If you refer to the graph below you'll see that there isn't much separating soluble, supersaturated or heavily supersaturated. They are also variable depending on your environment. The lines represent general assumptions about solubility.
When making your sugar water solution you need to do a few things. Start out with an equal ratio of sugar and water (1 cup water to 1 cup sugar). You need to boil the sugar water mixture, raising the temperature of the solvent (water) increases the solubility of the solute (sugar). You can also aid solubility by stirring. I'm not going to go into why, that's too much chemistry than even I enjoy. Once the first cup of sugar has dissolved, start adding more sugar 1/2 cup at a time. Wait for it to dissolve. Add another 1/2. At some point the sugar is going to stop dissolving, meaning the solute is going to stay solid in the solvent. You'll start to see the sugar. At this point the solution is most likely supersaturated and you should stop adding the sugar. When I made my supersaturated solution it worked out to be a ratio of roughly 3 cup sugar to 1 cup water. So, a 3:1 ratio. This will or can vary for you.
Another note, when making this solution, be sure you are using pure sugar and clean water. Any other solid solutes in the water can lead to changes in your sugars solubility. Also, it will provide other sites for nucleation (starts) of the crystals. Meaning the crystals will form on anything solid they can get their tiny little crystal fingers on, even a tiny dust particle. So, if you have dirt in your water crystals will form throughout the water and just not on the stick or string. We will talk a little more about this in a bit.
Moving along to....crystals. What are they? Basically they are the arrangement of the atoms of a substance in an ordered pattern in all dimensions. Snowflakes are good examples of crystals. Remember how no two are the same? That is the same for crystals. Which can tell you a lot about the composition of a substance. That is why we use crystallography to study substance composition. If we can determine the 3D crystal shape we can figure out the composition of the atoms within the substance. In a long drawn out round about way, but nonetheless it's very useful. I could go on and on about crystals, but I will spare you all. I'm well aware that not everyone loves science as much as I do.
In the case of rock candy, you are making sugar crystals. The sugar atoms never reach equilibrium because the crystallization steps will occur too quickly, forming the crystals.
That leaves us with evaporation. The whole process that triggers crystal formation. We all know that evaporation is the change of liquid to vapor. Evaporation occurs when we add enough energy (heat from the sun) to the water molecules that they overcome their polar and cohesion properties to break away. When we remove the liquid (water) from our solution through evaporation, we are left with the sugar atoms. They are left to begin the crystallization process. Evaporation is also why you need a clear container, a sunny spot and good air flow. All of these things will increase the rate of evaporation. The clear open container allows the sun to penetrate supplying energy to the water molecules slowly and allowing it to be exposed to the air, allowing the water molecules to break away and escape to form vapor/gas.
The string or popsicle stick is absorbent, so it will draw in the solution. The string or stick is also a solid material and the crystals are looking for something to attach to or a nuclei site since the water is evaporating. You can also increase the chance of crystal formation on the stick or string by seeding it, covering it in sugar before placing it in the solution. They'll attach to this solid and instead of building a bunch of new sites they will grow on each other. Nature will always try to take the easiest possible way. It is more efficient for them to build upon each other than to start new nucleation. The answers are in all the laws of thermodynamics; that I am not going to into here. Unless of course you ask, then I most definitely will. This is also why you need a very clean container and water. So there are not other solid sources for nucleation. You also do not want to disrupt the solution, to avoid new nucleation sites.
I think that sums up the why and how. It turned out to be a lot lengthier than I intended. Oh well. Hopefully now you know.
So here is what you'll need:
sugar
water
glass/clear containers
food coloring
popsicle sticks
clothes pins
a cool/sunny/breezy area
Create your sugar solution as I stated above. Let cool slowly for about 15 mins. You still want it to be warm to touch because that slow cooling process aids in the formation of crystals. Funnel the sugar solution into your clear containers. Add drops of food coloring to your desired color consistency. Stir. Dip the stick in water and then roll in sugar to coat (seeding the stick). Attach a clothespin to the end of the popsicle stick and place it into the solution. Move the containers to a window seal or something of the like. And wait....days. Maybe even 2 weeks. Without touching it. You should start seeing crystal formation within 5 days if everything is right. If you do everything exactly as stated above it should turn out well. If you start to see a crust on the top of your container, break it away so that evaporation can continue to occur and that site does not allow more crystal formation, and taking away formation on your stick.
Here are some photos from our Rock Candy Experiment.
Preparing (seeding) the sticks.
Attaching the clothes pins and placing the sticks in the solution.
The end product!
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