Adventures in home STEM experiments: Crystal fever
My Twitter feed reveals an interesting obsession of mine.
Even though I have a plenty of material waiting on my desk to write about, I’m still finding time to wrap it up into a coherent newsletter story. But at the same time, I continuously document on my Twitter feed the fun stuff I’m playing with. If you crave for STEM ideas and content, it might be fun to follow me on Twitter. Hence, I decided to write a short review of things that have appeared there.
And while going through my tweets I discovered that I am obsessed with … crystals. So, let’s start with that now and then in the next episode I’ll review the other STEM activities mentioned in my tweets.
There is something magical about crystals, as if they are triggering some deep primordial feeling that such objects are unnatural. Our intuition tells us that perfectly smooth surfaces enclosing perfect geometrical shapes can exist only if made by human hands. And yet, we encounter beautiful crystals all around us in nature.
It takes very long time for big crystals embedded in rocks to grow. Given enough time and perfect conditions for growth, crystals can reach enormous proportions. For example, it took 1 million years for gypsum crystals in the Cave of Crystals in Mexico to grow 12 meters in length and 1 meter wide. On the other hand, there are situations were growth is surprisingly fast, such as in cooling volcanic magma when a burst of extremely fast crystallization can happen and produce a meter sized crystal formation within a day.
Even better feeling is when you create conditions to grow your own crystals and see with your own eyes how perfect geometrical shapes grow seemingly out of nothing. Kids are fascinated by this, which makes growing crystals an opportunity for learning that should not be missed. A word of caution, though: once you start growing crystals, it can become an obsession – there is always a new way to change something and make the crystals bigger, prettier, more perfect, more impressive, more, more…. However, if your kids react to this, keep on! Explore together!
The goal is to make big monocrystals – single geometrically perfect shapes, not fused together into a big messy conglomerate. Or to explore some unusual properties of crystals, like color change or extremely fast crystallization. Instructions on the Internet will typically guide you to work with sugar or salt, but they are not a good start for this task (even worse, you might end up disappointed). If you have never done it before, it is better to buy a crystal growing toy set, which has a bag of monoammonium phosphate and instructions how to grow big clusters of crystals in the shape of elongated prisms or needles.
The best start for more serious monocrystal growing activity, and often used in school chemistry class, is copper(II) sulfate (CuSO₄; also known as copper sulphate or blue vitriol). It easily forms beautiful blue crystals. It has been traditionally used as a fungicide and herbicide. If you can’t find it in your local garden store, you can easily buy it online.
Growing crystals requires time, patience, and a certain level of skill that you will gain over time. The simplest process of making monocrystals goes like this:
- SAFETY: Use gloves and goggles – let the kids know that copper sulphate is toxic if swallowed. If it gets on the skin, just wash it off.
- Warm about 50mL of water (not hot, just warm).
- Dissolve as much of your compound as possible (stir the mixture for a long time until it appears that it does not want to dissolve any more)
- Filter the mixture through a coffee filter into a container (jar) that will be used for crystallization.
- Wait for the first crystals to form. As the mixture cools, and then slowly evaporates, it loses its ability to keep all the dissolved material. The solution becomes “saturated” and the dissolved material is pushed out of the mixture back into a sold form. If the process is slow, the solid will take the form of crystals.
- Crystal formation depends critically on how much stuff you managed to dissolve. It can take a couple of hours to overnight to see decent small crystals.
- Now comes the key moment for growing monocrystals – you must prevent crystals from fusing together. Select the biggest perfect crystal among the tiny ones. This will be the “seed” for your big monocrystal. Filter the mixture again to remove other tiny crystals.
- Put the seed crystal back into the filtered liquid. Repeat this procedure whenever you notice that tiny crystals are appearing. Only one crystal should grow, only one should rule them all!
- After a while, tiny crystals will almost stop growing, while your crystal will get bigger and bigger as the liquid slowly evaporates.
- Note that the shape will differ if you let it grow on the bottom of container or if you suspend it on a string or a fishing line.
Make sure that you keep this at some place where temperature is not changing. Even a small increase in the room temperature can increase the temperature of liquid and the crystal will start dissolving. It can take weeks to make a big monocrystal of copper sulphate. But you and your kid can watch every day how it gets bigger. If you want more detailed instructions, see HERE under “growing”.
I immediately wanted to produce a timelapse video that would show how the crystal grows. I bought a decent second-hand camera with a time-lapse controller and started taking images. It took one week of images every hour for a satisfactory time-laps video of copper sulphate crystal growth.
We played a lot with copper sulphate, and have a collection of crystals, but more versatile has been our experience with potassium alum KAl(SO₄)₂·12H₂O. Alum can be easily purchased online.
Alum forms beautiful transparent octahedral crystals, but only if growing while suspended on a string. If you let it grow from the bottom of container, the shape will not be octahedral. You can understand the reason if you consider how to cut octahedron parallel to one of its surfaces. We built an octahedron using small magnetic spheres and then split it into two parts along a plane parallel to its side. This reveals the shape of such crystals.
Magnetic balls help us visualize the key principle behind the physics of formation of flat surfaces in crystals. If you build a structured flat surface with magnetic balls, it becomes clear that an extra ball can easily slide all over the surface until it gets locked at kinks where other surface balls attract it. This simple approach in explaining the crystal growth is known as the Terrace Ledge Kink model. Atoms and molecules interact mutually with electric forces in a complicated way, but the key principle stays the same. Depending on the shape of molecules and their interaction, the growth can proceed only into well-defined mathematical shapes.
An interesting, unexpected thing happened when we added printer ink from old inkjet cartridges to alum. Initially, the crystals started to grow into ordinary alum shapes. But then they reshaped themselves into cubes! It turns out that adding inkjet printer dyes into the alum solution leads to the beveling of crystal corners.
Another fun experiment was possible when I got potassium ferricyanide (Prussian red) from a chemist. We mixed it with alum and got yellow crystals from this solution. They resembled alum crystals, but the shape was not perfect. The crystals kept in refrigerator (in dark and cold environment) maintained their bright yellow color, but those exposed to light and room temperature turned first green and then blue. The reason is transformation of red pigment into Prussian Blue pigment. This blue pigment is used in paint and ink cartridges. The green color appears as a mixture of initial yellow and the rise of blue pigment over time. From the time-lapse photos I created a video that shows this color change.
Crystallization can be pushed into extreme speeds in some cases, where crystallization can happen instantaneously. Sodium acetate (NaC₂H₃O₂) can do this in a simple experiment. Sodium acetate is a salt of acetic acid. Since vinegar is diluted acetic acid, you can get sodium acetate by mixing it with baking soda (NaHCO₃). However, this will not produce sodium acetate clean enough for this experiment to work. I had to purchase a clean sodium acetate and then the recipe is simple: melt 88g of sodium acetate in 50mL of water (heat it up in a water bath), let it cool, and then pour it onto specks of sodium acetate crystals (they serve as crystallization seeds).
The poured liquid immediately turns into sold as it touches the crystal. This happens because the mixture is supersaturated – a situation where a significant amount of dissolved material is ready to be transformed into solid. It just needs a trigger, a seed to grow from. You can recycle the solid by melting it (heat it up in a water bath).
This experiment generates heat, so we put a thermometer to observe how the growing pile of sodium acetate trihydrate crystals (NaC₂H₃O₂x3H₂O) is warming up. The temperature reached almost 40ºC. The experiment is so weird that we have repeated it many times for fun.
Another example of fast crystallization is supersaturated solution of sodium sulfate (Na₂SO₄). This was a bit difficult to make because I didn’t have an exact recipe how much to dissolve. The key was to stir the solution for a long time to force enough sodium sulfate to dissolve. I used water bath again, to avoid overheating on a direct fire. Then you filter the solution to remove any remaining solids and simply let the solution cool down to the room temperature (cover it to avoid dust falling in and prevent evaporation). Once it cooled down, a vibration can trigger crystallization, so make sure you do not shake this.
To induce crystallization, I used a wooden skewer (or a long toothpick) with dried sodium sulfate on its top (after dipping it in warm solution). The process is so fast that you can see with your own eyes how long crystal needles grow. The container is filled with crystals in a matter of seconds.
Another example of crystal beauty is bismuth. This metal is the chemical element of the highest atomic mass that is (essentially) stable. Interestingly, even though it has just one proton more than lead, which is toxic, bismuth has been used in the stomach relief drug Pepto-Bismol for 120 years. It is not entirely clear how bismuth manages to help in digestion problems, but the chemical structure of Pepto-Bismol has been reveled only recently.
I purchased a chunk of bismuth on the Internet and melted it on the kitchen stove. Bismuth melts at 272ºC, and when it starts to cool it forms crystals shaped as rhombohedron. When these hot crystals are exposed to air, a nanolayer of bismuth oxide forms on their surface. Light falling on this layer undergoes interference, where variations in layer thickness enhance different colors. The result is metal crystals colored in rainbows.
There are many more amazing phenomena still investigated in the science of crystals. For example, the latest news is that ordinary monoammonium phosphate, mentioned in the beginning as a children’s crystal growing toy, has also time crystal properties – a weird quantum effect. Or a mundane phenomenon of salt creeping out of the container and crystalizing all over it. Put salt solution to crystalize and you will see how it creeps up the container walls and then out of it. This process is still not fully understood.
We have right now several jars filled with different solutions with growing crystals. A couple of them are glowing in dark under UV light (black light). You can make it by mixing ink from fluorescent highlighter (yellow is the best) with alum. We have also made copper acetate from copper and vinegar and now we are waiting for the first crystals to appear.
If you end up pursuing this activity more seriously, here are websites where you can find a plenty of ideas: LINK, LINK.