The tin hedgehog is a chemistry experiment that produces a rounded spiky mass of sparkling tin metal crystals that resembles a hedgehog. It’s a great project for interesting people in science. In chemistry, it illustrates the metal reactivity series and is an example of a single replacement reaction. Here are two methods for growing a tin hedgehog and a look at the chemistry.
How to Grow a Tin Hedgehog
The two common methods of growing a tin hedgehog involve an aqueous solution of tin(II) chloride and either zinc or iron metal.
Tin Hedgehog From Zinc
Zinc is a good choice for a “hedgehog” because rounded pellets of the metal are readily available.
- 0.5 M tin(II) chloride solution (SnCl2)
- zinc pellet
- test tube, vial, or jar that is larger in diameter than the zinc
The concentration of the tin(II) chloride solution is not critical. If the solution is less dilute, the crystals grow more slowly. The rounded hedgehog shape forms around a pellet of zinc, but you can substitute any chunk of zinc metal. Since the reaction occurs at the surface of the metal, you may also use a galvanized (zinc coated) object in place of the zinc pellet.
- Pour tin chloride solution into a vial. Don’t fill it up all the way because you need room for the zinc.
- Add the zinc pellet. Set the vial somewhere stable, so it won’t get bumped or jarred.
- Watch the delicate tin crystals grow! You’ll see the beginning of a spiky hedgehog shape in the first 15 minutes, with good crystal formation within an hour.
Tin Hedgehog From Iron
Another way to grow tin crystals is using tin chloride solution and iron. Unless you use a round chunk of iron, you won’t get a “hedgehog”, but you can get the crystal growth, just the same.
- iron wire or nail
- 0.1 M tin chloride
- test tube
Again, the concentration of the tin chloride solution isn’t important. If you have 0.5 M solution from the zinc version of the project, that is fine. Crystal growth is slower with a less concentrated solution.
- Suspend the iron wire or nail in a test tube containing tin chloride.
- After about an hour, crystals start to form. You can examine these with a magnifying glass or by removing the wire and looking at the crystals under a microscope.
- Allow the iron to remain in the solution overnight for more/larger crystals.
How the Tin Hedgehog Works
The two key concepts in the tin hedgehog experiment are the metal reactivity series and a single replacement or displacement reaction.
Tin is lower down on the metal reactivity series than either iron or zinc. In other words, iron and zinc are more reactive than tin. So, iron or zinc readily replace the tin in its chloride compound, leaving tiny solid tin particles that precipitate as crystals. Another way of looking at it is that zinc or iron are reducing agents that give tin the electrons it needs to become stable and precipitate as a solid metal.
The equation for the reaction is as follows:
SnCl2(aq) + Zn(s) → Sn(s) + ZnCl2(aq)
SnCl2(aq) + Fe(s) → Sn(s) + FeCl2(aq)
The tin atoms stack in a crystal pattern that is a characteristic of the white tin allotrope. This is a fernlike or needle-like pattern. The other common allotrope or form of the element is gray tin, which is crumbly rather than crystalline.
Can You Keep the Tin Hedgehog?
The tin crystals you make in the tin hedgehog experiment are sparkling and beautiful, so you may want to keep or preserve the project. Sadly, the crystals are too delicate to remove from their container. Eventually, the crystals collapse under their own weight. Plus, the crystals lose their shine and the solution turns cloudy. Since you can’t keep the tin hedgehog, take pictures or video.
Safety and Cleanup
- Wear gloves and safety goggles.
- Choose an empty glass jar, not good dishes. Don’t use glassware for food afterwards.
- Don’t touch or drink any of the chemicals.
- After you complete the experiment, you can rinse the chemicals down the drain with plenty of water.
- Use a magnifying lens to compare tin crystals grown on the zinc and iron surfaces.
- Experiment with different zinc(II) chloride solution concentrations. How does this affect the rate of crystal growth? Does it affect the appearance or structure of the crystals?
- Try growing other metal crystals using this technique. Keep in mind, not all metals are good candidates for this experiment. Choose a subject that has a water-soluble metal salt, does not oxidize too quickly in air, and is further down the metal reactivity series than zinc or iron (or whatever common metal you have as a starting material). Consult the solubility chart for ideas of soluble salts. Hint: Copper and silver are good options!
- Brown, Theodore; et al. (2017) Chemistry: The Central Science (14th ed.). Pearson. ISBN 9780134414232.
- Holleman, Arnold F.; Wiberg, Egon; Wiberg, Nils (1985). “Tin.” Lehrbuch der Anorganischen Chemie (in German). Walter de Gruyter. ISBN 3-11-007511-3.
- Schwartz, Mel (2002). “Tin and Alloys.” Encyclopedia of Materials, Parts and Finishes (2nd ed.). CRC Press. ISBN 1-56676-661-3.
- Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.