How to Make Potassium Nitrate Crystals


Potassium nitrate crystals form needle shapes when crystallized from water.
Potassium nitrate crystals form needle shapes when crystallized from water. (photo: Adam Rędzikowski)

Potassium nitrate crystals are among the safest and easiest crystals to grow. Potassium nitrate (KNO3) is also known as saltpeter. It’s used in processed meats, fertilizers, gunpowder, fireworks, and many science projects. Here’s how to make potassium nitrate crystals and a look at how to up your game to grow a big crystal.

Materials

It’s fine to do this project in your kitchen. All you need is:

  • Potassium nitrate
  • Water
  • Cooking pot or microwave-safe bowl

You can order potassium nitrate online or find it in stores, if you know where to look. I use Spectracide Stump Remover from Home Depot’s garden section. Just be sure to check the label of your product. It should list potassium nitrate or saltpeter as the only ingredient.

Grow Potassium Nitrate Crystals

To grow potassium nitrate crystals, dissolve it in water to make a saturated solution. Potassium nitrate is a salt, like table salt (sodium chloride). Like other ionic solids, its solubility greatly depends on temperature. So, when you dissolve as much potassium nitrate as you can at a high temperature, it’s forced to crystallize as the liquid cools. Here is the solubility of potassium nitrate, so you see what I mean:

133 g/L (0 °C)
242 g/L (20 °C)
2439 g/L (100 °C)

From room temperature to boiling, there is a ten-fold increase in how much you can dissolve in water! You can dissolve a lot of potassium nitrate in boiling water (2.5 kg or about 5 lbs per liter of water). The reason I point this out is because you’ll need lots of potassium nitrate if you use a large volume of water. So, do yourself a favor and keep the project manageable. A good starting point is:

  • 3 cups water
  • 2 cups potassium nitrate

or

  • 1-1/2 cups water
  • 1 cup potassium nitrate

Keep in mind, you could dissolve a lot more potassium nitrate, but if you do, you’ll end up with a slushy mass, along with some crystals, and use a lot more materials than necessary. Similarly, you can go light on the salt and freeze the liquid to knock the salt out of solution. Feel free to experiment with the proportions and use the effect of temperature on solubility to get results that please you.

  1. Boil the water and stir in potassium nitrate. If it doesn’t all dissolve, you can cook it on the stove or microwave it until the water boils again.
  2. Remove the solution from heat, but let it cool slowly for the best crystal formation. If you like, you can transfer the solution to a clear container to observe crystallization. Needle-like shards of crystals form as the liquid cools.
  3. Once you’re happy with the crystal formation, pour off the water and let the crystals dry. You can rinse the solution down the drain and can wash your dishes as you normally would.

Tips for Success

  • Controlled cooling is the key to good crystal formation. If you’re in a hurry, you can get a mass of slender crystals immediately upon cooling (within minutes) if you either increase the amount of potassium nitrate you add to the water or you cool the solution in a refrigerator or freezer.
  • For larger crystals, slow the cooling process as much as possible. One way to do this is to set up a hot water bath and place the container of potassium nitrate/water inside it. An easy set-up is a 9×13 pan filled with boiling water.
  • If you don’t observe any crystal growth, try refrigerating the liquid, If that doesn’t work, reheat the liquid to boiling and add more potassium nitrate. (It’s highly unlikely you’ll have this problem.)
  • Play with the crystal structure. At room temperature, potassium nitrate has the orthorhombic crystal structure. If you heat the crystals (as in an oven) to 129 °C (264 °F), the crystals transition to the trigonal system. Don’t worry about melting the crystals. The melting point of potassium nitrate is 334 °C (633 °F).
  • Make a crystal geode! After mixing the potassium nitrate and boiling water, pour the liquid into an empty egg shell or a plaster of Paris mold. Pro tip: Both egg shells and plaster of Paris are easily dyed with food coloring. While potassium nitrate crystals are clear, they appear colored if grown on a dyed surface.
See how to grow potassium nitrate crystals. Even though he tastes the crystals, it’s probably not a great idea.

Grow a Large Potassium Nitrate Crystal

The crystal needles are cool, but it’s easy to grow a larger crystal. First, grow the crystals using the recipe. Use tweezers to remove one or more of the nicest crystals from the batch. These will be seed crystals around which larger crystals will grow. Set them aside for now.

  1. Reheat the crystal solution (or re-make it, if you like).
  2. After you remove the liquid from heat, let it cool slightly. This is to prevent the seed crystal(s) from dissolving.
  3. Add the seed crystal to the liquid. You may still get some growth on the sides and bottom of the container, but the seed provides the perfect conditions for additional growth. If you like, you can remove competing crystals as they form.

Safety and Clean-Up

Potassium nitrate is a common food additive, plus it’s found in everyday products, such as toothpaste for sensitive teeth, plant fertilizer, and black powder. In foods, it’s most often seen in salami, corned beef, and other cured meats. So, as far as crystal-growing chemicals go, it’s among the safest to try! It’s fine to use your everyday cooking utensils for this project.

Clean-up is easy. All you need is a bit of warm water. Remove large deposits from cookware by soaking in hot water.

References and Further Reading

  • Binkerd, E. F; Kolari, O. E (1975). “The history and use of nitrate and nitrite in the curing of meat.” Food and Cosmetics Toxicology. 13 (6): 655–661. doi:10.1016/0015-6264(75)90157-1
  • Gustafson, A. F. (1949). Handbook of Fertilizers – Their Sources, Make-Up, Effects, And Use. Read Books Ltd. ISBN 9781473384521.
  • Kosanke, B. J.; Sturman, B.; et al. (2004). Pyrotechnic Chemistry. Journal of Pyrotechnics. ISBN 978-1-889526-15-7
  • Spencer, Dan (2013). Saltpeter: The Mother of Gunpowder. Oxford, UK: Oxford University Press. ISBN 9780199695751.

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