Science Buddies: "Ask an Expert"

Would adding food coloring change the purity of a crystal?

The short answer is yes.

There is a definite relationship between crystal purity and it's growth. As a substance crystallizes, the molecules will align and bond in a such a way as to result in a certain crytalline form. If the substance is pure and crystallizes under the same conditions, the same crystalline form will result. However, impurities in the substance interfere with the crystal growth. As a crystal forms, molecules of other substances, such as the food coloring, can co-deposit with the molecules of the desired "pure" substance. It would be like building a structure, like a house or tower, using 6ft boards and every now and then, randomly, used an 8ft board. Things wouldn't line up exactly as expected and your structure would look kinda strange.

For example, if you are growing salt crystals and add food coloring, you should get very colorful crystals that contain both salt and food coloring (whatever that stuff is made of) molecules, but they will be of different size and shape than if you just made pure salt crystals. However, it's important to note that there is no such thing as an absolutely pure substance and so no absolutely perfect crystals.

For more on this, you should read up on Atom/Molecule size and Bond Length.

I'm not a chemistry expert, so if another expert with more knowledge of this subject would like to weigh in on anything I missed, that would be great.

Hi Daysia,

Theborg has given you a very good answer. The shape and size of crystals depend on the geometric shape of the elements use to make the crystals. Food coloring molecules are usually organic molecules with very different structure compared to the sugar, sodium chloride, borax, or other molecules being used to form the crystals. The food coloring is really an impurity in the crystal; although it can be incorporated into the crystals are they are formed. The highest purity crystals will be formed from molecules that don’t contain food coloring.

Here are two science buddies projects on growing crystals. You can find good background information on crystals in both projects.

https://www.sciencebuddies.org/science- ... p082.shtml

https://www.sciencebuddies.org/science- ... p082.shtml

What kind of crystals are you growing? Have you thought about comparing the shape and size of crystals grown with and without food coloring?

Donna Hardy