What is Formaldehyde? - Definition, Uses & Structures

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  • 0:02 Definition and…
  • 2:08 Preservation
  • 3:20 Killing Bacteria
  • 3:45 Industrial Uses
  • 4:24 Drug Testing
  • 4:52 Lesson Summary
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Lesson Transcript
Instructor: Nissa Garcia

Nissa has a masters degree in chemistry and has taught high school science and college level chemistry.

Do you remember that liquid in a jar that you used in biology class to preserve the frog you dissected? That liquid contains a substance known as formaldehyde. In this lesson, we will learn more about formaldehyde, its structure and its other uses.

Definition and Structure of Formaldehyde

When I think about formaldehyde, I think about my biology class back in high school when I first encountered it. The teacher had this jar with a pungent smelling liquid and told us that we needed to immerse the biological specimens we are examining in this liquid when class was done so that they can be preserved. This liquid is called formalin, a mixture that contains formaldehyde dissolved in a solution of water.

What is formaldehyde? Formaldehyde, also known as methanal, is a colorless and flammable gas that has a pungent smell and is soluble in water. I always thought that formaldehyde was a liquid, but its basic form is actually a gas. The chemical formula of formaldehyde is CH2O: it has one carbon (C) atom, two hydrogen (H) atoms and one oxygen (O) atom. Its chemical formula can also be written as HCHO, and its chemical structure is shown here.

Chemical Structure of Formaldehyde
Chemical Structure

The chemical structure of formaldehyde that is shown on screen can be written two ways. On the left (1), it shows all the carbon, hydrogen and oxygen atoms present in the structure. Another way to write it is shown on the right (2). Here, instead of writing in the carbon (C) atom, it is represented by an edge, or bend, in the line. When looking at its chemical structure, you'll see that formaldehyde has a carbon atom that has single bonds with two hydrogen atoms on both sides, and it has a double bond with oxygen.

Formaldehyde is an organic compound because it has a carbon atom present in its chemical formula. It is also classified as an aldehyde, which is an organic compound (shown here as RC=OH) that contains a formyl group (boxed in red) and an R group, which is a group that contains carbon and/or hydrogen.

A formyl group is the group in the chemical structure that contains (-CHO), also shown on screen. Because there are only hydrogen atoms bonded to the carbon atom, we can also remember formaldehyde as the simplest aldehyde. We can see that the R group in formaldehyde contains one hydrogen atom.

Formaldehyde Is the Simplest Aldehyde
Aldehyde and formaldehyde

We mostly know that formaldehyde is used for preserving biological specimens and also in embalming, but it has other uses as well. In this section, we will learn all about the different uses of formaldehyde. We encounter it more often than we think!


When I think of formaldehyde, I also think about embalmed bodies and coffins. Embalming is a funeral custom of treating human remains to delay their decomposition. The embalming fluid that is used contains formaldehyde. The primary purpose of formaldehyde in the embalming fluid is for the disinfection and preservation of the remains.

We often confuse formaldehyde and formalin, but there is most definitely a distinct difference. Formaldehyde, as we mentioned earlier, is a gas. Formalin, on the other hand, is a liquid solution that contains formaldehyde. Formalin is a liquid we have used in biology class to preserve our biological specimens. Because of the presence of formaldehyde in formalin, it delays decay, so this helps us to study our biological specimens for a longer period of time. Formalin is also used in preserving tissue samples, so it is used in histology, which is the study of microscopic tissues, and similar fields as well.

Killing Bacteria

Did you know that some personal care and hygiene products like soap, shampoo and toothpaste contain formaldehyde? It is actually present in minute amounts, and the reason for this is because formaldehyde inhibits the growth of bacteria and fungi, and also doubles as a preservative.

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