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Teacher Resources

Are formaldehyde and formalin the same substance?

Formaldehyde, in its basic form, is a gas. Most science teachers think of formaldehyde as a liquid. The liquid is actually a mixture of formaldehyde gas and water. The most common concentration used in school science departments (particularly biology) is a 37% solution. The solution consists of 37 grams of formaldehyde gas to 100 mL of water. To prevent polymerization of formaldehyde solution about 10-15% of methyl alcohol is added. It is the addition of the methyl alcohol that causes the substance to be called formalin as opposed to formaldehyde. It is correct to use the terms formalin and formaldehyde solution interchangeably. When diluting full-strength (37%) formaldehyde solution, assume it to be 100%. A fixative-strength (10%) solution is therefore a 3.7% solution of formaldehyde gas in water.

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