* Element: A pure substance that cannot be broken down into simpler substances by chemical means. Examples: hydrogen (H), carbon (C), nitrogen (N).

* Compound: A substance formed when two or more different elements are chemically bonded together. Examples: water (H₂O), carbon dioxide (CO₂), sodium chloride (NaCl).

* Mixture: A combination of two or more substances that are not chemically bonded. The substances in a mixture can be separated by physical means. Examples: salt and pepper, sand and water.

Hydrogen cyanide is formed by the chemical bonding of hydrogen, carbon, and nitrogen atoms. Therefore, it is a compound.