Chemical (noun, “KEM-ih-kul”)
This is any substance formed by two or more atoms. Atoms are the smallest units of any element. For the substance to be a chemical, and not just a molecule, those atoms must be bound together in a fixed structure and ratio. That fixed structure and ratio gives the chemical a specific set of properties.
The universe is full of chemicals. They’re everywhere you look. You can find chemicals in the soil under your feet, in the food you eat and in the air you breathe. You are made of chemicals too.
Every chemical can be written as a chemical formula. That is a description of the ratio of the atoms that make it up. Water, for example, is a chemical. It has a chemical formula of H2O. That means it has two hydrogen atoms and one oxygen atom.
A particular combination of atoms will always react with other chemicals in the same way. Combine water from anywhere in the universe with pure sodium and you get an explosion. Using water from different sources won’t change the reaction. H2O will always be water. That’s true whether you make it in a lab or get it from a lake.
Sometimes people misuse the word chemical to mean a dangerous substance, often one created by humans. They may warn about chemicals in your food, house, clothes or environment. And it’s true that some chemicals can have negative effects. Some are made in labs or factories. But that isn’t true for all chemicals.
Chemical (adjective, “KEM-ih-kul”)
This word can be used to describe something related to chemicals or chemistry. For instance, a chemical reaction is when two compounds react and re-arrange their atoms.
In a sentence
Ancient monks used a beautiful blue chemical in paintings in medieval manuscripts — and scientists finally figured out where it comes from.