An essential feature is a feature that determines a material's identity. The essential features of a material are what make it the material it is and not some other material.
Materials have many features, but not all features count equally. Some are essential; change them and you have a different material. Others are accidental; they vary without changing what the material is. Composition is a typical essential feature. Water without its composition is no longer water. The shape of a particular sample of water (a drop, a puddle, a glass) is accidental. The water is the same.
The distinction is older than chemistry; it goes back to early philosophy of substance. What chemistry contributes is a way of testing it: change a feature, observe whether the material is now a different material, and you have evidence about whether the feature was essential or accidental. The essential features are the basis for classifying materials into kinds, the work the field begins with.
Question
How can you tell if a feature of a material is essential?
Reveal Merrill's answer →Hide answer
Change it and see what you get. If you change the feature and you have a different material, the feature was essential. If you change it and you have the same material in a different state, the feature was accidental. The test is conducted by observation.