What best describes a substance that is made up of a single type of particle?

A substance made up of a single type of particle is best defined as a pure substance. This means that the material consists of only one kind of constituent, whether that be atoms, molecules, or ions. Pure substances exhibit consistent and uniform properties throughout, as they are not mixed with any other types of particles.

In contrast, mixtures contain two or more different particles that may be combined physically but not chemically bonded. Solutions are a specific type of mixture where one substance (the solute) is dissolved in another (the solvent), creating a homogeneous mixture at the molecular level. Colloids also represent a mixture but with particles that are larger than those in solutions, leading to a cloudy appearance and unique properties that allow them to scatter light.

Therefore, the definition of a pure substance directly aligns with the concept of being composed of single types of particles, distinguishing it clearly from other forms like mixtures, solutions, or colloids.