Final answer:
If a substance cannot be separated by physical or chemical means, it is an element since elements consist of only one type of atom and cannot be chemically decomposed.
Step-by-step explanation:
If a substance cannot be separated by physical or chemical means, it is termed an **element**. Elements are fundamental substances that consist of only one type of atom. Each element is uniquely defined by the number of protons in the nucleus of its atoms, known as the atomic number. Examples of elements include oxygen (O), gold (Au), and iron (Fe). Physical means of separation typically involve physical changes such as phase transitions (solid to liquid to gas) or state changes.
If a substance cannot be separated by these processes, it suggests that it is not a mixture of different components. Chemical means of separation involve breaking or forming chemical bonds, and if a substance is not separable by such processes, it is indicative of its elemental nature. This contrasts with compounds, mixtures, and solutions. Compounds are substances composed of two or more elements chemically bonded in fixed ratios.
Mixtures are combinations of substances that retain their individual properties, and solutions are homogeneous mixtures where one substance (solute) is uniformly distributed in another (solvent). The inability to separate a substance by physical or chemical means is a key characteristic distinguishing it as an element.