What is a compound?

A compound is defined as a substance formed when two or more elements chemically bond together. This means that the individual atoms of the elements interact through chemical reactions, resulting in the creation of a new substance with properties that are distinct from those of the original elements.

In contrast, a mixture, as referenced in one of the other choices, consists of two or more elements or compounds that are physically combined but not chemically bonded. These components can often be separated by physical methods, such as filtration or evaporation, without breaking any chemical bonds, which distinguishes them from compounds.

The other options further clarify the nature of compounds. A substance made from a single type of atom would be classified as an element rather than a compound. Lastly, a solution that cannot be separated by physical means typically refers to homogeneous mixtures where the solute is completely dissolved, but again, this is different from a compound since the components in a solution can still be separated under certain conditions, highlighting the key distinction between mixtures and compounds.