Which of the following correctly describes a compound?

A compound is a pure substance formed by the chemical combination of two or more different elements.

A compound may be splitted into simpler substances by chemical reactions, and has different properties to those of the elements that form it.

The composition of a compound is fixed: every piece of a compound has the same kind of atoms, bonded in the same way and proportion.

Some examples of compounds are H₂O, NaCl, H₂O₂, CH₃COOH. As you see, they have a chemical formula which states the kind and number of the atoms that form them.

They are different to mixtures, which are formed by two or more compounds, in a variable proportion, and can be separated by physical media. Some examples of mixtures are the solutions (e. g. NaCl dissolved in H₂O), and some solid mixtures (e. g. a mixture of marbles and sand).