When two or more elements unite chemically, they form what?

When two or more elements unite chemically, they form a compound. A compound is defined as a substance that results from the chemical combination of two or more different elements in a fixed ratio. The chemical bonds that hold them together give compounds distinct properties that are different from those of the individual elements.

For instance, when hydrogen and oxygen combine chemically, they form water (H2O), which has totally different characteristics from the gases hydrogen and oxygen that make it up. This process requires a chemical reaction, resulting in a stable, new substance.

In contrast, a solution is a homogeneous mixture of two or more substances but does not involve a chemical reaction; similarly, a mixture comprises two or more substances that retain their individual properties and can be separated physically. An element, on the other hand, refers to a pure substance made up entirely of one type of atom and cannot be simplified into a simpler form through chemical means. Thus, the formation of a compound through the chemical union of elements signifies a new entity with unique characteristics, which is why that option is the correct answer.