How is a substance defined in scientific terms?

The definition of a substance in scientific terms refers specifically to matter that has a uniform and definite composition. This means that a substance is consistent throughout its entirety and retains the same properties and characteristics. For example, pure water is a substance because it has a specific chemical composition (H2O) and consistent properties, regardless of its state (solid, liquid, or gas).

In contrast, the other options do not accurately define what a substance is in the context of science. While man-made materials can be substances, not all substances are man-made; many occur naturally. A basic element in chemistry refers to a fundamental type of substance that cannot be broken down into simpler substances, but it does not encompass all substances, as not all substances are elements. Lastly, the outcome of a chemical reaction may involve substances but does not define what a substance is. Therefore, the correct definition is the one that highlights the uniform and definite composition of matter.