What does a chemical substance refer to?

A chemical substance is defined as a form of matter that has a constant chemical composition and characteristic properties. In the context of this question, the correct choice highlights that chemical substances cannot be separated into simpler substances by physical means without breaking the chemical bonds that hold their atoms together.

This definition encompasses pure substances such as elements and compounds, which have fixed ratios of their constituent elements and maintain consistent properties throughout. For example, water (H2O) is a chemical substance because it consists of hydrogen and oxygen in a specific ratio, and separating it into hydrogen and oxygen gas involves breaking chemical bonds.

The other options, while describing different types of matter, do not align with the definition of a chemical substance. For instance, physical means of separation pertain to mixtures, which can be separated without altering their chemical composition, making them distinct from pure substances. Mixtures can be either homogeneous or heterogeneous, consisting of two or more substances that retain their individual properties. The choice dealing with gases at standard temperature describes a physical state rather than the nature of a chemical substance itself. Therefore, the most accurate characterization is that a chemical substance cannot be separated without breaking the chemical bonds.