What is it called when the composition of a substance is altered and it is no longer the same substance?

When the composition of a substance is altered and it is no longer the same substance, this process is referred to as a chemical change. A chemical change involves a transformation that alters the internal structure of the molecules, resulting in the formation of one or more new substances with different physical and chemical properties.

For instance, when iron rusts, it reacts with oxygen and moisture to form iron oxide, which is a completely different substance than elemental iron. This indicates a fundamental change in the composition of the material involved.

In contrast, options related to physical properties and changes describe scenarios where the substance's form or appearance may change, but its chemical identity remains the same. Physical properties and changes include alterations like phase changes (e.g., ice melting to water) that do not result in new substances being formed. Thus, recognizing the distinction between physical and chemical changes is essential in understanding how substances interact and transform in various processes.