Final answer:
The composition of a substance remains the same when it transitions between states such as solid, liquid, and gas. This is referred to as a physical change, which does not affect the chemical composition of the substance.
Step-by-step explanation:
The composition of a substance will always be the same even though the substance can transition from one state, such as solid, liquid, or gas, to another state. This concept is crucial in understanding that a physical change, such as the transition between different states of matter, does not alter the substance's chemical composition. For instance, water remains H2O whether it's ice, liquid water, or steam.
Matter consists mostly of mixtures of pure substances, which may be either homogeneous (having uniform composition throughout) or heterogeneous (where different regions have different compositions). A pure substance can be an element—composed of a single type of atom and unable to be chemically broken down further—or a compound, consisting of two or more types of atoms.
It's also key to recognize that the properties of substances can be classified as physical or chemical. Physical properties, such as melting point and color, can be observed without changing a substance's composition, while chemical properties are all about how a substance reacts under various conditions, potentially changing its composition through chemical reactions. As substances change states, they absorb or release energy, but this process of changing states is different from a chemical reaction because it does not alter the substance's inherent chemical composition.