Here's why:

* States of Matter: States of matter (solid, liquid, gas) are determined by the arrangement and movement of molecules, not by their chemical makeup.

* Chemical Composition: The chemical composition of a molecule refers to the types and number of atoms that make up that molecule.

Example:

Water (H₂O) remains water whether it's in solid form (ice), liquid form (water), or gaseous form (steam). The chemical composition (two hydrogen atoms and one oxygen atom) stays the same. The only difference is the arrangement and movement of the water molecules.

Exceptions:

* Chemical Reactions: While changing states doesn't alter chemical composition, chemical reactions do. During a chemical reaction, new substances with different chemical compositions are formed. For example, when you burn wood, the wood reacts with oxygen, creating ash, carbon dioxide, and other substances.

* Phase Transitions that involve Chemical Change: Some phase transitions, like the conversion of graphite to diamond, involve a change in the arrangement of atoms within a molecule. While the chemical composition of carbon remains the same, the arrangement changes, resulting in different physical properties.

In summary: Changing the state of matter (solid, liquid, gas) doesn't change the chemical composition of the molecules. It only affects the arrangement and movement of those molecules.