* Chemical reactions involve the breaking and forming of chemical bonds. This means that the atoms within the reactants (starting materials) are rearranged to form new molecules, resulting in different products.

* The chemical composition of a substance refers to the types and ratios of atoms that make up that substance. When bonds are broken and formed, the composition changes.

Examples:

* Burning wood: Wood (mostly cellulose) reacts with oxygen, breaking down into carbon dioxide, water, and ash. The composition of the wood has changed dramatically.

* Baking a cake: Flour, sugar, eggs, and other ingredients undergo chemical reactions when heated, resulting in a completely different substance with a new texture, taste, and composition.

Other options are incorrect because:

* The mass of the substances: The law of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. While the form of matter changes, the total mass remains the same.

* The physical state of the substances: A chemical reaction can change the physical state (solid, liquid, gas) of the substances involved. However, the underlying chemical composition is what is changing.

* The temperature of the substances: Temperature is a measure of heat energy, and while chemical reactions often involve heat release or absorption, it doesn't fundamentally alter the chemical composition.