Substances in a chemical reaction have new properties because the atoms within the substances rearrange and form new bonds. This creates entirely new molecules with different structures, and therefore, different properties.

Here's a breakdown:

* Atoms are the building blocks of matter: They are the smallest units that retain the chemical properties of an element.

* Chemical bonds hold atoms together: These bonds are formed by the sharing or transferring of electrons between atoms.

* Chemical reactions involve breaking and forming bonds: When substances react, the bonds holding the atoms together are broken, and new bonds are formed.

* New molecules are formed: The rearrangement of atoms and the formation of new bonds result in the creation of entirely new molecules.

* New molecules have different properties: These properties can include:

* Physical properties: Appearance, melting point, boiling point, density, etc.

* Chemical properties: Reactivity, flammability, acidity, etc.

Example:

* Reactants: Sodium (Na) and Chlorine (Cl) are highly reactive elements.

* Chemical reaction: Na + Cl → NaCl

* Product: Sodium chloride (NaCl), also known as table salt, is a stable compound that is not reactive and has a completely different appearance and properties than sodium and chlorine.

In summary, chemical reactions change the arrangement of atoms and bonds, leading to the formation of new molecules with distinct properties.