Answer:
Explanation:
There are two kind of substances that may be classified as pure substances: elements and compounds.
Elements are the pure substances whose atoms are all of the same kind (same atomic number). Some xamples of pure substances are sodium (Na), oxygen gas (O₂), hydroge gas (H₂), nitrogen gas (N₂), helium (He), and gold (Au). There are 118 known elements. You can find all of them in a periodic table.
Elements cannot be broken down into simpler substances by chemical or physical means because the simplest substance is the atom.
The only way to break elements into other substances is by nuclear reactions: fission and fusion reactions transform the atoms of an element into different atoms, but those reactions are not considered either chemical or physical changes. Nuclear reactions are a completely different kind of changes.
On the other hand, the other kind of pure substances, compounds, are formed by two or more different elements, and they can be broken down into simpler substances by chemical means. For example, water molecules (H₂O) can be broken down into the elements that constitue them, i.e. hydrogen and oxygen.
