Question

What are the simplest pure substances that cannot be broken down chemically into other substances?

Answer 1

Pure substances cannot be broken down by physical means. Pure substances contain only one kind of molecule. Molecules are small groups of atoms that make up matter. For example: Water $\left( {{H_2}O} \right)$ is a molecule of two hydrogen atoms and one oxygen atom and atoms are the smallest part of the elements. Water $\left( {{H_2}O} \right)$ is also known as universal solvent.

Complete answer:
Each kind of atom is an element. Elements are the pure substances that cannot be broken down into simpler substances by ordinary chemical or physical means. Each element has unique physical and chemical properties whereas a compound is a substance formed when two or more chemical elements are chemically bonded together and in mixtures, the substances present are not chemically bonded together.

Note:
In chemistry, an element is a pure substance consisting only of atoms that all have the same number of protons in their atomic nuclei. The number of protons in the nucleus is the defining property of an element, and is referred to as its atomic number $\left( Z \right)$. All atoms with the same atomic number are atoms of the same element. There are $118$ elements known on the earth. When different elements undergo chemical reactions, atoms are rearranged into new compounds held together by chemical bonds. Only a minority of elements, such as silver and gold, are found uncombined as relatively pure native element minerals. Nearly all other naturally occurring elements occur in the Earth as compounds or mixtures.