Answer:Substances on the periodic table cannot be broken down into other substances and are therefore elements.
Periodic table is an arrangement of elements. This arrangement is based on the similar chemical and physical properties. Though elements can be broken down to subatomic particles like electrons, protons etc. but these cannot be broken down into other substances. Hence, all substances on the periodic table classified as elements.
Substances on the periodic table cannot be dissolved in water or other liquids: This is not true. Substances on the periodic table can be dissolved in water and other liquids. For example, Aluminium, Iron, Arsenic etc can dissolve in water.
All elements are pure substances: Even compounds are pure substances.
Substances on the periodic table can be combined to form compounds and are therefore elements: This is true but not the ultimate reason.