you can prove it by breaking water down to collect hydrogen and oxygen gas. This is done via electrolysis
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An element is an item that has the same atoms throughout (Gold, uranium, silver, potassium, carbon, radon). A compound is a substance that is formed by two or more elements combining. For example Salt is NaCl Sodium chloride (Sodium and Chlorine). Water is H2O Hydrogen and Oxygen. Since it is formed by two elements, it is a compound.