The compound is a pure product consisting of two or more different elements. The different atoms that make up this compound must be present in a certain proportion. In other words, the compound has a certain composition regardless of its source. In daily life, sodium chloride and distilled water (water) are common compounds. From these compounds, it was found that their properties are different from each other. Salt is composed of sodium atoms and chlorine atoms; sugar is composed of atoms such as carbon, hydrogen, and oxygen; and hydrogen gas reacts to produce water when it is burned in oxygen. These facts mean that two or more substances can react to produce a new substance. This new substance is a compound. The nature of the new material is completely different from that of the original material. It is usually chemically used to determine whether a substance is a compound or not. And if a pure substance can be broken down into two or more than two qualities, the original substance must be a compound. For example, molten salt can be completely decomposed into sodium and chlorine atoms by electric current. Therefore, salt is a compound.