H2O refers to the chemical formula for water. It is defined as the binary compound that occurs at room temperature as a clear, colourless, odourless and tasteless liquid. It usually freezes into ice below 0 degrees centigrade and boils above 100 degrees centigrade.
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H2O is the chemical formula of water. It means that each molecule of water is comprised of two parts of hydrogen and one part of oxygen.
This is basic chemistry. H2O means that there are two parts hydrogen to one part oxygen. This is the chemical formula that makes up water.