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When molecules or compounds are chemically changed it is called a chemical reaction.

Molecules formed by a reaction are products.

Water is the single most important molecule of earth All organisms are 70-90% water.

Atoms differ in their electronegativity, or their attraction for electrons in a covalent bond.

Oxygen has a very strong attraction for electrons, so when oxygen is sharing electrons with two hydrogen atoms, it gets the negative electrons slightly more than its fair share of the time. Since the negative electrons are near the oxygen end, more of the time, the oxygen is slightly negative. The hydrogen ends of water are slightly positive because the hydrogen atoms each have a positively charged proton that is left by itself when oxygen is sharing the electrons unfairly.

The unequal sharing of electrons in a molecule such as water makes the molecule polar.

Polar water molecules are attracted to one another and can form hydrogen bonds.