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Answer: H2O (water) is a polar molecule because charge is not equally distributed around the molecule. 

As mentioned previously when discussing the Lewis Dot Structure for H2O, the oxygen molecule contains four free electrons organized into two lone pairs. As a result, these create regions of negative charge within the molecule. Electron-electron repulsion (since they have the same charge) forces the molecule into a "bent" structure where the hydrogens are only approximately 104.5˚ apart. Furthermore, due to the great electronegativity difference between hydrogen and oxygen (2.20 and 3.44, respectively), the oxygen generally pulls hydrogen's electron towards itself increasing the partial positive charge on each of the hydrogen atoms. These partial positive charges are typically represented by a lowercase greek letter delta (δ).

Due to the "bent" confirmation of the water molecule, dipole-dipole interactions among H2O molecules are especially strong, leading chemists to term these kinds of intermolecular forces "hydrogen bonds." Hydrogen bonds can only form between HF, H2O and NH3 molecules due to the large electronegativity difference between these most electronegative elements (F, O, and N) and hydrogen. In large part this is what gives water many of its essential qualities such as a high melting and boiling point.

H2O Ball and Stick Model
H2O Ball and Stick Model. Created with MolView.
Water as a "universal solvent"

H2O, since it has high polarity, is very good at dissolving other polar and ionic substances. For example, NaCl (or table salt) will dissolve into aqueous ions within solution (Na+ and Cl-) as will all other ionic salts. However, H2O is not a great solvent for nonpolar substances (such as those with a lot of C-H hydrocarbon bonds) which will separate from water based on their density. Those which are less dense than water (less than 1 mg/mL) will float above and those that are more dense will float below. This is a generalization of the typical "oil and water" effect whereby nonpolar and polar solvents separate into distinct layers over time (shown below).

Image of the Oil and Water Effect. Source
Water as a solid, liquid and gas

One of most important qualities of water which drives its behavior is the existence of water in all three states of matter in the natural environment. Water vapor is constantly in the atmosphere driving weather patterns. Liquid water serves as a critical resource for life on earth and constitutes a majority of the surface of the planet. Solid water is most prevalent at the Arctic ice caps trapping a large amount of greenhouse gases.

Water in all three of its natural states (solid, liquid, gas). Source
To learn more about the relationship between these states, check out the article about the Lewis Dot Structure for H2O.

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