Why does a water molecule have different electrical sides?

A water molecule has no overall electric charge, yet its oxygen side is slightly negative and its hydrogen side slightly positive. Its electrons are distributed unevenly. These partial charges make water a polar molecule and help explain why water interacts strongly with many ions and other polar substances.

Oxygen pulls shared electrons toward itself more strongly than hydrogen does. Each oxygen–hydrogen bond therefore has an uneven distribution of negative charge. The nuclei have not changed, and an ordinary water molecule has not turned into separate positive and negative ions. Instead, one region has more electron density and another less. The small partial-charge signs in the drawing mark this unevenness within an electrically neutral molecule.

Shape matters as much as unequal sharing. Water's two O–H bonds point at an angle, so their electrical effects do not cancel. Carbon dioxide provides a useful comparison. Each carbon–oxygen bond is polar, but the molecule is straight with equivalent oxygen atoms at opposite ends. Their effects cancel overall. The presence of polar bonds therefore does not by itself establish that the whole molecule is polar.

Nearby water molecules tend to orient so that a partly positive hydrogen region is attracted toward a partly negative oxygen region of another molecule. They keep moving, turning and exchanging neighbors, rather than locking into one permanent arrangement in liquid water. These attractions help account for water's surface behavior and the energy needed to separate its molecules. Polar water can also surround an ion with the appropriately charged region facing inward.

Whether a substance dissolves involves the attractions being disrupted and those being formed, as well as how many arrangements become available. The phrase 'like dissolves like' is a useful first guide, not a universal rule. A large molecule may contain both water-attracting and water-avoiding regions. That is why molecular shape and the distribution of charge are more informative than simply calling every substance wet or dry. Water's remarkable behavior starts with an uneven electrical pattern in a very small bent molecule.