Water

Water is one of the most important molecules as it is essential to all life. Water is important in metabolic reactions, and it also helps in many other ways by providing a solvent to dissolve substances into, lubricating joints, and providing cellular structure and support.

Water consists of two hydrogen atoms bonded to one oxygen atom (H₂O_(l)). The bond between the hydrogen atom and oxygen atom is polar due to the strong electronegativity of the oxygen atom. The resulting structure of the water molecule is asymmetrical not only due to the polarity but also due to the repulsion of same charged atoms away from each other. The area around the oxygen atoms is negative while the area around the hydrogen atom tends to be positive.

Because of this polarity, the water molecules strongly attract other molecules including other water molecules. A strong attractive force found between two molecules is called an intermolecular force. Intermolecular forces are weaker than ionic and covalent bonds and are affected when matter changes from a solid to a liquid state. When ice melts, energy (heat or pressure) is being added to the system. This energy causes the water molecules in the ice to move. This disrupts the strong intermolecular forces in the solid form (ice), and results in the liquid form of water. There are three types of intermolecular forces: London forces, dipole-dipole forces, and hydrogen bonds  are the three types of intermolecular forces. These three different forms are collectively called van der Waals forces.

London forces, also known as London dispersion forces, are the most common form of bond and are found between all atoms and molecules. They are the only way to hold non-polar molecules together.

Next, dipole-dipole forces are found between polar molecules. The slightly positive part of one molecule is attracted to the slightly negative part of another molecule.

The strongest of the intermolecular forces is the hydrogen bond, which is an especially strong dipole-dipole attractive force. A hydrogen forms strong attractive forces when the hydrogen nucleus (essentially a proton) is simultaneously attracted by its shared pair of electrons in its own molecule and a lone pair of electrons exposed on a neighbouring molecule. In order for these strong hydrogen attractive forces to occur, the H atom must be bonded to an atom of N, O or F. Water molecules are attracted to one another through strong hydrogen attractive forces.

Water has a relatively high boiling point when compared to other liquids such as the alcohols. The reason for the elevated boiling point is the electron arrangements that result in the electronegativity and consequent polarity of the water molecule.

The oxygen atom shares a pair of electrons with each of the hydrogen atoms. This sharing of electrons makes each atom relatively stable and bonds all three atoms together into a water molecule.

However, this sharing of a pair of electrons is not equal. The oxygen is much more electronegative and so attracts each pair of electrons closer to its nucleus and away from the hydrogen's nucleus.

Because electrons are negative, the result is that the region around the oxygen atom in the water molecule is partially negative and a partially positive region exists around each of the hydrogen atoms. Identify the partial positive and negative charges on the water molecule below:

We subsequently refer to water molecules as being polar or having dipoles. The dipole regions on one water molecule will tend to attract the opposite dipole region on a neighbouring molecule. This is highly significant as it will tend to bind the molecules together.

Due to the unequal sharing of electrons among the atoms of a water molecule, parts of the water molecule are positive and one part is negative. Because polar molecules have dipoles, they may be attracted to each other or repelled from each other. If they have oppositely charged dipoles—they are attracted to each other; if they have the same dipoles—they are repelled from each other.

Water is an excellent solute, or solution in which to dissolve chemicals. Because of its high polarity there is a great opportunity for molecules to bind with the slightly negative or slightly positive parts of the water molecule. When ionic compounds are dissolved into solution, they break into their ionic parts (cation and anion). These positive and negative ions bind eagerly to water molecules. A molecule does not necessarily have to be ionic to dissolve in water; polar molecules such as sugars are also  soluble in water. However, some substances will not dissolve in water. Here are some important definitions:

Miscible liquid: A liquid that dissolves into another liquid.

Immiscible liquid: A liquid that forms a separate layer instead of dissolving into another liquid (e.g. oil sits on top of water in a “layer”).

Hydrophobic: Having a dislike to water; the tendency of a non-polar molecule to stay as far away from water as it can (oil is “hydrophobic”).

Hydrophilic: Having a liking for water; the tendency of polar molecules and ionic substances to dissolve in water.

Small and large non-polar molecules generally do not dissolve into water. Try washing vegetable oil (large non-polar molecule) off with just water—no soap! A basic rule of chemistry with regard to this is “like dissolves like”. This means one type of oil will dissolve into another type of oil, and a polar solution will dissolve into any other polar solution, etc.