GED Science: Polarity and Hydrogen Bonds

While water is not the only compound to contain polar covalent bonds, the polar covalent bonds in water explain features that are unique to water.  The weak charges in polar molecules cause attractions between molecules (intermolecular attractions) called hydrogen bonds.  Hydrogen bonds are not “true” bonds, in that electrons are not given, taken, or shared between atoms, but still exert influence on the behavior of polar compounds.

In a hydrogen bond, the partial negative charge (δ-) near the oxygen atom in water interacts with the partial positive charge (δ+) near the hydrogen atom on a different water molecule.

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This interaction creates a number of properties unique to water. Water molecules stay close to each other (cohesion), due to the collective action of hydrogen bonds between water molecules. Cohesion also causes water to have surface tension, which can be seen in droplet of water on a penny, or when waterstriders and other insects are able to “walk” on the surface of a lake or stream.  Water also can stick to other surfaces (adhesion) because of its polar nature. For example, on extremely clean/smooth glass the water may form a thin film because the molecular forces between glass and water molecules (adhesive forces) are stronger than the cohesive forces. Due to an interplay of the forces of adhesion and surface tension, water exhibits a phenomenon called capillary action whereby water rises into a narrow tube against the force of gravity. Water is also a good solvent, due to its polarity. Substances that will mix well and dissolve in water (e.g. salts) are known as hydrophilic (“water-loving”) substances, while those that do not mix well with water (e.g. fats and oils), are known as hydrophobic (“water-fearing”) substances. The ability of a substance to dissolve in water is determined by whether or not the substance can match or better the strong attractive forces that water molecules generate between other water molecules. In general, ionic and polar substances such as acids, alcohols, and salts are relatively soluble in water, and non-polar substances such as fats and oils are not.

Most interestingly, though, is that hydrogen bonds explain why the solid form of water (ice) is less dense than its liquid form.  This is extremely unusual, as most substances have solid forms that are more dense than their liquid forms.  Hydrogen bonds create a much more open, and less dense structure within solid ice, as seen in the diagram below:

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