Water

Water is the molecule that sustains all kinds of life forms. Its structure allows it to interact with other molecules and water molecules itself, resulting in many unique emergent properties that help make Earth suitable for life.

Polar covalent bond

Water is a polar molecule, meaning that its overall charge is unevenly distributed. In an H₂O molecule, the oxygen has two regions of partial negative charge (δ⁻) and the hydrogen has a partial positive charge (δ⁺) due to the difference in electronegativity between the two elements. Oxygen is more electronegative than hydrogen, so the electrons of the covalent bonds spend more time closer to oxygen than to hydrogen, leading to the formation of polar covalent bonds as a result in the unequal sharing of electrons.

The partial charges of water molecules make it possible for it to form hydrogen bonds with one another, through intermolecular interaction between the hydrogen of one molecule and the oxygen of another. Hydrogen bonds, as you will later see, contribute to a variety of important properties in which water possesses.

Cohesive behavior

Hydrogen bonds hold water molecules together, making it more structured than most other liquids. Several phenomena arise as a result of this behavior, such as cohesion, adhesion, and surface tension.

Cohesion: the attraction between different substances through hydrogen bonds, in this case, water molecules and other substances. Think of a straw. Without cohesion, you would not be able to suck up any water because the water molecules are unable to cling onto the surface of the straw. This is cohesion.

Adhesion: the clinging of one substance to another, in this case, water molecules to water molecules. Let’s take the straw as an example again. Without adhesion, you would again be unable to suck up any water, because the water will fall down the straw due to gravity since the molecules below are not attached to the ones above.

Surface tension: a measure of how difficult it is to stretch or break the surface of a liquid. Water has an unusually high surface tension due to its hydrogen bonding, and this can be observed in a water droplet, which has the shape of a dome.

Moderation of temperature

Water moderates air temperature by absorbing heat from the air that is warmer and releasing the stored heat to air that is cooler. Several characteristics of water allow it to do so effectively, and that is its unusually high specific heat and heat of vaporization.

Specific heat: defined as the amount of heat that must be absorbed or lost for 1 g of a substance to change its temperature by 1°C. The specific number and calculations of water’s specific heat (1 cal/g．°C) is of little importance here, and you only need to know that compared with most other substances, water has an unusually high specific heat, making it less likely to change temperature than other liquids when absorbing or losing a given amount of heat. This is due to its hydrogen bonds: a lot of heat are spend on disrupting water’s hydrogen bonds before the water molecules can begin moving faster, and when the temperature drops slightly, many additional hydrogen bonds are formed, releasing heat into its surroundings.

The heat of vaporization: the quantity of heat a liquid must absorb for 1 g of it to be converted from liquid to gas. For the same reason, that water has a high specific heat, it also has a high heat of vaporization relative to other liquids. This characteristic of water allows it to moderate Earth’s climate, through evaporative cooling and circulation of air currents across the globe.

Ice v. liquid water

Water is one of the few substances that are less dense as a solid than as a liquid. In other words, ice floats on liquid water. Water expands when solidify because its hydrogen bonds form crystalline lattices when freezing, making ice about 10% less dense than liquid water, at 4 °C. This property has many values, including the sustentation of polar ice caps and its ecosystems.

Water as a solvent

Water is a very versatile solvent, due to its characteristic of being a polar molecule. The partial charges of water molecules, both positive and negative, can surround solute ions and form a sphere of water molecules called the hydration shell. That is, a compound is identified as “water-soluble” as long as water is able to establish a hydration shell around it. This characteristic is very vital since many different kinds of polar compounds are dissolved in the water of such biological fluids like blood, the sap of plants, and the liquid within all cells.

Water as a polar molecule

Reference: 

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.