Water

Systematic Naming

The accepted IUPAC name of water is oxidane (H₂O) or simply water.

The simplest and best systematic name of water is hydrogen oxide.

The polarized form of the water molecule, H⁺OH^-, is also called hydron hydroxide by IUPAC nomenclature.

Dihydrogen monoxide (DHMO) is an overly pedantic naming of water.  Other systematic names for water include hydroxic acid, hydroxylic acid, and hydrogen hydroxide. Both acid and alkali names exist for water because it is amphoteric (able to react both as an acid or an alkali). While these names are technically not incorrect, none of them are used widely.

Electrical Properties

Dipole

Water is 1 molecule of oxygen covalently bonded to 2 hydrogen atoms, forms an angle ( 104.5° ), with H at the tips and O at the vertex.

Since O has a higher electronegativity than H, the side of the molecule with O has a partial negative charge.  An object with such a charge difference is called a dipole.   The charge differences cause water molecules to be attracted to each other and to other polar molecules.  This attraction contributes to hydrogen bonding, and explains many of the properties of water, eg. universal solvent, pH, adhesion, cohesion, capillary action, electric conductivity…   

Hydrogen bonding

A water molecule can form a maximum of 4 hydrogen bonds, because it can accept 2 and donate 2 hydrogen atoms. 

In water local tetrahedral order due to the 4 hydrogen bonds gives rise to an open structure and 3-dimensional bonding network, resulting in the anomalous decrease of density when cooled below 4°C.

Hydrogen bonding is responsible for a number of water’s physical properties, eg. high melting point, high boiling points, large specific heat capacity, large specific heat of vaporization, surface tension & capillary waves, density …

Physical Properties

Taste & Odor

Water is a tasteless, odorless liquid at standard temperature and pressure.

Transparency

Water is transparent to visible light, near ultraviolet light, and far-red light, but it absorbs most ultraviolet light, infrared light and microwaves. 

Most photoreceptors and photosynthetic pigments utilize the portion of the light spectrum that is transmitted well through water.  Thus aquatic plants can live within the water because sunlight can reach them.

Colour

The very weak onset of absorption in the red end of the visible spectrum lends water its intrinsic blue hue.

Density

The density of water is approximately 1 gram per cm³.   Yet it is very inconsistent. 

Although it is dependent on its temperature, the relation is not consistent.  At high temperature, it is less dense.  When it cooled from room temperature, the density increases.  At approximately 4°C, pure water reaches its maximum density.   

Upon freezing, the density of water decreases by about 95%.  Thus, the less denser solid water will float on denser liquid form.   

The reason of this is the water molecule forming hexagonal Ice l_h formation.

These properties of water have important consequences in its role in the Earth’s ecosystem.  Water at 4°C will accumulate at the bottom of fresh water lakes. Ice will floats over the water surface, creating a blanket which prevent loss of heat of the liquid water beneath.  Thus it is unlikely a deep lake will freeze completely, thus help preserve aquatic life.

Surface Tension

Water has a high surface tension of 72.8 mN/m at room temperature, cause by the dipolar interactions, the highest of the non-metallic liquids.  

The effects of surface tension can be seen with ordinary water :

-         beading of water on  sorption-free surface ( non-adsorbent and non-absorbent, eg. polyethylene ).  Surface tension give water their near-spherical shape, because a sphere has the smallest possible surface are to volume ratio.

-         formation of drops.

-         flotation of object denser than water occurs when the object is non-wettable and its weight is small enough to be borne by the forces arising from surface tension.

-         separation of oil and water

-         tears of wine

-         emulsions are a type of solution in which droplets of oil suspended in water ( or vice versa ).

Capillary Action

Due to an interplay of the forces of adhesion and surface tension, water is able to flow against gravity where it spontaneously rises in a narrow space such as a thin tube, or in porous material such as paper or cloths.

Examples of capillary actions:

-         kerosene lamp’s wick

-         chromotography

-         water movement in plant’s xylem

Universal Solvent

Water is a good solvent.  Substances that dissolve in water includes : salts, sugars, acids, alkalis, some gases.

However, water cannot dissolve all substances, especially those that are non-polar or hydrophorbic in nature.

Electrical Conductivity

Pure water does not conduct electricity, thus an excellent insulator.  But there are no such pure that it is free of ions, even deionized water.  As water is such a good solvent, it  almost always has some solute dissolved in it. 

pH

Pure water is pH neutral, that is pH 7.0. 

But since there are no such pure water, as there are a lot of substances that dissolve easily in water.  It is thus, water’s pH is very dependent on dissolved chemicals and minerals, as does it changes over time.

Normally, water pH is < 7.0.   That explains metals rust upon contact with water.  Surface water are mostly acidic.

Water with pH > 7.0 is considered basic.  Sea water is basic in nature.

Water with pH > 8.5 is considered hard water.  Hard water contains high mineral content, especially Ca²⁺ and Mg²⁺.  Hard water is difficult to suds – difficult to produce soap bubbles.  It also forms deposits that clog plumbing.

High Boiling Point

Boiling point is when a substance reach a temperature at which the liquid state of the substance change to vapor, under atmospheric pressure.  Water has a high boiling point ( 100°C ) for its strong hydrogen bonding.  High boiling point means water is not easily vaporized by heat, thus a stable medium for living organism.

Specific Heat Capacity & Heat of Vaporization

Water has the 2^nd highest specific heat capacity of all known substances, after ammonia, as well as high heat of vaporization, both of which are a result of the extensive hydrogen bonding between its molecules.

Specific heat capacity is the amount of energy required to change a substance’s temperature by a given amount. 

Ice’s specific heat capacity at -10°C is about 2.05 J•g^-1•K^-1.

Steam’s specific heat capacity at 100°C is about 2.08 J•g^-1•K^-1.

Heat of vaporization refers to amount of energy required to transform a given quantity of a substance into a gas at a given pressure. 

Water’s heat of vaporization is 2257 kJ•kg^-1.

The high specific heat capacity and heat of vaporization of water help to moderate Earth’s climate by buffering large fluctuations in atmosphere temperature.