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The Water Molecule

 

SEE ALSO

The Water and Ice Module

Properties
General
Name Water
Diagram A diagram of a water molecule, with dimensions
Chemical formula H2O
Appearance Colourless liquid
Physical
Formula weight 18.01528 amu
Melting point 273 K (0 C)
Boiling point 373 K (100 C)
Critical temperature 674 K
Critical Pressure 22.1x10^6?? Pa
Density 1.0 103 kg/m3
Thermochemistry
”fH0gas -241.83 kJ/mol
”fH0liquid -285.83 kJ/mol
”fH0solid -291.83 kJ/mol
S0gas, 1 bar 188.84 J/molK
S0liquid, 1 bar 69.95 J/molK
S0solid 41 J/molK
Safety
Ingestion Necessary to life; excessive consumption can cause headache, confusion, and cramps, and can be fatal in athletes
Inhalation Non-toxic. Can dissolve surfactant of lungs. Suffocation in water is called drowning.
Skin Prolonged immersion may cause flaking (desquamation).
Eyes Not dangerous.
SI units were used where possible. Unless otherwise stated, standard conditions were used.

Disclaimer and references

Water is a chemical compound and polar molecule, which is liquid under STP conditions. It has the chemical formula H2O, meaning that one molecule of water is composed of two hydrogen atoms and one oxygen atom. Water is found almost everywhere on earth and is required by all known life. About 70% of Earth's surface is covered by water.

General

The solid state of water is known as ice; the gaseous state is known as water vapor (or steam). The units of temperature (formerly the degree Celsius and now the Kelvin) are defined in terms of the triple point of water, 273.16 K (0.01 C) and 611.2 Pa, the temperature and pressure at which solid, liquid, and gaseous water coexist in equilibrium. Water exhibits some very strange behaviors, including the formation of states such as vitreous icea noncrystalline (glassy), solid state of water.

At temperatures greater than 647 K and pressures greater than 22.064 MPa, a collection of water molecules assumes a supercritical condition, in which liquid-like clusters float within a vapor-like phase.

Body of water is a term for an ocean, sea, lake, river, stream, canal, pond, or the like. See water (resource)s for information about fresh water supplies. Also see: sea water, fresh water, and underwater.

Earth's approximate water volume (the total water supply of the world) is 326,000,000 cubic miles. Of this volume:

  • 316,900,000 is in the oceans
  • 6,000,000 is in glaciers and icecaps
  • 3,000,000 is in underground storage in aquifers
  • 60,000 is in fresh water in lakes, inland seas, and rivers.
  • 3,100 is water vapor at any one time.

The dipolar nature of water

An important feature of water is its polar nature. The water molecule forms an angle, with hydrogen atoms at the tips and oxygen at the vertex. Since oxygen has a higher electronegativity than hydrogen, the side of the molecule with the oxygen atom has a partial negative charge, relative to the hydrogen side. A molecule with such a charge difference is called a dipole. The charge differences cause water molecules to be attracted to each other (the relatively positive areas being attracted to the relatively negative areas) and to other polar molecules. This attraction is known as hydrogen bonding.

This relatively weak (relative to the covalent bonds within the water molecule itself) attraction results in physical properties such as a very high boiling point, because a lot of heat energy is necessary to break the hydrogen bonds between molecules. For example, Sulphur is the element below oxygen in the periodic table, and its equivalent compound, hydrogen sulphide (H2S) does not have hydrogen bonds, and though it has twice the molecular weight of water, it is a gas at room temperature. The extra bonding between water molecules also gives liquid water a large specific heat capacity.

Hydrogen bonding also gives water an unusual behaviour when freezing. Just like most other materials, the liquid becomes denser with lowering temperature. However, unlike most other materials, when cooled to near freezing point, the presence of hydrogen bonds means that the molecules, as they rearrange to minimise their energy, form a structure that is actually of lower density: hence the solid form, ice, will float in water i.e. water expands as it freezes (most other materials shrink on solidification). Liquid water reaches its highest density at a temperature of 4 C. This has an interesting consequence for water life in winter. Water chilled at the surface becomes denser and sinks, forming convection currents that cool the whole water body, but when the temperature of the lake water reaches 4C, water on the surface, as it chills further, becomes less dense, and stays as a surface layer which eventually forms ice. Since downward convection of colder water is blocked by the density change, any large body of water frozen in winter will have the bulk of its water still liquid at 4C beneath the icy surface, allowing fish to survive. (this is one of the principal examples of finely-tuned physical properties that support life on Earth that is used as an argument for the Anthropic Cosmological Principle).

Another consequence is that ice will melt if sufficient pressure is applied.

Water as a solvent

Water is also a good solvent due to its polarity. The solvent properties of water are vital in biology, because many biochemical reactions take place only within aqueous solutions (e.g., reactions in the cytoplasm and blood). In addition, water is used to transport biological molecules.

When an ionic or polar compound enters water, it is surrounded by water molecules. The relatively small size of water molecules typically allows many water molecules to surround one molecule of solute. The partially negative dipoles of the water are attracted to positively charged components of the solute, and vice versa for the positive dipoles.

In general, ionic and polar substances such as acids, alcohols, and salts are easily soluble in water, and nonpolar substances such as fats and oils are not. Nonpolar molecules stay together in water because it is energetically more favorable for the water molecules to hydrogen bond to each other than to engage in van der Waals interactions with nonpolar molecules.

An example of an ionic solute is table salt; the sodium chloride, NaCl, separates into Na+ cations and Cl- anions, each being surrounded by water molecules. The ions are then easily transported away from their crystalline lattice into solution. An example of a nonionic solute is table sugar. The water dipoles hydrogen bond to the dipolar regions of the sugar molecule and allow it to be carried away into solution.

Cohesion and surface tension

The strong hydrogen bonds give water a high cohesiveness and, consequently, surface tension. This is evident when small quantities of water are put onto a nonsoluble surface and the water stays together as drops. This feature is important when water is carried through xylem up stems in plants; the strong intermolecular attractions hold the water column together, and prevent tension caused by transpiration pull. Other liquids with lower surface tension would have a higher tendency to "rip", forming vacuum or air pockets and rendering the xylem vessel inoperative.

Conductivity

Pure water is actually an insulator, meaning that it does not conduct electricity well. Because water is such a good solvent, it often has some solute dissolved in it, most frequently salt. If water has such impurities, then it can conduct electricity well.

Electrolysis

Water can be split into its constituent elements, hydrogen and oxygen, by passing a current through it. This process is called electrolysis. Water molecules naturally disassociate into H+ and OH- ions, which are pulled toward the cathode and anode, respectively. At the cathode, two H+ ions pick up electrons and form H2 gas. At the anode, four OH- ions combine and release O2 gas, molecular water, and four electrons. The gases produced bubble to the surface, where they can be collected.

Reactivity

Chemically, water is amphoteric: able to act as an acid or base. Occasionally the term hydroxic acid is used when water acts as an acid in a chemical reaction. At a pH of 7 (neutral), the concentration of hydroxide ions (OH-) is equal to that of the hydronium (H3O+) or hydrogen ions (H+) ions. If the equilibrium is disturbed, the solution becomes acidic (higher concentration of hydronium ions) or basic (higher concentration of hydroxide ions).

 
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