Liquids can exist because of the Kinetic Molecular Model. This model describes the relationship between energy, motion, and phase. H2 and O2 can combine together chemically to form any of the three states. The relationship between pressure and temperature determine state. Have you heard of the triple point? No, then I will tell you (if yes ignore). If you have water in a vacuum, it will boil away (even at room temperature). Pressure is just as important as temperature, since both affect the motion of molecules. If you adjust temp and pressure just right then you can have a flask of liquid water with chunks of ice in it boiling away. I have seen this and performed it myself, it happens. Chemists use graphs called phase diagrams to report the regions where the three phases exist.
Water is perhaps the best understood molecule in chemistry. More research has been poured (no pun) into it then anything else. An overview:
The Chemistry of Water
The polarity of water Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen.
Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge () near the oxygen atom due the unshared pairs of electrons, and partial positive charges () near the hydrogen atoms.
An electrostatic attraction between the partial positive charge near the hydrogen atoms and the partial negative charge near the oxygen results in the formation of a hydrogen bond as shown in the illustration.
The ability of ions and other molecules to dissolve in water is due to polarity. For example, in the illustration below sodium chloride is shown in its crystalline form and dissolved in water.
Many other unique properties of water are due to the hydrogen bonds. For example, ice floats because hydrogen bonds hold water molecules further apart in a solid than in a liquid, where there is one less hydrogen bond per molecule. The unique physical properties, including a high heat of vaporization, strong surface tension, high specific heat, and nearly universal solvent properties of water are also due to hydrogen bonding. The hydrophobic effect, or the exclusion of compounds containing carbon and hydrogen (non-polar compounds) is another unique property of water caused by the hydrogen bonds. The hydrophobic effect is particularly important in the formation of cell membranes. The best description is to say that water "squeezes" non-polar molecules together.
Acids and Bases, Ionization of Water
Acid release H+
Bases accept H+
We define the pH of a solution as the negative logarithm of the hydrogen ion concentration.
at pH 7.0, a solution is neutral
at lower pH (1-6), a solution is acidic
at higher pH (8-14), a solution is basic
The following image shows the hydrogen bond between two or more water molecules. This is why water has a higher boiling point, surface tension, and differing densities.
<img src="
www.biology.arizona.edu/biochemistry/tut...y/graphics/water.gif
" border=0>
Now notice how water behaves in an electrolyte solution between table salt and water. Watch how the amphiphoteric relationship of water allows for this ion relationship.
<img src="
www.biology.arizona.edu/biochemistry/tut...y/graphics/nacl2.gif
" border=0>
I hope this is helpful.
