Water

Water is a compound of hydrogen and oxygen H₂O. The H-O-H bond angle is 104.4°. Due to the charge separation the molecule is highly polar, the positive end toward the hydrogens, the negative toward the oxygen. (More accurately it is a quadrupole with two occupied nonbonding orbitals on the oxygen in a tetrahedral arrangement with the hydrogens.) Because of the polar nature of the molecule it is a particularly good solvent for ionic compounds, the ions being stabilized in solution by oriented water dipoles. It has a very high dielectric constant, 80 at room temperature. Because of the high dielectric constant the force of attraction and repulsion of electric charges are very weak. Two opposite charges will attract each other with one 80th of the force that would exist in a vacuum. This along with the polarity and hydration sphere formed around ions make it a very good solvent for ionic compounds (electrolytes). It boils at 100°C and freezes at 0°C. The solid form, ice, is unusual in that it is less dense than the liquid. Thus lakes freeze over on the top.

Water ionizes to a small degree, the concentrations of hydrogen ions H⁺ and hydroxide ions OH^- at room temperature are 1 x 10^-7 moles/liter. Addition of acid increases the hydrogen ion concentration and decreases the hydroxide ion concentration. The hydrogen ions don't exist by themselves but combine with water molecules to form the hydronium ion H₃O⁺. This exerts a structural influence on more water molecules in the vicinity. Addition of bases will increase the hydroxide ion concentration and decrease the hydrogen ion concentration. The product of the two will always equal 1 x 10^-14. Water is called neutral when the concentration of hydrogen ion is 1 x 10^-7M. It is neither acidic or alkaline (basic) since the concentration of hydroxide ion would also be 1 x 10^-7M. The term "pH" means the power of the hydrogen ion concentration. pH 7 means that the hydrogen ion concentration is 1 x 10^-7M. At pH 5 the hydrogen ion concentration is 1 x 10^-5M and the hydroxide ion concentration is 1 x 10^-9M. The solution with pH 5 is acidic. Acidic has pH<7, basic pH>7.