Water/ H2O

Water is the most abundant molecule present in all living organisms. All chemical reactions within an organism take place in the presence of water. Several characteristics unique to water contribute to its vital importance in the processes of life, such as its properties as a solvent and tendency to form ions.

Water as a Solvent

document.write('');

Solvents dissolve other molecules, called solutes, to form solutions, which are homogeneous mixtures of molecules. The hydrogen bonds that hold water molecules make water a versatile solvent that can form solutions with polar (hydrophilic) molecules. Water cannot form solutions with nonpolar (hydrophobic) molecules.


Water Ionization

Occasionally, water molecules spontaneously ionize, or break apart, into hydroxide ion (OH-) and hydrogen ion (H+), as illustrated in the following chemical equation.


The pH scale, which ranges from 0 to 14, expresses the relative concentrations of OH- and H+ in a solution. The pH value determines whether a solution is acidic, basic, or neutral:

Neutral solutions have equal concentrations of OH- and H+ and a pH of 7.
Acidic solutions have a greater concentration of H+ and a pH of less than 7.
Basic solutions have a greater concentration of OH- and a pH of greater than 7.
Acids lower the pH of solutions, while bases raise the pH. Buffers are substances that reduce the effect of acids and bases on the pH of a solution.

Calculating pH

The pH of a given solution expresses the negative logarithm of the hydrogen ion concentration, as represented in the following equation.

document.write('');

pH = -log[H+]

For example, the [H+] of wine equals 10-3. Because the negative logarithm (represented by the exponent in this equation) is three, wine pH = 3 and is therefore acidic.