pH

Water molecules contain covalent bonds. When the covalent bond of a water molecule breaks, one of the protons from the hydrogen atom dissociates from the molecule without an electron. Thus, the hydrogen ion (H⁺) carries a positive charge and the remaining part of the water molecule, the hydroxide ion (OH^-), retains the electron and carries a negative charge.

• How does a covalent bond differ from a hydrogen bond?

Any substance that dissociates or ionizes, and releases hydrogen ions is called an acid. The more hydrogen ions that are released, the more acidic a solution is. An example of an acid is HCl. The HCl ionizes completely in water resulting in complete release of the hydrogen ion. Conversely, a substance that dissolves in water and combines with hydrogen ions is called a base. Sodium hydroxide ionizes in water and the hydroxide ion combines with hydrogen ions and there by reduces the concentration of hydrogen ions.

• What is formed when hydrogen ions combine with hydroxide ions?

The concentration of hydrogen ion is expressed as pH. Simply stated, pH refers to the balance between hydrogen and hydroxide ions. When pH is less than 7.0 there are more hydrogen ions than hydroxide ions. At a pH of 7.0, the concentration of hydrogen ions and hydroxide ions is similar. When the pH is greater than 7.0, the concentration of hydrogen ions is less than the concentration of hydroxide ions because the hydrogen ions are combining to form other compounds. But it gets even more complicated. There are strong acids and bases and the concentration of hydrogen ions can vary by a factor of 100 trillion or more.

• What is an example of a weak acid and a strong acid?
• What are the criteria for calling an acid strong or weak? 

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