Glossary
Acidic
Describes a solution that has a pH less than 7, indicating a higher concentration of H+ ions than OH- ions.
Example:
Vinegar, with a pH of around 2.4, is a common household example of an acidic substance.
Autoionization of Water
The process by which water molecules spontaneously react with each other to form hydrogen ions (H+) and hydroxide ions (OH-). This equilibrium is crucial for understanding pH.
Example:
Even in pure water, a small fraction of molecules undergo autoionization of water, creating a neutral pH of 7.
Basic
Describes a solution that has a pH greater than 7, indicating a higher concentration of OH- ions than H+ ions.
Example:
Baking soda dissolved in water creates a basic solution, often used to neutralize acidic spills.
Complete Dissociation
The process where a compound, typically a strong acid or base, breaks apart entirely into its constituent ions when dissolved in a solvent, with no intact molecules remaining.
Example:
When table salt (NaCl) dissolves in water, it undergoes complete dissociation into Na+ and Cl- ions.
Concentration of protons (H+)
The molar concentration of hydrogen ions (H+) in a solution, which directly determines its acidity. A higher concentration means a more acidic solution.
Example:
When you add lemon juice to water, the concentration of protons (H+) increases, making the water more acidic.
Equivalence Point
The point in a titration where the moles of acid exactly equal the moles of base, resulting in the complete neutralization of the analyte.
Example:
During the titration of a strong acid with a strong base, the equivalence point occurs at a pH of 7.
Kw
The ion-product constant for water, representing the equilibrium constant for the autoionization of water. At 25°C, Kw = [H+][OH-] = 1 x 10^-14.
Example:
The value of Kw allows us to calculate the [H+] if we know the [OH-], or vice versa, in any aqueous solution.
Strong Acids
Acids that completely dissociate (ionize 100%) in water, releasing all their hydrogen ions into the solution. Their concentration directly determines the [H+].
Example:
Hydrochloric acid (HCl) is a strong acid; if you have 0.1 M HCl, you have 0.1 M H+ ions.
Strong Bases
Bases that completely dissociate in water, releasing all their hydroxide ions into the solution. Their concentration directly determines the [OH-].
Example:
Sodium hydroxide (NaOH) is a strong base commonly used in titrations because it fully dissociates in water.
Titrations
A quantitative chemical analysis method used to determine the concentration of an identified analyte by reacting it with a precisely known concentration of a reagent (titrant).
Example:
A chemist might perform titrations to determine the unknown concentration of acetic acid in a sample of vinegar.
Weak Acids/Bases
Acids or bases that only partially dissociate in water, establishing an equilibrium between the undissociated molecule and its ions.
Example:
Unlike strong acids, acetic acid (CH3COOH) is a weak acid, meaning only a small fraction of its molecules ionize in water.
pH
A measure of the concentration of hydrogen ions (H+) in a solution, indicating its acidity or basicity. It is calculated as the negative logarithm of the H+ concentration.
Example:
A solution with a hydrogen ion concentration of 1 x 10^-7 M has a pH of 7, indicating it is neutral.
pH Scale
A logarithmic scale ranging typically from 0 to 14, used to specify the acidity or basicity of an aqueous solution. A pH of 7 is neutral, below 7 is acidic, and above 7 is basic.
Example:
Understanding the pH scale helps chemists classify substances like stomach acid (low pH) or bleach (high pH).
pOH
A measure of the concentration of hydroxide ions (OH-) in a solution, indicating its basicity. It is calculated as the negative logarithm of the OH- concentration.
Example:
If a cleaning solution has a hydroxide ion concentration of 1 x 10^-2 M, its pOH would be 2, making it very basic.