Definitions: Acids & bases
Related Examples and Practice Problems
Check out the practice on the Conjugate Acid & Base Pair Topic Page.
Topic Summary & Highlights
and Help Videos
Core Concept
Acids are substances that increase the concentration of hydrogen ions ($\text{H}^+$) or donate protons, while bases increase the concentration of hydroxide ions ($\text{OH}^−$) or accept protons. These definitions are captured in the Arrhenius model (focused on aqueous solutions), the Brønsted-Lowry model (proton transfer), and the Lewis model (electron pair exchange).
Practice Tips
Acids donate protons ($\text{H}^+$); bases accept protons.
Strong acids and bases completely dissociate, while weak ones only partially dissociate.
Neutralization reactions result in water and salt.
Understanding acid and base strength is essential for predicting reaction behavior.
Definitions
Arrhenius Definition:
Acid: A substance that increases $\text{H}^+$ (or $\text{H}_3\text{O}^+$) concentration in aqueous solution.
Base: A substance that increases $\text{OH}^−$ concentration in aqueous solution.
Example:
Acid: $\text{HCl} \rightarrow \text{H}^+ + \text{Cl}^-$
Base: $\text{NaOH} \rightarrow \text{Na}^+ + \text{OH}^-$
Brønsted-Lowry Definition:
Acid: A proton donor.
Base: A proton acceptor.
Example:
Acid: $\text{HCl} + \text{H}_2\text{O} \rightarrow \text{Cl}^- + \text{H}_3\text{O}^+$
Base: $\text{NH}_3 + \text{H}_2\text{O} \rightarrow \text{NH}_4^+ + \text{OH}^-$
Lewis Definition:
Acid: An electron-pair acceptor.
Base: An electron-pair donor.
Example:
Acid: $\text{BF}_3$
Base: $\text{NH}_3$
$\text{BF}_3 + \text{NH}_3 \rightarrow \text{F}_3\text{B-NH}_3$
Properties of Acids and Bases
Acids:
Taste: Sour (e.g., lemon juice, vinegar).
Reaction with Metals: React with active metals to produce hydrogen gas.
Example: $\text{Zn} + 2\text{HCl} \rightarrow \text{ZnCl}_2 + \text{H}_2$
pH: Less than 7.
Indicators:
Turn blue litmus paper red.
Methyl orange: Red in acidic solutions.
Examples:
Strong acids: \text{HCl}, \text{H}_2\text{SO}_4}, \text{HNO}_3
Weak acids: $\text{CH}_3\text{COOH}$ (acetic acid), $\text{H}_2\text{CO}_3$
Bases:
Taste: Bitter and slippery (e.g., baking soda, soap).
Reaction with Acids: Neutralize acids to form water and a salt.
Example: $\text{NaOH} + \text{HCl} \rightarrow \text{NaCl} + \text{H}_2\text{O}$
pH: Greater than 7.
Indicators:
Turn red litmus paper blue.
Phenolphthalein: Pink in basic solutions.
Examples:
Strong bases: NaOH,KOH\text{NaOH}, \text{KOH}NaOH,KOH
Weak bases: NH3,C5H5N\text{NH}_3, \text{C}_5\text{H}_5\text{N}NH3,C5H5N