Definitions: Acids & bases

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Check out the practice on the Conjugate Acid & Base Pair Topic Page.

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Core Concept

Acids are substances that increase the concentration of hydrogen ions (H+) or donate protons, while bases increase the concentration of hydroxide ions (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 (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.

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Definitions

Arrhenius Definition:

  • Acid: A substance that increases H+ (or H3O+) concentration in aqueous solution.

  • Base: A substance that increases OH− concentration in aqueous solution.

  • Example:

    • Acid: HCl→H++Cl−

    • Base: NaOH→Na++OH−

Brønsted-Lowry Definition:

  • Acid: A proton donor.

  • Base: A proton acceptor.

  • Example:

    • Acid: HCl+H2O→Cl−+H3O+

    • Base: NH3+H2O→NH+4+OH−

Lewis Definition:

  • Acid: An electron-pair acceptor.

  • Base: An electron-pair donor.

  • Example:

    • Acid: BF3

    • Base: NH3

      BF3+NH3→F3B-NH3

Properties of Acids and Bases

Acids:

  1. Taste: Sour (e.g., lemon juice, vinegar).

  2. Reaction with Metals: React with active metals to produce hydrogen gas.

    • Example: Zn+2HCl→ZnCl2+H2

  3. pH: Less than 7.

  4. Indicators:

    • Turn blue litmus paper red.

    • Methyl orange: Red in acidic solutions.

  5. Examples:

    • Strong acids: \text{HCl}, \text{H}_2\text{SO}_4}, \text{HNO}_3

    • Weak acids: CH3COOH (acetic acid), H2CO3​

Bases:

  1. Taste: Bitter and slippery (e.g., baking soda, soap).

  2. Reaction with Acids: Neutralize acids to form water and a salt.

    • Example: NaOH+HCl→NaCl+H2O

  3. pH: Greater than 7.

  4. Indicators:

    • Turn red litmus paper blue.

    • Phenolphthalein: Pink in basic solutions.

  5. 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​,C5​H5​N

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