A weak acid in water
Grey = undissociated CH₃COOH, red = H₃O⁺, blue = CH₃COO⁻. Illustrative, not to scale: in 0.1 M acetic acid only about 1 molecule in 75 is ionised; add CH₃COO⁻ (its salt) and you have a buffer.
Grey = undissociated CH₃COOH, red = H₃O⁺, blue = CH₃COO⁻. Illustrative, not to scale: in 0.1 M acetic acid only about 1 molecule in 75 is ionised; add CH₃COO⁻ (its salt) and you have a buffer.