Students in an upper-level physical chemistry course used a polarimeter to measure the molar optical rotation of different optically active amino acids to confirm the Clough, Lutz, and Jirgensons (CLJ) rule. This rule states that an amino acid is in the l-configuration if the molar optical rotation becomes more positive upon the addition of acid or the acid is of the d-configuration if the optical rotation becomes more negative in an acidified solution. Students demonstrated this rule for known configurations of amino acids and then applied this rule to determine the configuration of an amino acid with an unknown configuration.