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Narrative
A basic solution of the milk protein casein is titrated with 2 molar hydrochloric acid. The solution begins to turn cloudy as the pH decreases. The protein precipitates about pH 6, (which is the isoelectric point). Continued addition of hydrochloric acid causes the protein to redissolve.

Discussion
Proteins are made of amino acids whose side chains sometimes contain acidic or basic groups. At the isoelectric point, the protein is uncharged and displays minimum water solubility. At high pH, the protein carries a negative charge, and at low pH a positive charge. These ionic forms of the protein are water soluble.