Which statement is not true under low pH conditions?

• A. Ammonia is less toxic to fish
• B. Metals tend to precipitate
• C. Cyanide and sulfide are more toxic to fish
• D. Acid mine drainage conditions exist

Answer: (B)

Low pH makes water acidic, which pushes many metals into dissolved, highly mobile forms. When metals are dissolved, they’re more bioavailable and can more readily enter fish, increasing toxicity. Precipitation, in contrast, happens as pH rises and dissolved metals form solid hydroxides or carbonates, causing them to drop out of solution. So under low pH, metals tend to stay dissolved rather than precipitate, making the statement about precipitation not true in acidic conditions.

Other points fit the scenario of acidic water: ammonia shifts toward ammonium, which is less toxic to fish; the uncharged, more toxic forms of cyanide and sulfide (like HCN and H2S) are favored at low pH; and acid mine drainage is a classic source of low-pH conditions.