Hardness in water is primarily due to which ions, and how does it affect soap use and aquatic organisms?

• A. Sodium and potassium; increases soap lather.
• B. Calcium and magnesium; reduces soap lather but has no effect on aquatic life.
• C. Chloride and sulfate; has no effect on soap.
• D. Calcium and magnesium; reduces soap lather and can affect some aquatic life and piping.

Answer: (D)

Hard water gets its name from dissolved calcium and magnesium ions (Ca2+ and Mg2+). These ions come from minerals like calcium carbonate and magnesium carbonate that dissolve in water. When soap is used, these multivalent ions react with the fatty acid part of soap to form insoluble salts such as calcium stearate and magnesium stearate. The solid salts precipitate as scum, so the water lathers less and you need more soap to get bubbles.

These same calcium and magnesium ions also influence water chemistry and can lead to scaling in pipes and appliances, which is an everyday consequence of hardness. For aquatic life, the presence of Ca2+ and Mg2+ affects ion balance and osmoregulation in organisms; while some species tolerate a range of hardness, very hard water can stress certain aquatic organisms and alter their habitat. So the description that best matches both effects—reduced soap lather and potential impacts on some aquatic life and piping—is that hardness is due to calcium and magnesium, which cause soap scum and can influence piping and aquatic ecosystems.