Neutralizing acidic lake water safely with excess addition

Steps:

• Acidic lake water has excess H+ ions harming fish; neutralization requires a base to react with H+.
• The base must be addable in excess without over-alkalizing or toxifying the water.
• Evaluate options: bases react to form water and salts; acids worsen pH; neutrals do nothing.
• Select calcium carbonate, as it buffers mildly and excess remains harmless.

Why A is correct:

• Calcium carbonate neutralizes acids via CaCO3(s) + 2H+(aq) → Ca2+(aq) + H2O(l) + CO2(g), forming non-toxic products; excess is insoluble limestone, safe for aquatic life.

Why the others are wrong:

• B: Phosphoric acid (H3PO4) is an acid, adding more H+ and increasing acidity.
• C: Potassium hydroxide (KOH) is a strong base; excess raises pH too high, harming fish.
• D: Sodium nitrate (NaNO3) is a neutral salt, inert and unable to neutralize acids.

Final answer: A