What causes toxic metal leaching?

• A. Heat causes metal to expand.
• B. Oxygen forms rust on metal.
• C. Acids react with the metal.
• D. Water dissolves metal.

Answer: (C)

Toxic metal leaching occurs when the metal is dissolved into water as ions, and the strongest driver of that dissolution is acidic conditions. When an acid encounters a metal, a reaction happens that forms soluble metal ions and hydrogen gas (for example, a metal reacting with hydrogen ions to produce metal ions in solution). Those metal ions then move with the water, spreading through soil or groundwater.

Heat can speed up reactions but doesn’t by itself ensure metals enter solution. Oxygen can cause surface corrosion and form oxides (rust), but that oxide layer isn’t the same as dissolving the metal into water. Water alone may not dissolve many metals unless aided by acids or complexing agents. So the key mechanism that best explains leaching of toxic metals is acids reacting with the metal to form soluble ions.