Final answer:
The key step in extracting magnesium from seawater involves the reaction: MgCl₂ + Ca(OH)₂ → Mg(OH)₂ + CaCl₂, producing solid magnesium hydroxide and aqueous calcium chloride (b).
Step-by-step explanation:
The key step in the novel process of obtaining magnesium from seawater involves a reaction between solid calcium hydroxide and dissolved magnesium chloride. This reaction yields solid magnesium hydroxide and aqueous calcium chloride. The balanced chemical equation for this key step is:
MgCl₂ + Ca(OH)₂ → Mg(OH)₂ + CaCl₂
This corresponds to the process described where seawater, which contains magnesium chloride, is treated with calcium hydroxide to precipitate magnesium hydroxide. Subsequently the magnesium hydroxide can be treated with hydrochloric acid to leave behind magnesium chloride, which can then be electrolyzed to obtain magnesium metal.