If mixing two aqueous ionic solutions forms a white precipitate, what does this indicate about the ions?

• A. They form a soluble compound.
• B. They form a gas.
• C. They form an insoluble compound (a precipitate).
• D. No reaction occurs.

Answer: (C)

When two aqueous solutions mix and a solid appears, it means an insoluble salt has formed. Ions in solution can combine in different ways, and if the new compound is not soluble in water, it will come out as a solid—this is a precipitation reaction. A white solid is a common example of such a precipitate, though the color can vary with different salts (for instance, AgCl is a white precipitate). An example helps: silver ions and chloride ions meet and form solid silver chloride, while the other product remains dissolved in solution.

So the visible solid shows that a reaction occurred and produced an insoluble compound. If everything stayed dissolved, there would be no precipitate; if a gas formed, you’d see fizzing; and if no reaction happened, nothing would change.