Question

which of the following is a sign a chemical reaction has occurred? a solid dissolves in a liquid. a solid forms when two liquids are mixed. a solid melts to a liquid when it is heated. a solid metal changes shape.

Explanation:

To determine a chemical reaction sign, we analyze each option:

• "A solid dissolves in a liquid": Dissolving is often physical (e.g., sugar in water, no new substance).
• "A solid forms when two liquids are mixed": This is a precipitate formation, indicating a new substance (chemical reaction, like double displacement).
• "A solid melts to a liquid when heated": Melting is a physical state change (no new substance).
• "A solid metal changes shape": Shaping metal is physical (no new substance).

Answer:

A solid forms when two liquids are mixed.