You are presented with the quiz "What changes a solid into a liquid using heat energy?" But you don't know the answer, so you turn to your teacher to ask for hints. He says that "melting means changing from a solid into a liquid by adding heat energy" and "melting is a phase change". So, what's the best answer to the question?

(1). electricity.
(2). It gets heated up.
(3). boiling.
(4). aqueous solution.
(5). a phase change.
(6). a hydraulic system.
(7). heating liquids.
(8). Water expanding.
The answer is:
(5).