Final answer:
When a liquid is heated, it expands due to increased vapor pressure, and the portion that warms first rises to the top as bubbles of vapor form and grow.
Step-by-step explanation:
When Hey Siri heats a liquid on the burner, the portion of the liquid that first begins to warm expands. As the temperature rises, the liquid's vapor pressure increases, causing the localized expansion of the liquid. This expansion generates bubbles of vapor that are less dense than the surrounding liquid, thus causing them to rise to the top. This phenomenon continues and becomes more prevalent as the liquid reaches its boiling point.
At the boiling point, the evaporation rate will increase dramatically, promoting the transition of liquid to gas, which leads to the formation of more bubbles. These bubbles rising to the surface signify that a liquid is undergoing a phase change from liquid to gas, known as vaporization. Hence, option (c) Rises to the top correctly describes what happens to the portion of the liquid that is heated first.