Question

the diagram shown is a heating curve for a pure substance. which of the following describes the change in the substance in the period from point x and point y on the curve?
a the substance is changing from a solid to a liquid.
b the substance is in the solid phase, and the temperature of the solid is increasing.
c the substance is in the liquid phase, and the temperature of the liquid is increasing.
d the substance is in the gas phase, and the temperature of the gas is increasing.

Explanation:

• A heating curve shows the phase changes of a substance as heat is added.
• The flat sections of the curve represent phase changes (solid - liquid or liquid - gas).
• Between the flat sections, the substance is in a single phase (solid, liquid, or gas) and its temperature is increasing as heat is absorbed.
• In the period from point X to point Y, the curve is sloping upwards (not flat), so there is no phase change.
• Since the first flat section (left - most flat part) is the solid - liquid phase change (melting), after that (in the sloping part before the next flat section which is liquid - gas phase change (boiling)), the substance is in the liquid phase.

Answer:

C. The substance is in the liquid phase, and the temperature of the liquid is increasing.