Question

1. which of the following is always true when a substance undergoes a phase change?

a. a new substance with new properties will form.
b. the substance becomes a liquid as it heats up.
c. the melting point of the substance changes.
d. the substance is made up of the same type of molecules.

Explanation:

• Option A: Phase change (like melting, freezing, vaporization) is a physical change, no new substance forms. So A is false.
• Option B: Heating a substance can cause it to become gas (e.g., water vaporizing) or solid (if cooling, but heating here—e.g., heating ice to liquid, but heating liquid can go to gas). So it’s not always liquid. B is false.
• Option C: The melting point of a pure substance is constant (e.g., water melts at 0°C at standard pressure). Phase change occurs at the melting point, which doesn’t change. C is false.
• Option D: A substance (element or compound) undergoing phase change is made of the same molecules (e.g., H₂O in ice, liquid, gas—same molecules). So D is always true.

Answer:

D. The substance is made up of the same type of molecules.