Question

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in this graph, the substance initially starts out as a solid
at point a, the temperature of the substance is increasing
at point b, the temperature of the substance is
during this time, the substance changes from a
at point c, the temperature of the substance is
at point d, the temperature of the substance is
during this time, the substance changes from a
at point e, the temperature of the substance is

Explanation:

Step1: Analyze Point B

In a heating curve (for a substance changing state), horizontal segments represent phase changes (melting, boiling) where temperature is constant. Point B is on a horizontal segment (between 150 - 350 sec approx), so temperature is constant. During this time, since the substance started as solid, it's melting (solid to liquid).

Step2: Analyze Point C

Point C is on an increasing slope (after the first horizontal segment), so temperature is increasing. The substance is now in liquid phase (after melting at B) and being heated.

Step3: Analyze Point D

Point D is on a horizontal segment (500 - 750 sec approx), so temperature is constant. This is the boiling phase, changing from liquid to gas.

Step4: Analyze Point E

Point E is on an increasing slope (after the second horizontal segment), so temperature is increasing. The substance is now in gas phase and being heated.

Answer:

• At point B: temperature is constant; substance changes from solid to liquid.
• At point C: temperature is increasing.
• At point D: temperature is constant; substance changes from liquid to gas.
• At point E: temperature is increasing.