Freezing and Boiling Point Graph Worksheet with Questions

Let’s go step by step to answer each question based on the heating and boiling point graph.

The graph shows temperature (°C) on the vertical axis and energy added on the horizontal axis. The line goes up, then flat, then up again — this is typical for a substance being heated: it warms up, melts (stays at same temp), warms more, boils (stays at same temp), then warms as gas.

We’ll label the parts of the graph mentally:
- From start to first flat part: solid warming up
- First flat part: melting (solid → liquid)
- Rising after first flat: liquid warming up
- Second flat part: boiling (liquid → gas)
- Rising after second flat: gas warming up

Now let’s answer each question:

1. What is the freezing point of the substance?
Freezing point = melting point (same temperature). Look at the first flat part — that’s where melting happens. The temperature there is 5°C. So freezing point is also 5°C.

2. What is the boiling point of the substance?
Boiling point is the second flat part — where liquid turns to gas. That’s at 15°C.

3. What is the melting point of the substance?
Same as freezing point — 5°C.

4. What letter represents the region where the solid is being warmed?
Before any phase change — from start to point B (first flat part starts). So that’s segment A→B. Letter: A

5. What letter represents the region where the liquid is being warmed?
After melting, before boiling — between end of first flat (point C) and start of second flat (point D). So segment C→D. Letter: C

6. What letter represents the region where the vapor is being warmed?
After boiling — after point E. Segment E→F. Letter: E

7. What letter represents the melting of the solid?
Melting happens during the first flat part — constant temperature while solid turns to liquid. That’s segment B→C. Letter: B

8. What letter represents the vaporization of the liquid?
Vaporization (boiling) happens during the second flat part — liquid turning to gas. Segment D→E. Letter: D

9. During which letters do changes in potential energy occur?
Potential energy changes during phase changes — when bonds are breaking/forming, not when temperature rises. So during melting (B) and boiling (D). Letters: B and D

10. During which letters do changes in kinetic energy occur?
Kinetic energy changes when temperature changes — so when the line is going UP. That’s segments A, C, E. Letters: A, C, E

11. “Heat” refers to what kind of energy?
Heat is thermal energy — energy transferred due to temperature difference. Answer: thermal energy

12. Which letter represents condensation?
Condensation is gas → liquid — opposite of vaporization. On this graph, we’re adding energy (heating), so condensation isn’t shown directly. But if we were cooling, it would be the reverse of D. Since the graph only shows heating, condensation doesn’t appear. However, sometimes questions expect you to know that condensation occurs at the same temp as boiling — so if forced to pick a letter associated with that temp, it’d be D (but technically not shown). Wait — actually, since the graph is about *adding* energy, condensation (releasing energy) isn’t plotted. But maybe they want the letter corresponding to the boiling/condensing temperature — which is D. Let’s say D — because condensation happens at the same temp as boiling.

Wait — let me double-check #12. In many textbooks, even though the graph shows heating, they ask “which letter represents condensation?” meaning “at which stage would condensation occur if we reversed the process?” So yes — condensation occurs at the boiling point temperature, which is represented by segment D. So answer: D

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Final Answer:

1. 5°C
2. 15°C
3. 5°C
4. A
5. C
6. E
7. B
8. D
9. B and D
10. A, C, E
11. thermal energy
12. D