The energy involved in change of state
4 marks · NormalA group of students heated a small sample of solid naphthalene. They recorded the temperature every minute for 15 minutes. The melting point of naphthalene is 80°C. Their results are shown below.
| Time (min) | Temperature (°C) |
|---|---|
| 0 | 40 |
| 2 | 55 |
| 4 | 69 |
| 6 | 80 |
| 8 | 80 |
| 10 | 80 |
| 12 | 86 |
| 15 | 93 |
Use the table to answer the questions.
For how many minutes did the naphthalene remain at 80°C?
Explain why the temperature stayed at 80°C for this time even though the substance was being heated.
What was happening to the naphthalene during this period?
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) From 6 min to 10 min: 4 minutes min
(b) The naphthalene temperature stayed at 80°C because all energy supplied was used to change state from solid to liquid, not to raise temperature. The energy went towards breaking forces between particles.
(c) It was melting (changing from solid to liquid).
Common mistakes: Counting the time incorrectly; Not mentioning breaking of bonds
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly identifies 4 minutes
M1 (b) · 1 mark · Explains temperature constant due to energy used for state change
B1 (b) · 1 mark · Mentions breaking forces/bonds or molecules separating
B2 (c) · 1 mark · States melting/changing state occurred
Worked solution
a) The temperature was 80°C from 6 min to 10 min = 4 minutes.
b) The temperature stayed the same because all the energy supplied was used to break the forces holding the solid together, so the naphthalene could change from solid to liquid. This is the energy needed to change state, not raise temperature.
c) The naphthalene was melting during this time.