P6.3
Student practical
Name ......................................................................
Class ..................
Date ......................
Measuring the melting point of a substance
Specification references:
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
P3.2.3 Changes of heat and specific latent heat
MS 4a, 4c
WS 2.4, 2.6, 2.7, 3.1, 3.2, 3.5
Aims
In this practical, you will monitor the temperature of a substance as it changes
state, and discover the main features of the heating curve of a pure substance.
Learning outcomes
After completing this practical you should be able to:
 state the meanings of ‘melting point’ and ‘boiling point’
 state that the melting and boiling points of a pure substance are fixed
 describe the processes of melting and boiling
 use the term ‘latent heat’
 find the melting or boiling point of a substance using a graph
 describe how the melting and boiling points of a substance can be changed
 evaluate data produced by a heating experiment to discuss the reproducibility
of the measurement of a boiling point.
Safety
Wear eye protection in case of hot liquid splashes.
Read the CLEAPSS hazcard for precautions on handling stearic acid
(octadecanoic acid), CH3(CH2)16COOH.
Clear up any spillages immediately, to prevent slipping.
Do not try to remove the thermometer from the solidified stearic acid, as this could
break the thermometer.
Equipment
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

Heating apparatus (Bunsen burner,
heatproof mat, tripod and gauze)
250 cm3 beaker
Stirrer
© Oxford University Press 2016


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Clamp stand with clamp
Boiling tube containing a thermometer in solid
stearic acid
Stopwatch
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1
P6.3
Student practical
Name ......................................................................
Class ..................
Date ......................
Method
1
2
3
4
5
6
7
Heat about 150 ml (150 cm3) of water in the beaker until it is just before the
point of boiling.
Adjust the Bunsen burner so that the water stays close to boiling. (If the water
starts boiling at any time, slightly reduce the heat from the Bunsen burner.)
Secure the boiling tube containing the stearic acid using a clamp stand and
carefully lower it into the boiling water, as shown in the diagram. It should not
be touching the bottom of the beaker.
Start the stopwatch and record the temperature of the stearic acid.
Every 30 s, carefully stir the water and then record the temperature of the
stearic acid, until the temperature reaches about 80 °C. (Once the stearic acid
has melted, you can also stir the stearic acid using the thermometer. When
measuring temperature, do not let the thermometer bulb touch the sides of the
boiling tube.)
Turn off the Bunsen burner.
Wait for the apparatus to cool before clearing it away.
Results
Record all results in a suitable table.
Questions
1
2
Plot a line graph of time on the x-axis against temperature on the y-axis,
remembering to include units in the axis labels.
(6 marks)
Describe your graph, including any key features.
(3 marks)
© Oxford University Press 2016
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This resource sheet may have been changed from the original.
2
P6.3
Student practical
Name ......................................................................
3
a
Class ..................
Date ......................
What was the resolution of the thermometer?
(1 mark)
b
What was the resolution of the stopwatch?
(1 mark)
4
Describe and explain one precaution you took to make your thermometer
reading as accurate as possible.
(2 marks)
5
The accurate melting point of pure stearic acid is 69 °C, to the nearest degree.
Comment on the accuracy of your measurement.
(2 marks)
6
By comparing the results of different groups, comment on the reproducibility of
the melting point measurement.
(2 marks)
7
Suggest two reasons why the melting point of stearic acid measured in this
experiment may be inaccurate.
(2 marks)
Student follow up
1
Define the ‘melting point’ and the ‘boiling point’ of a pure substance.
(2 marks)
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3
P6.3
Student practical
Name ......................................................................
2
Class ..................
Date ......................
Describe what happens to the temperature of a pure substance when it is
being heated, and:
a it is not changing state
(1 mark)
b
it is changing state.
(1 mark)
3
A piece of ice is taken from a freezer, put into a saucepan and heated until the
saucepan is empty. The graph shows the temperature against time for the
ice/water. Match the following labels to the letters A–D on the graph:
water heats up
ice melts
ice warms up
water boils
(4 marks)
4
A bowl of pure water at room temperature (20 °C) is placed in a freezer
(whose temperature is −18 °C) until the water has completely frozen. Sketch
a graph of temperature against time for the water/ice as it cools down to the
temperature of the freezer.
(4 marks)
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4
P6.3
Student practical
Name ......................................................................
5
Class ..................
Date ......................
Usually when heat is transferred to a substance, the temperature of the
substance increases. However, while a substance is melting or boiling, its
temperature does not change, even though heat is being transferred to it.
What is the scientific term used for the heat transferred in this situation?
(1 mark)
Extension
6
Describe any changes that happen to the motions, positions and bonding of
particles in a pure substance when:
a the substance is solid and getting hotter (but not yet melting)
(2 marks)
b
the substance is melting
(2 marks)
c
the substance is liquid and getting hotter (but not yet boiling)
(1 mark)
d
the substance is boiling
(2 marks)
e
the substance is gas and getting hotter.
(1 mark)
7
a
If an impurity is introduced to a pure substance, that lowers its melting point.
Use this fact to explain why we sometimes scatter salt on icy roads or paths.
(2 marks)
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5
P6.3
Student practical
Name ......................................................................
b
Class ..................
Date ......................
Suggest why scattering salt on icy roads or paths might not be useful if the
temperature is a long way below 0 °C.
(1 mark)
© Oxford University Press 2016
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This resource sheet may have been changed from the original.
6