Melting and crystallization diagram of a pure substance
6.3.1.5
Safety precautions
Naphthalene is harmful if swallowed.
May cause cancer.
Is further very toxic to aquatic organisms and can have long-term harmful effects in bodies of water.
Equipment
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Cobra4 Wireless Manager
Cobra4 Wireless-Link
Cobra4 Sensor-Unit Chemistry
Software Cobra4 –
Single user and school licence
Holder for Cobra4 with support rod
Immersion probe NiCr-Ni, steel, –
50…1000 °C
Retort stand,
210 mm × 130 mm, h = 500 mm
Right angle clamp
Universal clamp
Ceran protection plate,155 mm × 155 mm
Holder for ceran protection plate
®
Glass beaker DURAN , short, 250 ml
12600-00
12601-00
12630-00
14550-61
12680-00
13615-03
37692-00
37697-00
37715-00
33281-00
33283-00
36013-00
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Glass beaker DURAN , short, 600 ml
Powder spatula, steel, l = 180 mm
Teclu burner, DIN, natural gas
Safety gas tubing, DVGW, sold by metre
Hose clip for 12–20 mm diameter tube
Lighter for natural/liquified gases
Boiling chips, 200 g
Test tube, 180 mm × 18 mm, 100 pieces
Glass rod, boro 3.3, l = 200 mm, d = 5 mm
Naphthalene, white, 250 g
36015-00
47561-00
32171-05
39281-10
40995-00
38874-00
36937-20
37658-10
40485-03
48299-25
Additional material
1 PC with USB port, Windows XP or higher
Water
Ice
Fig. 1: Experimental set-up for recording the heating and cooling curves of naphthalene
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6.3.1.5
Melting and crystallization diagram of a pure substance
Notes
Naphthalene (s)
R: 22 – 40 – 50/53
S: 36/37 – 46 – 60 – 61
Naphthalene forms colourless crystals that have a peculiar smell. It is insoluble in water but dissolves
easily in most organic solvents. It can be sublimed and burns with a luminous and very smoky flame.
Risks: Harmful if swallowed. May cause cancer. Is very toxic to aquatic organisms, can have long-term
harmful effects in bodies of water.
Safety precautions: Avoid contact with skin and eyes. Do not breathe dust. When using wear suitable
protective clothing, gloves and eye protection. If swallowed immediately seek medical advice and show
packaging or label. Dispose of naphthalene and/or container as hazardous waste. Do not release to the
environment. Ask for special instructions / Consult the safety data sheet.
First aid: If swallowed immediately seek medical advice and show packaging or label.
Waste disposal: Collect flammable, halogen-free organic solvents in an appropriately labelled container.
Principle
When a pure substance is heated or cooled, the temperature of it does not continually increase or decrease when it undergoes a change in the state of aggregation. Instead of this, and despite the continuing external supply or removal of heat respectively, the temperature of it remains constant until the
change in phase has been completed. Two examples of this behaviour are shown in this experiment.
Fig. 2: Experimental set-up for recording a heating curve of ice / water.
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Melting and crystallization diagram of a pure substance
6.3.1.5
Preparatory work
Plug the Cobra4 Sensor-Unit Chemistry on the Cobra4 Wireless-Link and use the holder for Cobra4
to fasten it to the stand rod (see Fig. 1).
Connect the immersion probe to input T1 of the Cobra4 Sensor-Unit Chemistry.
Start the PC and Windows®.
Connect the Cobra4 Wireless Manager to the USB port of the computer.
Start the “measure” software package on the PC.
Switch the Cobra4 Wireless-Link on. The sensor is now automatically recognised and is allocated an
ID number (01) which can be seen in the display of the Cobra4 Wireless-Link. Communication between the Cobra4 Wireless-Manager and the Cobra4 Wireless-Link is shown via the Data LED.
Load the “Melting and freezing point curves of pure substances” experiment. (Experiment > Open
experiment. All pre-settings that are necessary for measured value recording are now loaded.
Part 1 – Melting and freezing point curve of naphthalene
Set-up and procedure
Set the experiment up as shown in Fig. 1.
Fill 200 ml of water into the 600 ml beaker and add a few boiling chips.
Transfer approx. 2 ml of naphthalene to one of the test tubes.
Fit the temperature probe so in this test tube that the probe tip is completely surrounded by naphthalene.
Use a universal clamp to position the test tube in the water bath.
Gently heat the water with the burner.
Click on
in the icon strip to start measurement.
When a temperature of approx. 90 °C has been reached, stop measurement with a click on in the
icon strip.
Transmit the measured values to “measure”.
Click on “File” and “Save measurement” to save the measured data for evaluation after all measurements have been made.
Remove the water bath and click on
to have the cooling curve for naphthalene recorded.
When a temperature of approx. 50 °C has been reached, stop measurement with a click on in the
icon strip.
Transmit the measured values to “measure”.
Click on “File” and “Save measurement” to save the measured data for evaluation after all measurements have been made.
Part 2 – The heating curve of ice / water
Set-up and procedure
Set the experiment up as shown in Fig. 2.
Put ice that has been crushed as small as possible in the 250 ml beaker and add a little water.
Fit the temperature probe so in the ice / water mixture that sufficient contact is made with the liquid.
Additionally add a few boiling chips.
Gently heat the mixture with the burner while gently stirring it with the glass rod.
Click on
in the icon strip to start measurement.
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6.3.1.5
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Melting and crystallization diagram of a pure substance
When the water markedly begins to boil, stop measurement with a click on in the icon strip.
Transmit the measured values to “measure”.
Click on “File” and then on “Save measurement” to have the measured data saved in the “measure”
sub-programme. All measured data can now be evaluated.
Results
The temperature curves recorded during heating and during cooling do not slope evenly upwards and
downwards respectively. Instead of this, the graphs show 3–5 portions in which the temperature remains
constant for some time.
Evaluation
Melting point of naphthalene
A look at Fig. 3 shows that there are two portions in which the temperature hardly changes. At the
start, the temperature does not increase from the approx. 23 °C although the water is being heated
by the burner. This is because the heat supplied by the burner must first be conducted to the inside
of the test tube and this takes a little time.
Subsequent to this, the temperature increases until about 80 °C is reached. During measurement
recording, it could be observed that the naphthalene starts to melt at this point. The temperature remains constant over the whole melting process. It first increases again in the test tube when all of
the naphthalene has melted.
This process can be described on a molecular level as follows: The supply of energy increases the
natural oscillation of the particles in the crystal lattice (solid naphthalene). The higher the temperature, the stronger the particles oscillate around their rest position, i.e. the energy supplied is converted into energy of motion. At a certain temperature, however, the oscillations are so strong that
particles leave the crystal structure, i.e. the crystal disintegrates and begins to melt. The temperature at which this occurs is called the melting point. During the melting process, the whole of the energy supplied is used to separate the individual particles from the crystal structure. The temperature
therefore remains constant during this process (see Fig. 3, horizontal course of the curve at 80 °C).
The further supply of energy only again leads to an increase in temperature when all of the particles
have left the crystal, i.e. the crystal has completely disintegrated.
Fig. 3: Melting point curve of naphthalene
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Melting and crystallization diagram of a pure substance
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6.3.1.5
Note: The apparent plateau at 95 °C in Fig. 3 results because this temperature is very near to the
boiling point of water, so that here heat is only very slowly transported from the water to the naphthalene. When the recording of the measured points is made at a constant time interval, then it appears as though the increase in temperature continually lessens.
Freezing point of naphthalene
The freezing process is analogous to the melting process but the other way around. When a melt
cools, the oscillation amplitude of the individual particles decreases with the decreasing temperature
(see Fig. 4, portion 1). It is hereby possible for the melting temperature of the substance to be gone
below without freezing taking place, giving what is called a supercooled melt. Spontaneous solidification then raises the temperature back to the melting temperature of the substance (Fig. 4, portion 2), whereby the particles again cluster to a crystal structure. The heat energy that was required
to melt the substance is hereby released as latent heat of fusion. This counteracts a further cooling
effect and leads to a constant temperature during the following course of the process (Fig. 4, portion 3). The amount of heat that is released during freezing is hereby exactly the same as the
amount that was required for the sample to melt. When no further heat of fusion is released because
the freezing process has been completed, then cooling again occurs due to a thermal interaction
with the environment (Fig. 4, portion 4).
Fig. 4: Freezing point curve of naphthalene
Melting of ice and evaporation of water
The melting process of water (see Fig. 5, horizontal course of the curve at 0 °C) corresponds principally to the melting process of naphthalene (see above).
The vaporizing process of water does not fundamentally differ from the melting process. Here also,
only a transition from one state of aggregation to another takes place. The energy of motion of the
individual particles that results from the external supply of energy is hereby so large, that they are
able to release themselves from their association in the liquid and pass into the gaseous phase,
where each can move free of the others. During the vaporizing process, the whole of the energy that
is supplied is consumed as heat of evaporation, so that the temperature remains constant (see
Fig. 5, horizontal course of the curve at 100 °C).
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6.3.1.5
Melting and crystallization diagram of a pure substance
Fig. 5: Heating curve of ice / water.
Notes
In general, cooling curves are easier to record than heating curves. In the latter, inhomogeneous
temperature distribution in the liquid phase always leads to greater fluctuations in the temperature
values than those of cooling curves.
Deviations of the temperatures obtained from the measurement from the literature values are mainly
caused by the NiCr-Ni thermocouples used. These can show higher or lower temperatures than the
actual ones because of their production process. Such probes should therefore be initially calibrated
against a different type of temperature measurement device, such as a thermometer, which is
known to show temperatures accurately.
This experiment can also be carried out using Cobra4 Sensor-Unit 2 x Temperature, NiCr-Ni (article
number 12641-00) instead of the Cobra4 Sensor-Unit Chemistry.
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