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[MUSIC PLAYING]

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PROFESSOR: Throughout your
laboratory experiences,

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you will frequently need to
assess the purity or identity

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of a crystalline solid.

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Both of these goals can be
accomplished by determining

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a compound's melting point.

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When a compound is in a
crystalline solid state,

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the intermolecular forces
that are holding the molecules

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together greatly outweigh
the kinetic energy that

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is trying to pull them apart.

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Therefore the molecules appear
to vibrate in a fixed location.

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If energy is introduced into
the system in the form of heat,

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the kinetic energy
of the molecules

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and thus the temperature
of the system is increased.

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At a certain point, the
molecular kinetic energy

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is high enough to overcome
the attractive forces that are

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holding the molecules together.

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As a result, the crystal
lattice breaks down

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and the molecules
enter a liquid state.

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The temperature at which
the solid and liquid phases

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of a compound are in equilibrium
at a certain pressure

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is called the melting point.

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In reality, a solid melts
over a temperature range

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instead of at a
specific temperature.

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Because this range is a
physical characteristic,

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it is reproducible
for a pure compound.

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For example, N-phenylacetamide
melts between 114.2 and 114.9

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degrees Celsius.

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Therefore the term melting point
really implies a melting range.

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Although a variety of
melting point apparatuses

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are commercially
available, this video

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will demonstrate the
determination of melting point

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by using the Mel-Temp.

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The Mel-Temp apparatus consists
of an on/off switch, a sample

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holder, a light source that
illuminates the sample,

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an eye piece that magnifies the
sample, a digital thermometer,

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which indicates the
temperature of the system,

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and a voltage control.

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The voltage control
dictates the rate

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of heating with a higher setting
corresponding to a faster rate.

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The voltage control
does not control

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the temperature of the system.

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Use caution when
touching the Mel-Temp

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and always assume that
the aperture is hot,

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especially the part that
is labeled "caution.

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Unit hot."

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The first step in
determining a melting point

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is the preparation of
the crystalline solid.

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To prepare the sample, you will
need a spatula, a watch glass,

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a mortar and pestle,
capillary tubes that

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are sealed at one end, and a
long piece of glass tubing,

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such as a long stem funnel.

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Before proceeding, ensure
that the crystalline solid

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is clean and dry.

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If the solid is wet,
you can try to pat

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it dry with filter paper.

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Do not place the
sample in the oven

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where it may melt or decompose.

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If your sample consists
of large crystals,

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you must first crush them into
smaller crystals with a mortar

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and pestle.

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Large crystals cannot
pack together well,

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which creates air pockets.

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If air pockets are
present in your sample,

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then a time lag
for heat transfer

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will exist, causing the
crystals to heat unevenly.

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If you do not have
a mortar and pestle,

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you can use the flat
end of a spatula

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to grind the sample instead.

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After crushing the crystals,
place them on a watch glass.

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Turned the capillary
tube upside down

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and thrust the end
over the crystals.

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A small amount of
crystals should be

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trapped inside the capillary.

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Invert the tube to its
correct orientation

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and pack the crystals
down to the bottom

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by gently stroking the tube or
by tapping it on your benchtop.

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If the crystals fail to fall
to the bottom of the capillary,

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then drop the capillary through
a length of glass tubing.

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A long stem funnel works
well for this application.

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You may have to repeat
this action several times

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to ensure that the crystals
are effectively packed down.

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Loose material
contains air pockets

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which cause the sample
to heat unevenly.

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Ideally, the smallest
manageable amount of material

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should be used, which correlates
to a depth of approximately 1

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millimeter of packed material.

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If the depth exceeds
2 millimeters,

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inaccuracies may result
due to uneven heating.

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Prepare four capillary
tubes of your sample.

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One filled capillary tube
will be used to determine

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a crude melting point.

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And the other three will
be used to determine

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your actual melting
point in triplicate.

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If you do not know the expected
melting point of your compound,

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you must first determine
a crude melting point.

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Finding the approximate
melting point of a sample

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will save a lot of
time when performing

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the actual measurement.

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To determine the
crude melting point,

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set the voltage on
the melt temp to zero

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and wait until the aperture
has cooled to room temperature.

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Insert a sample.

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Increase the temperature at
a rate of 10 to 20 degrees

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Celsius per minute and
record a rough temperature

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when the sample melts.

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To determine the melting
point of a crystalline solid,

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set the voltage on
the Mel-Temp to zero,

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turn on the power and
the digital thermometer,

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and then insert a
filled capillary tube

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into the sample holder.

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If the temperature is lower
than 20 degrees Celsius

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below the expected
melting point,

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then dial the voltage
to a setting that

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enables the
temperature to increase

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at a rate of 10 to 20
degrees Celsius per minute.

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After the temperature
is within 20 degrees

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Celsius of the
expected melting point,

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dial down the voltage so that
the temperature increases

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at a rate of 1 to 2
degrees Celsius per minute.

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Watch the sample
through the eyepiece

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and record the temperature
when the first crystal starts

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to melt, which means when
the first droplet of liquid

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appears.

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Finally, record the
temperature when

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the last crystal disappears.

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Turn down the voltage,
remove the melted sample,

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and throw it into the
proper waste receptacle.

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Allow the Mel-Temp apparatus
to cool to 20 degrees

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below the melting
point of the compound

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and repeat the process two
more times with the remaining

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samples to ensure
reproducibility.

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Never remelt a
compound because it

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may have undergone a
chemical change while

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heating such as
oxidation, decomposition,

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or rearrangement.

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The point where the first
crystal starts to melt

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can seem ambiguous to someone
taking a melting point

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for the first time.

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As stated earlier,
this is the point

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where the first droplet
of liquid forms.

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It is not when the
material begins

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to settle, soften, or shrink.

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Only the appearance
of a liquid droplet

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indicates the
beginning of melting.

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One common use for
melting point is

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to assess the purity
of a known compound.

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If an impurity is
present in a sample,

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it will interrupt
the crystal lattice

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and weaken the forces
of attraction that are

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holding the molecules together.

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Therefore the sample will
melt at a temperature

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that is lower than expected.

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The sample will also melt
over a wide temperature range.

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As a general guideline,
if a compound

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melts over a range greater
than 2 to 3 degrees Celsius,

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it is most likely impure.

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Another common application
of melting point

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is the identification of an
unknown crystalline solid.

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To identify a solid, simply
compare the melting point

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of the unknown with the melting
points of known compounds

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that are listed in
a reference book.

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Typically, just the upper limit
is reported in these reference

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books.

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When combined with other
spectroscopic data,

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you may be able to
identify your compound.

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To confirm that your compound is
indeed what you think, prepare

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three samples
containing mixtures

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of your unknown compound
and the proposed compound.

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20 to 80, 50/50, and 80 to 20
are good ratios to start with.

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Determine the melting
point of each mixture.

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If all the mixtures melt
at the same temperature

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and within the same range, you
may have correctly identified

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your compound.

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If they do not,
then you definitely

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have not identified
your compound.

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There are several ways to
obtain inaccurate melting point

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readings.

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The most common mistake is
increasing the temperature

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too rapidly.

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In this situation,
the capillary will not

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have enough time to equilibrate
with the heated block.

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As a result, the melting
range will appear narrower

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than it actually is and the
melting temperature will

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appear higher.

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A sample that
disappears upon heating

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indicates that it is
subliming instead of melting.

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To remedy this problem, seal
the open end of the field

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capillary with a Bunsen
burner before placing it

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in the sample holder.

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A sample that changes
color upon heating

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is probably undergoing
decomposition.

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Decomposition must
be reported along

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with the melting
point of the compound.

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If the melting point is still
incorrect after troubleshooting

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or if you need to confirm
the accuracy of your result,

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you must calibrate the
Mel-Temp apparatus.

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To do this, find pure compounds
with known but dissimilar

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melting points.

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This video will use
naphthalene, N-phenylacetamide,

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salicylic acid,
and succinic acid.

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Determine the melting
point of each pure compound

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and compare the value you
received with what is expected.

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Plot the actual melting
point of each compound

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against its observed
melting point.

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Use the resulting
calibration curve

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to determine the expected
melting point of your compound.

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Determining the correct melting
point of a crystalline solid

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involves good technique
in both sample preparation

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and in taking the melting point.

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To ensure accurate results,
use a minimal amount of solid,

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pack the crystals
tightly together,

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and increase the
temperature slowly.

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Determining the melting
point of a crystalline solid

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is the best way to
assess its purity

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or establish its identity.

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[MUSIC PLAYING]