Melting Point Lab
CLAB 263
Physical and Chemical Properties
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All chemical compounds have both physical and
chemical properties
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Chemical properties – how a compound or element
interacts with another compound or element
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A change in composition of the interacting compounds
occurs
Example: Combustion reaction
CH4 + 2 O2  CO2 + 2 H2O
Photosynthesis
6 CO2 + 6 H2O  C6H12O6 + 6 O2
Physical properties – can be measured or
observed; no change in chemical composition
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Two types of physical properties: intensive and
extensive
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Intensive – independent of the amount of material present
(density, melting point, boiling point)
Extensive – dependent upon the amount of material present
(mass)
Melting points (Freezing points) –
intensive property

The temperature at which a substance changes from the solid state into
the liquid state. An equilibrium exists between liquid state  solid
state.
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Organic Compounds
 Organic compounds have a lower melting point than inorganic (ionic)
compounds.
 Interactions between organic compounds are weaker than the ionic
attraction between ions in inorganic salts.
 Interactions are intermolecular (between molecules) vs. intramolecular
(between atoms/ions in a compound)
Inorganic Salts
 High melting points due to the intramolecular force of attraction
 Not combustible due to intramolecular forces
 Melting points of organic compound will also vary with size and branching
of the compounds
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Determining if a compound is organic…
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Combustion of Organic Compounds
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All organic compounds combust
This property can be used to determine if a compound is
organic.
If a flame is applied
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compound burns with no residue left, compound is organic.
compound doesn’t burn, it is not a carbon compound
Compound burns but leaves a residue, it may be organometallic (calcium propionate, a food preservative)
Observations at Melting Points

The sample will appear wet. This is called sweating.
Note this temperature as this is the beginning of the
melting point range.

With continued heat more solid will melt until no solid
left only liquid. Note this temperature as this is the
melting point.

Melting point range – temperature range between the
beginning of the melt (sweating) and the melting point.
Melting Point Ranges
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Generally pure crystalline compounds have
sharp or narrow melting point range.
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Two compounds can have the same melting
point.
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Two things can affect this sharpness:
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Poor technique
Impurities
Areas that are affected by technique
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Size of crystals
Amount of material being heated
Density of packing
Rate of heating
Avoid adverse effects due to
poor technique
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Size of crystals and density of packing
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Crystals should be small and tightly packed. This will
avoid a difference in the heat applied to the sample at
different points
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Capillary tube – use ~0.5 mm of material
Hot plate apparatus – use smallest amount of material that fits
into the center of the stage
HEAT SLOWLY
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Temperature should not rise more than 1-2 oC per
minute
If you have a good technique and still the melting
range is not narrow, your sample is probably not
pure…
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Impurity lowers the melting point and widens the
melting range!
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Equilibrium vapor pressure of the melt are affected by
impurities just as a solute* affects the vapor pressure of
a solution.
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*Colligative properties – vapor pressure lowering, boiling point
elevation, freezing point depression, osmotic pressure
Caution
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Melting Point Apparatus can be very hot!
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Capillary tubes and glass slides can be very hot!
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Place all used capillary tubes in Glass Disposal
Box.
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Allow the melting point apparatus to cool at least
10o below the observed melting point of your
compound before repeating procedure.
TODAY
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Get a melting point on pure benzoic acid, fast
ramp, then a slow ramp.
Unknown – Get a “fast” melting point, b/c this will
allow you to have a general idea what the
melting point is. Allow the apparatus to cool to
approximately 10o below the initial sweat. Then
repeat SLOWLY two times.
Get a melting point on impure benzoic acid, fast
then slow.