Melting Points and Mixed Melting Points
Experiment 1: Identify a compound by its melting
point and mixed melting points.
Acetamide
113 - 115 oC
p-Aminobenzoic acid
188 - 189 oC
Camphoric Acid
183 - 186 oC
trans-Cinnamic Acid
133 - 134 oC
Malonic Acid
135 - 137 oC
p-Nitrophenol
113 - 115 oC
Resorcinol
110 - 113 oC
Succinic Acid
187 - 189 oC
Urea
133 - 135 oC
Melting Point Tube
A sample is put in
the bottom of a
melting point tube.
Filling Tube
Put a small
amount of the
compound in the
open end of the
melting point
tube.
Turn over and tape the closed end on the desk
top until the compound falls to the bottom.
Sample in the melting point tube.
Thiele Tube
Use a Thiele tube filled with mineral
oil to heat your sample.
Using Thiele Tube
Attach the melting
point tube to a
thermometer.
Heating Rate
Heat about 5o per minute
until within about 10o of
the melting point.
Near the melting point
heat at 1 - 2o per minute
Mel-Temp in the lab
Temperature
Starts to melt
Finished melting
m.p. = Start - Finish
Salt on Roads
Why is salt put on snow covered roads?
Ice melts at
What happens to the
melting point if salt
is added?
Ice melts!
o
0C
Impurities
Impurities such as salt lowers the melting
point of water.
Putting salt on icy roads causes the ice to
melt because it lowers the melting point of
water.
Impure compounds usually melt lower than
pure compounds so the melting point may
be used as a measure of the purity of a
compound
Acetic Acid
Acetic Acid, CH3COOH,
is a colorless liquid that
melts at 16.6oC.
Let’s look at the melting point of
mixtures of water and acetic acid.
Eutectic Point
Plot of melting point vs. mole fraction water
for mixtures of water and acetic acid.
Eutectic point
Cool
Cool a acetic acid - water solution
with a mole fraction water of 0.9C
Heat
Heat a acetic acid - water solution
with a mole fraction water of 0.9
Heat
Heat a acetic acid - water solution
with a mole fraction water of 0.2
Pure - Impure
Pure compounds usually melt over
a narrow temperature range, often
1o or less.
Impure compounds melt lower
than pure compounds and over a
wider temperature range.
Purity
Melting points are a measure of purity
m.p. = 115o - 119o
m.p. = 118o - 120o
m.p. = 121o - 122o
Unknown
Two of these bottles contain benzoic
acid and one m-nitrophenylacetic acid.
m.p. = 120o - 122o
m.p. = 120o - 122o
m.p. = 120o - 122o
How do you tell what is in each bottle?
Mixed Melting Points
Grind samples together to be sure they are
mixed and then measure the melting point.
Results
1
2
3
Mixed 1 and 2
m.p. = 120o - 122o
Mixed 1 and 3
m.p. = 114o - 117o
Mixed 2 and 3
m.p. = 115o - 118o
Summary
1. Measure the melting point
of your unknown
2. Run mixed melting points
to confirm identification