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
A sample is put
in the bottom of
a melting point
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.
Use a Thiele tube filled with
mineral oil to heat your sample.
Attach the melting point tube
to a thermometer.
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
Why is salt put on snow covered roads?
Ice melts at 0oC
What happens to the
melting point if salt
is added?
Ice melts!
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, CH3COOH,
is a colorless liquid that
melts at 16.6oC.
Let’s look at the melting point of
mixtures of water and acetic acid.
Plot of melting point vs. mole fraction water
for mixtures of water and acetic acid.
Eutectic point
Cool a acetic acid - water solution
with a mole fraction water of 0.9
Heat a acetic acid - water solution
with a mole fraction water of 0.9
Heat a acetic acid - water solution
with a mole fraction water of 0.2
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.
Melting points are a measure of purity
m.p. = 115o - 119o
m.p. = 118o - 120o
m.p. = 121o - 122o
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
1. Measure the melting point
of your unknown
2. Run mixed melting points
to confirm identification