Melting Points and Mixed Melting PointsExperiment 1: Identify a compound by its meltingpoint and mixed melting points.

Acetamide

p-Aminobenzoic acid

Camphoric Acid

trans-Cinnamic Acid

Malonic Acid

p-Nitrophenol

Resorcinol

Succinic Acid

Urea

113 - 115 oC

188 - 189 oC

183 - 186 oC

133 - 134 oC

135 - 137 oC

113 - 115 oC

110 - 113 oC

187 - 189 oC

133 - 135 oC

A sample is putin the bottom ofa melting pointtube.

Put a small amount of the compound in theopen end of the melting point tube.

Turn over and tape the closed end on the desktop until the compound falls to the bottom.

Sample in themelting pointtube.

Use a Thiele tube filled withmineral oil to heat your sample.

Attach the melting point tubeto a thermometer.

Heat about 5o per minuteuntil within about 10o ofthe melting point

Near the melting pointheat at 1 - 2o perminute

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 themelting point if saltis added?

Ice melts!

Impurities such as salt lowers the meltingpoint of water.

Putting salt on icy roads causes the ice tomelt because it lowers the melting point ofwater.

Impure compounds usually melt lower thanpure compounds so the melting point maybe used as a measure of the purity of acompound

Acetic Acid, CH3COOH,is a colorless liquid thatmelts at 16.6oC.

Let’s look at the melting point ofmixtures of water and acetic acid.

Plot of melting point vs. mole fraction waterfor mixtures of water and acetic acid.

Eutectic point

Cool a acetic acid - water solutionwith a mole fraction water of 0.9

Heat a acetic acid - water solutionwith a mole fraction water of 0.9

Heat a acetic acid - water solutionwith a mole fraction water of 0.2

Pure compounds usually melt overa narrow temperature range, often1o or less.

Impure compounds melt lowerthan pure compounds and over awider temperature range.

m.p. = 115o - 119o m.p. = 118o - 120o m.p. = 121o - 122o

Melting points are a measure of purity

m.p. = 120o - 122o m.p. = 120o - 122o m.p. = 120o - 122o

Two of these bottles contain benzoicacid and one m-nitrophenylacetic acid.

How do you tell what is in each bottle?

Mixed Melting Points

Grind samples together to be sure they aremixed and then measure the melting point.

m.p. = 114o - 117o

m.p. = 120o - 122o

m.p. = 115o - 118o

Mixed 1 and 2

Mixed 1 and 3

Mixed 2 and 3

1 2 3

Results

1. Measure the melting pointof your unknown

2. Run mixed melting pointsto confirm identification