A2 Melting point of Naphthalene
Definition
When a solid substance is heated, typically it will melt; that is to say, at some temperature the solid will
begin to liquify and by some slightly higher temperature all of the solid will have become liquid. The melting
point (actually melting point range) of a compound is then defined as the temperature at which an observer
can first see liquid forming from the solid to the temperature where the last particle of solid has become
liquid.
For example, the melting point of pure sucrose (table sugar) is 185o-186oC. This means that as a small
sample of sucrose is slowly heated some of the crystals begin to liquefy at 185o and all of the crystals have
become liquid by 186oC. Sometimes only the second number (completely melted) is reported as the melting
point. In general, this is not a good idea and should be avoided.
Purpose
There are several purposes for doing this experiment. The first is to learn how to determine the melting
range of a solid substance accurately. When a solid substance is prepared its melting point is usually
determined to aid in its identification and to get some idea of its purity. The second is to observe the effect of
the purity of a substance on its melting behavior. Pure substances usually have melting point ranges of a
degree or two; impure substances (which are mixtures of two or more substances) often have wider ranges.
Thus, a sharp melting point is often taken as prima facie evidence that a substance is pure, while a wide
range suggests that it is not. It is often, but not always, true that an impure substance will have a lower
melting point than the same substance when it is pure. Thus, pure benzoic acid has a melting point of 122-
123oC, while an impure sample might have a melting point of, say, 115-120oC. The third is to use a
physical constant of an unknown substance – its melting point – to identify it from among several
possibilities. In most cases, the melting point alone will not enable you to identify a compound. Millions of
solid organic compounds, and their melting points, are known. Perhaps 10,000 of these will have the same
melting point as your unknown compound. You don’t know what the compound is yet, but you have
narrowed the field. In this experiment you will attempt to determine the identity of a compound, from among
ten contenders, based on its melting point.
Determination of Melting Points – Practical Aspects
First, the term “melting point” is a misnomer; what we really mean is melting range, the temperature from
which you first observe liquid in the sample to the temperature at which “all of the solid” has just melted.
Whenever – now and in the future – you are asked to take a “melting point”, you must determine and record the melting range.
[Remember – the two reasons for determining melting point are compound identification and estimation of
purity. If you do not report the range, you have provided no information about the latter and the former is
compromised since the melting point of an impure substance is usually different from the pure substance.]
Melting points are usually determined by placing one or two milligrams of the material to be tested into a
melting point capillary, and heating the capillary and a thermometer together, and observing over what
temperature range the material melts. The melting point capillary is a thin-walled glass tube, about 100 mm
in length and not more than 2 mm in outside diameter, sealed at one end.
Filling the Capillary Tube
Make sure the sample is dry. [This is not an issue in this experiment, but in the future you will want to
determine the melting point of a solid you have just separated from a liquid by filtration. If you try to do this
while the solid is damp it is impure and the melting point will reflect this.] Place a small spatula tip full of the
sample on a watch glass and pulverize it with the spatula until it is a powder. Form the powder into a mound
and push the open end of the capillary into the mound several times to force some of the powder into it. You
want between 1 and 2 millimeters of sample, lengthwise, in the capillary. Holding the melting point tube
between your thumb and forefinger, strike the closed end against the bench top perpendicularly until the
material is firmly packed against that end of the tube. Repeat this process if you do not have enough sample
(1-2 mm) in the tube. If it is difficult for you to force the sample from the open end to the closed end, try
dropping the capillary onto a hard surface (closed end down) through a 4 foot section of glass tubing. It is
surprising how well this works.
The sample should be well packed or it will not melt evenly. Insert your (dry) thermometer into one of the
small rubber bands you will find on the side bench. Push the rubber band up the thermometer so it will be
able to hold the capillary, near its open end, to the thermometer with the closed
end next to the thermometer bulb (see Fig. 1). Pinch the rubber band and slide
the top of the melting point tube under it; then adjust the position of the
capillary. Now you need to insert the bulb end of the thermometer, along with
the attached capillary tube, into oil that is contained in an apparatus known as
a Thiele tube. You will need to clamp the Thiele tube and the top of the
thermometer to a ring stand with clamps.
Both the Thiele tube and thermometer should be vertical. The thermometer
should be centered in the Thiele tube with its bulb below the side-arm of the
Thiele tube. The thermometer and capillary tube must not touch the Thiele
tube (see Fig. 1). (If your clamp will not hold your thermometer, you can attach
the thermometer adapter and clamp to the rubber piece of that. Safety tip: Be
sure to lubricate the top of the thermometer with a small amount of
stopcock grease before attempting to slide the adapter onto the
thermometer. Do not force the adapter or you may wind up breaking the thermometer and inserting its stub into your hand.)
The Thiele tube is warmed using a micro burner which heats the sidearm elbow (see Fig. 1). This causes the oil to flow around the tube and past the sample and thermometer bulb by convection, thus warming the sample and thermometer. To obtain an accurate melting point you must heat the oil at a rate of 1 to 2 degrees per minute as you approach and go through the melting range of the sample. If you heat faster your result will be in error because the bulb of the thermometer will not warm up as quickly as the oil, but your sample will; consequently, the thermometer and sample would be at different temperatures – not a good situation. If you have no idea what the melting point of your sample is, you have two choices at this point.
(1) You could start at room temperature and warm at the recommended rate until the solid melts. This could take more than an hour.
(2) You could start at room temperature and warm at about 10 degrees a minute and obtain an approximate melting point. Then, after the apparatus has cooled below the sample’s melting point, replace the sample (and capillary) with a new one, and heat slowly until the sample has melted, thus getting an accurate melting point. [Sometimes materials decompose as they melt, so you should use a new sample.] Neither of these options may thrill you, but the second is better, unless you have happened to have a low melting solid.
Procedures
Determination of Melting Point of Naphthalene
If your thermometer is wet, dry it. If there is water on your thermometer the oil in the Thiele tube will become
cloudy because oil and water are not miscible. Using a Thiele tube, determine the melting point (actually
melting range) of naphthalene
You will not need more than a small spatula tip full of sample since melting point requires only a couple of
milligrams of material. Record your data on the report sheet. Follow the instructions above for using the
Thiele tube. Remember that you should not allow the temperature to rise faster than 1º to 2º per minute as a
sample is melting. If you go faster than this the thermal lag of the more massive thermometer will be greater
than that of the capillary and they will not be at the same temperature! Too rapid heating is the most
common melting point mistake made by neophyte chemists.
Your setup should look like Figure 1 above.
N. B. Do not try to cool down a hot thermometer or Thiele tube using water. The thermometer will crack and the Thiele tube might.
Cleanup
You should dispose of used capillary tubes in a box on the side bench or in the broken glass box. You
should never put glass you are disposing of in the wastepaper baskets or regular garbage cans.
The mineral oil that we used to heat up the sample must be collected as per teacher instructions. You need
to wash the thiele tube with soap and water, return it to the storing box and return that to the teacher.
LAB REPORT
TITLE:
Purpose of the lab:
Hypothesis: IF applicable…in an “IF…THEN STATEMENT”
MATERIALS: (Listed in columns and Bulleted )
PROCEDURE: (Listed and Numbered)
LAB SETUP: (Diagram of the lab setup with labels
showing the apparatus and the chemicals- ON COMPUTER PAPER)
DATA TABLES, CHARTS ETC (On graph paper onlyUSE RULER) showing in this case the time and temperatures recorded for the heating and cooling curves
OBSERVATIONS: (Anything relevant observed during the lab)
CONCLUSION: (In a paragraph format) It must include the following answers:
- WHAT WERE THE MAJOR FINDINGS?
- WHAT POSSIBLE EXPLANATIONS CAN YOU OFFER FOR YOUR FINDINGS?
- WHAT RECOMMENDATIONS DO YOU HAVE FOR FURTHER STUDY AND FOR IMPROVING THE EXPERIMENT?
- WHAT ARE SOME POSSIBLE APPLICATIONS OF THE EXPERIMENT?
QUESTIONS AND CALCULATIONS FOR THE LAB (SHOW YOUR WORK):
