EXPERIMENT #1
Boiling Point and Melting Point Determination
I. Objectives
1. To be familiar with the principles of melting point and boiling point determinations.
2. To master the method of determination of melting point and boiling point using
capillary tube.
3. To determine the melting point and boiling point of organic solids and liquids.
II. Introduction
The determination of physical properties of organic compounds—such as boiling points,
melting points, density, solubility, and refractive index—is crucial for their characterization
and identification. In this experiment, you will learn techniques for measuring the melting
point of solids and the boiling point of liquids.
The boiling point is defined as the temperature at which the vapor pressure of a liquid
equals the pressure at its surface. This point can vary with changes in surface pressure.
Since pure substances have distinct boiling points, these measurements are often used
to assess purity.
A solid is considered to melt sharply if its melting point range is between 0.5 and 1.0 °C.
Pure solids typically melt sharply because the forces of attraction between their particles
are uniform. However, foreign particles in the crystal lattice disrupt this uniformity,
weakening these forces. Consequently, an impure solid melts at a lower temperature and
over a wider range. Thus, melting point is a valuable indicator of both identification and
purity in organic compounds. The melting point is reached when the thermal energy of the
particles overcomes the inter-crystalline forces holding them together.
In organic chemistry, which focuses on carbon-containing compounds, the melting point
signifies the transition from solid to liquid. This experiment will specifically explore the
melting points of organic compounds like naphthalene and benzoic acid.
In contrast, a liquid is characterized by its disordered arrangement of particles, allowing
them to move freely. A liquid boils when its thermal energy is sufficient to overcome the
cohesive forces holding its particles together. This occurs when the vapor pressure of the
liquid equals atmospheric pressure, which is significant for understanding a compound’s
physical properties and structural characteristics.
Vapor pressure is influenced by the kinetic energy of the molecules, which depends on
temperature, mass, and velocity. As temperature increases, so does the average kinetic
energy of the particles. When the boiling point is reached, this energy is sufficient for
molecules to escape the liquid state and enter the gas phase.
The boiling point of a liquid varies with atmospheric pressure; higher pressure results in a
higher boiling point. The normal boiling point serves as an indicator of volatility: a higher
boiling point signifies lower volatility, while a lower boiling point indicates higher volatility.
At atmospheric pressure, compounds with lower normal boiling points generally exist as
gases, whereas those with higher boiling points are likely to be liquids or solids.
III. Materials/ Reagents
(6) Capillary Tubes
Cooking Oil
(1) Thermometer
0.5 g Urea
(1) Bunsen Burner
0.5 g Benzoic Acid
(1) Watch Glass
0.5 g Benzoic Acid-Urea Mixture
(1) Stirring Rod
0.5 mL Isopropyl Alcohol
(5) rubber band
0.5 mL Acetone
(1) Iron Ring, Iron Stand, Wire Gauze
0.5 mL Acetic Acid
(1) Universal Clamp
(2)-5-mL test tube
(1) 50-mL Beaker
IV. Procedure
A. Melting Point Determination
1. Sealing Capillary Tubes
Put one end of a prepared capillary tube to the edge of a small hot flame and
rotate the capillary tube slowly so that the end is sealed. To ensure the capillary
tube sealed compactly and smoothly, you should operate it carefully and not make
it bended or into a small ball.
2. Filling capillary tubes.
Place a small sample of the solid pulverized finely on a clean watch glass, then
collect the solid into a small mound and push the open end of the capillary tube
down into the sample. The solid may be forced down by dropping the tube (sealed
end downward) through a long length of ordinary glass tubing onto the desktop.
Further increments of the sample are introduced in the same way until the material
forms a compact column 3-5 mm high at the bottom of the tube after repeated
dropping. It is essential that the material be packed firmly and densely into the end
of the tube.
Follow the method above: fill three tubes with benzoic acid, urea, and mixtures
of unknown sample.
3. Arranging assembly.
Introduce oil bath liquid into a 50-mL beaker and keep the bath level at the position
of the two- third of the beaker. The capillary tube containing the sample is attached
to a thermometer by means of a small rubber band. The rubber band must be kept
well above the level of the hot oil, or the oil could melt the rubber and break the
band. For accurate reading, the sample compound in the capillary tube is kept
close to and at the level of the thermometer bulb, which is fully submerged and
centered in the oil bath, as shown in Figures 1-2.
4. Taking melting point
When the apparatus has been arranged, properly heat the bottom portion of the
side arm with burner. Convection currents carry the heated oil up through the side
arm and down the main shaft of the apparatus for uniform heating. Apply heat at
a moderately rapid rate until the bath liquid is within 15-20 °C of the melting point.
Continue the heating with a very small flame adjusted so that the temperature
rises slowly and at a uniform rate (about 1 °C per minute). If necessary, hold the
burner by its base and move it back and forth under the bath. Observe carefully
the samples in the melting point tube and the thermometer reading. Record as the
observed melting point the range between the thermometer reading when sample
starts to liquefy and that when the melt is clear. After the samples have melted,
extinguish the flame and allow the bath to cool.
Capillary Tube
Oil Solid sample
Figure 1. Oil bath assembly
Figure 2. Test Tube assembly
B. Boiling Point Determination
1. Make a test tube assembly by using the following directions and illustration in
figure 1 and 2.
2. Place about 1 mL of Isopropyl alcohol in a 10-12 mm diameter test tube.
3. Using a small rubber band, attach a thermometer to the outside of the test
tube. The thermometer bulb should be even with the test tube's bottom.
4. Insert an inverted closed end capillary tube into the test tube.
5. Make an oil bath assembly similar to Figure 1.
6. Place the above test tube assembly in the oil bath so that the surface level of
the alcohol in the test tube is beneath the surface level of the oil bath.
7. Heat the oil bath carefully and observe the stream of bubbles emerging from
the capillary tube.
8. Remove the heat source and when the last bubble emerges from the capillary
tube, record the temperature.
Clean-up:
Dispose of the used capillary tubes by putting them in your trash can. If any isopropyl
alcohol is left in your test tube, you may pour it down the drain with water.
DATA AND ANSWER SHEET
1. MELTING POINT DETERMINATION
SAMPLE T1 (⁰C) T2 (⁰C) Average THEORETICAL %
MELTING ERROR
POINT
Benzoic Acid
Urea
Benzoic Acid-
Urea Mixture
2. BOILING POINT DETERMINATION
SAMPLE T1 (⁰C) T2 (⁰C) Average THEORETICAL %
BOILING ERROR
POINT
Isopropyl
Alcohol
Acetone
Acetic Acid
QUESTIONS
1. Which of the following solid samples has the largest melting point range? Explain.
2. Which of the three liquids has the highest boiling point? Explain.
3. How does the melting point of a mixture compare with that of the individual
components? Explain why this occurs
4. Compare and contrast the boiling points of isopropyl alcohol, acetone, and acetic acid
based on their molecular structures. How does intermolecular bonding influence these
differences?
5. Compare your experimental results to the theoretical values. What does the % error
suggest about the accuracy of your experiment?
6. How can melting point and boiling point data be used to identify unknown organic
compounds
