Melting and Boiling Points

Melting and Boiling Points

Date: 02/13/2015

Objective:

     To determine the purity of a compound, by illustrating the principles of melting points and boiling points with a simple apparatus.

Pre-Lab questions:

     Pre-lab questions were attached on the first page.

Introduction:

    -Melting points are defined as the temperature at which solid and liquid states coexist. If a solid is pure, it has a “sharp melting point”, and the range is usually from 0.5 °C to 1 °C. If the solid has a wide melting point range (>4 °C), it will be regarded as an impure substance. The melting point of an impure solid is usually lower than the accepted value.

    -Boiling points are defined as the temperature at which the liquid and gas states coexist. The boiling point of an impure liquid is usually higher than the accepted value.

    -The melting points of pure solids and the boiling points of pure liquids are usually used to figure out the purity of a compound, since their values are often used to point out the purity of a substance.

List of equipment:

     p-Dichlorobenzene; Acetone; Ethanol; Test tubes; Two-hole corks; Vernier interfaces; Temperature probes; Weighing papers; Capillary tubes, Hallow glass tube; melting point apparatus; Spatula; Beaker; Water; Hot plate; Ring stand/clamp; Boiling chips.

Procedure and observations:

     Part A. Melting Point Determination

     A dime size sample of p-dichlorobenzene was presented on a piece of paper. The large crystals were grinded into a powder by using a spatula. Then the power of crystals was filled in the end of the capillary by pushing the open end of a capillary tube. About ¼ inch of crystals were filled into the capillary, and then the crystals were shifted to the bottom of the capillary by using the hallow glass tube. This capillary was labeled as No.1. Then another capillary was filled with another sample, acetamide. Same way, the sample was grinded and ¼ inch of sample was filled in the bottom of the capillary. This capillary was labeled as No.2. Again, the mixture of p-dichlorobenzene and acetamide was grinded into powder and the same amount (1/4 inch) of mixture sample was filled into the bottom of another capillary, and this capillary was labeled as No.3. All of three capillaries were inserted into the melting point apparatus, and then the temperature probe and the vernier interface were placed with the melting point apparatus. At that time, the observation was started. When the first droplet of each capillaries were appeared, that temperatures were collected on the data sheet; then the half samples of each capillaries were melted, that data was collected on the data sheet also; finally, when the last droplet of each capillaries were produced, the temperatures were put on the data sheet.

     Part B. Boiling Point Determination Using a Temperature Probe

     Around ¾ amount of water was placed into the beaker. A hot water bath was set with a clamp and a ring stand. The hot plate was placed on the ring stand, and then the test tube was filled to ½ inch with the liquid, acetone. Then three particle of boiling chips were added into the test tube. The two-hole cork was placed into the test tube; then the temperature probe was inserted into one of the holes of the cork (the other hole was left unblocked). At the same time, the above of the test tube was clamped. The test tube was lower into the hot water bath, and the heating was started. The liquid in the test tube was observed; when the liquid started boiling and the temperature reading was kept as constant, the heating was stopped and that temperature was collected on the data sheet. Next, the acetone and the ethyl alcohol were mixed together, and were filled into another test tube. This mixture liquid was heated by repeating the same steps as before. When the mixture liquid started boiling, the heating was stopped and the data was collected on the data sheet.