Determining Melting Points Lab

Determining Melt= ing Points Lab

All pure substances are made up of atoms, molecules, or ions. These particles arrange themselves in order to minimize their energy at the temperature and pressure of the environment. The physical form of matter under these condit= ions is referred to as the state of a substance. The state of a substance at a specific temperature and pressure is a physical property of that substance.= In this activity, you will observe melting, a change in state from solid to liquid. You will determine the melting points of several pure substances and identify an unknown substance from its melting point alone.

OBJE= CTIVES

• Determine melting points of several substances, and compare with accept= ed values.

• Identify a substance from its melting point.

HYPO= THESIS:

MATE= RIALS

apron = &nb= sp; goggles

capillary tubes (4) = glass stirring rod

hot plate = &nb= sp; ring stand

100-mL beaker = fine wire

thermometers (2) = thermometer clamp

small rubber bands = pieces of paper (4)

laboratory burner = paper towel

PROC= EDURE

(Steps 1-5 were done for you to save time.)=

1. Place a small sample of a solid on a piece of paper.

2. Seal one end of a capillary tube (about 6 cm long) in the flame of a burner. CAUTION: Do not touch the hot end of the tube.=

3. When the capillary tube has cooled, push the open end into the solid.

4. When you have a small amount of solid in the tube, invert the tube and gently tap the closed end on the table. If the solid does not drop to the bottom, push the sample down the tube with a piece of fine wire.

5. Wipe off the open end of the tube, then seal= it in the burner flame.

(Steps for yo= u to complete)

6. Use a rubber band to attach the capillary tube to a thermometer. The sa= mple should be at the same level as the bulb of the thermometer.

7. Set up a water bath. Put the hot plate on the base of the ring stand, a= nd place the beaker one-half full of water on the hot plate.

8. Put the thermometer and attached capillary tube into the water bath. Cl= amp the thermometer to the ring stand so that it does not touch the bottom or s= ides of the beaker. Your setup should look like the one in the diagram.

9. Heat the water bath, stirring constantly with a glass stirring rod.

10. Observe the crystals of the sample. Quickly note the temperature when t= he crystals melt.

11. Stop heating the water bath and remove the thermometer and capillary tu= be. Cool the tube by running it under tap water.

12. Add cool tap water to the water bath to decrease the temperature noted = in step 10 by about 10°C. Check the temperature with the second thermomete= r.

13. Put the thermometer with the attached capillary tube and sample back in= to the water bath.

14. Slowly heat the water bath again. As the temperature of the water nears= the solid’s melting point, reduce the rate of heating and watch the sample carefully. When the sample melts, have a partner read the temperature to the nearest 0.1°C. Record this temperature in the table under Data and Observations.

15. Repeat steps 1– 14 with the other solids. Record each melting poi= nt in the table.

16. Your teacher will provide you with= the accepted values for the melting points of the known solids. They are record= ed on the table.

DATA= AND OBSERVATIONS

Sample	Melting Point (°C)	Error (%) Observed – accepted X 100 &nbs= p; Accepted
Observed	Accepted
1,4-dichlorobenzene		53.1°
Naphthalene		80.2°
Stearic acid		69.6°
Unknown		80.2°

ANAL= YSIS

1. Calculate the percentage error of each melting point you determined and fill in the last column of the table.

Obser= ved melting point – accepted melting point X 100

Accepted melting point

(numerator must be positive)

CONC= LUSIONS

1. What is the unknown substance?

2. How did you arrive at its identification?

EXTE= NSION AND APPLICATION

Suppose that naphthalene were mixed in with the 1,4-dich= lorobenzene. Do you think the impurity would affect the melting point?

How? Make a prediction. Then, with your teacher’s permission, test your prediction by repeating the activity with a sample of 1= ,4-dichlorobenzene contaminated with naphthalene.