Characteristic Properties

Introduction

One important aspect of science is to isolate and identify a pure substance. You can be identified by your fingerprints, which are unique to you. In this experiment you will examine some physical properties -- freezing and boiling points -- of an unknown liquid and use them to help you determine the identity of the unknown.

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Freezing Point

1. At your lab station, you will find a ring stand with ring, wire gauze, and a clamp.
2. Place enough ice in a 250-mL beaker to fill it half full and cover the ice with water. Place the beaker on the wire gauze.
3. Obtain a test tube containing an unknown liquid from your teacher and clamp it to the ring stand so that it is immersed in the ice bath.
4. Carefully place a thermometer in the test tube.
5. Stir the liquid gently with the thermometer and watch for crystal formation. Wait until about half of the solid freezes.
6. Warm the test tube until the liquid begins to melt and the temperature remains steady. When the crystals first appear, record the temperature to the nearest 0.2 degrees. This is the freezing point (the melting point) of the liquid.

Recorded Freezing Point: ______________

Boiling Point

1. Use the same sample and bath to determine the boiling point. Remove the ice and water, half fill the beaker with tap water, and replace it on the wire.
2. Place a small capillary tube into the test tube, sealed end up. Attach the thermometer to the outside of the test tube with a rubber band so that the bottom of the test tube is even with the thermometer bulb.

   

3. Clamp the test tube, with the attached thermometer, to the ring stand. Adjust the clamp so that the bottom of the assembly is about 1 cm from the bottom of the beaker.
4. Carefully light the Bunsen burner and place it under the bath.
5. Heat the water bath slowly, stirring it with a glass stirring rod, until capillary tube in the unknown liquid produces a very rapid steady stream of bubbles.
6. Turn off the Bunsen burner and keep stirring the bath until the stream of bubbles just stops and the liquid enters the small tube. This is the boiling point of the liquid. Carefully read and record the temperature to the nearest 0.2 degrees.

   Recorded Boiling Point: ______________

7. Return your sample to your teacher.

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Define the following terms:

Pure Substance: ___________________________

_______________________________________

_______________________________________


Freezing Point: ____________________________

_______________________________________

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Boiling Point: ____________________________

_______________________________________

_______________________________________

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Compare your data for the unknown liquid with the reference data provided by your teacher. Identify the unknown liquid and explain how you made your decision. (Remember, experimental error may cause slight differences in some values)

1. What was the accepted freezing point of the liquid?
2. Calculate the absolute error for your freezing point.
3. What was the accepted boiling point of the liquid?
4. Calculate the absolute error for your boiling point.
5. Discuss the possible reasons for the errors.
   

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Safety

• Wear safety goggles and aprons in the laboratory at all times. Bunsen burners and hot water baths can cause burns. Use caution when handling them. Use hot plates if the substances studied are flammable; do not use open flames .
• Cork test tubes of liquids when not being heated or cooled.

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A pure substance is an element or compound.

The freezing point is the temperature at which a liquid is in equilibrium with its solid. (It is not the temperature at which the liquid first starts to freeze; many liquids supercool, and form unstable supercooled liquids which may spontaneously freeze. The normal freezing point is the freezing temperature at one atmosphere pressure.)

The boiling point is the temperature at which a liquid is in equilibrium with its vapor. (It is not the temperature at which the liquid first starts to boil; a few liquids superheat, and form unstable superheated liquids which may spontaneously and explosively boil. The normal boiling point is the boiling temperature at one atmosphere pressure.)

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The absolute error is the difference between the literature and the experimental value of the temperature. Experimental readings tend to be low for the freezing temperatures. (Use fresh tertiary butyl alcohol when possible; when it gets wet, the freezing temperature is greatly lowered.)

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Cyclohexane and Butyl alcohol should be collected in containers in the hood for proper disposal.

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Materials (per student)

• ring stand, wire gauze, and a clamp
• 250-mL beaker
• ice
• test tube containing 1-2 mL of unknown liquid (cyclohexane - mp 6.5 °C, bp 80.7 °C; tertiary butyl alcohol - mp 25.6 °C, bp 82.6 °C; )
• thermometer
• capillary tube
• rubber band
• Bunsen burner
• glass stirring rod
• reference table
  

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Take two sealed bottles of tert-butanol. Place one in a refrigerator. Wait until it freezes. Remove it from the refrigerator, and set it alongside the liquid. Ask students to explain to result. (Usually the liquid remains liquid and the solid remains solid.) When students claim that the bottles are contaminated, switch the bottles. Place the liquid in the refrigerator, and melt the solid by holding it in your hand.

(The freezing temperature of tertiary butyl alcohol is near room temperature. The heat flow depends upon the difference in temperature between the system and its surroundings. Since this is small, it takes a long time for the frozen material to melt at room temperature.)

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Cyclohexane and tertiary butyl alcohol are flammable. Use these liquids with a flameless heat source. Use only in a well ventilated room. Dispense only the amount indicated, 1-2 mL per student. Check that fire extinguishers are available and charged.

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