Freezing point and melting point

Topics: Melting point, Liquid, Melting Pages: 7 (1331 words) Published: September 24, 2013


Title
Freezing and Melting Point
Objectives
1. To gain proficiency in constructing a graph and plotting data points. 2. To determine the freezing points of a compound from the graph of decreasing temperature versus time. 3. To determine the melting points of a known and unknown compound. Introduction

Physical changes are the changes in the physical properties of a substance (Moore, Stanitski & Jurs, 2009). Freezing point is the fixed temperature at which a pure liquid converted into crystalline solid. Melting point is the fixed temperature at which a crystalline solid converted to a liquid (Ebbing & Gammon, 2010). The melting point of a solid and the freezing point of its liquid is the same. It is the temperature at which the rate of freezing of its liquid is the same as the rate of melting of a solid under a given applied pressure (Whitten, Davis & Peck, 2009). A change of state involves the removal energy or addition energy to or from the substance (Ebbing & Gammon, 2010). In this experiment, the temperature of the stearic acid is needed to plot on a graph. When the stearic acid turned from liquid into solid and the freezing point of the stearic acid can be determined from the graph. The melting points of stearic acid and unknown compound can also be determined in this experiment. If the temperature of the stearic acid/time data plots on the graph, cooling curve will occur. If the temperature drops and remains unchanged at a certain point, then the freezing point will be found in the graph. If the stearic acid and unknown substance heat from solid form and turn to liquid form, then the melting point of the stearic acid and unknown substance can be found. If the melting point of the unknown compound is determined, then the unknown compound can be found. Materials and Apparatus

1. Wire gauze
2. Thermometer
3. Test tube
4. Mortar and pestle
5. 110 oC thermometer with split cork
6. Capillary tubes
7. Rubber bands
8. 400 mL beaker
9. Stearic acid crystal
10. Stearic acid solid
11. Unknown substance
12. Test tube
13. Thermometer
14. Stopwatch
15. Water
16. Bunsen burner
17. Iron wire
18. Retort stand
Methods
A. Cooling Curve and Freezing Point
1. 300 mL of 50 oC distilled water was added to 400 mL beaker. 2. The apparatus was set up as shown in figure 1.

Figure 1.Change of State Apparatus
3. 6g of stearic acid was obtained in a test tube and the test tube was heated well above 75 oC in the set up apparatus. 4. The test tube was then transferred to the 400 mL beaker of water at 50 oC. 5. The temperature of the stearic acid was recorded in every 30 seconds for ten minutes when the temperature drops until 65.0 oC. B. Melting Point of an Unknown

1. Stearic acid crystal was grinded by using a mortar and pestle. 2. 5 mm of crystal was inserted into the capillary tube by using a tiny iron wire to push it in. 3. The capillary was attached to the thermometer bulb with a rubber band. 4. The apparatus was set up as shown in figure 1.

5. The thermometer was placed into the beaker and heated until it melts. The approximate melting point was observed and the range of temperature was recorded. 6. This experiment was repeated again for trial 2 with a new capillary tube. 7. The experiment was then repeated twice with the unknown powder and result was recorded. Results

A. Cooling Curve and Freezing Point
Temperature
Time
Observation
65.0 oC
0:00
Liquid
60.5 oC
0:30
Liquid
58.0 oC
1:00
Liquid
56.5 oC
1:30
Liquid
55.5 oC
2:00
Solid and liquid
55.5 oC
2:30
Solid and liquid
55.5 oC
3:00
Solid and liquid
55.5 oC
3:30
Solid and liquid
55.5 oC
4:00
Solid and liquid
55.5 oC
4:30
Solid and liquid
55.5 oC
5:00
Solid and liquid
55.5 oC
5:30
Solid and liquid
55.5 oC
6:00
Solid and liquid
55.5 oC
6:30
Solid and liquid
55.5 oC
7:00
Solid and liquid
55.5 oC
7:30
Solid and liquid
55.5 oC
8:00
Solid and liquid
55.5 oC
8:30
Solid and liquid
55.5 oC...
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