Report- Identification of Substances by Physical Properties The purpose of this lab was to see how certain substances; Naphthalene, Toulene, and and 2 unknowns (one liquid, one solid) react with 3 different solvents. To identify the two unknowns, testing needs to be done to find the density of both the liquid and the solid, determine the melting point of the solid, and the boiling point of the liquid. The Physical Properties of Pure Substances Table can then be used to compare the observed results with the accurate properties from the table.
If testing is done correctly, accurate assumptions can be made to figure out what the unknown substances were. Introduction Being able to identify unknown substances by observing their physical properties is very thought provoking. By using such properties as melting point, boiling point, density, solubilty and refractive index of unknown substances identification is possible. Scientists have gathered and recorded data on the physical properties of pure substances for well over 100 years.
It is these data tables that allow us to do experiments on unknown substances and make reasonable assumptions as to the correct substance being investigated.
By using such physical properties as solubility, density, melting point, boiling point and refractive index results were narrowed down from 32 possible choices to just 1 or 2. Procedure Testing for solubility of Naphthalene (mothballs): 1. Add a few crystals of Naphthalene to 3 clean, dry test tubes. 2. Add 2-3 mL water to test tube #1, 2-3 mL cyclohexane to test tube #2, and 2-3 mL of ethyl alcohol to test tube #3. . Stir each test tube with separate mixing rods. 4. Record the results. S for soluble, SP for sparingly soluble and I for insoluble. 5. Repeat this experiment using the same 3 solvent: water, cyclohexane, and ethyl alcohol with the unknown solid and unknown liquid. 6. Record the results (refer to #4) Determine the densities of the two unknowns: Density of the solid unknown: 1. Weight about 1-gram of the unknown solid on the weighing paper. Record the mass. 2. Fill a 10-mL graduated cylinder with the solvent (in this case water) about ? way.
Read and record to the nearest 0. 1 mL. 3. Add the weighted solid to the liquid in the cylinder. 4. Measure the cylinder now that the solid has been added to the liquid. 5. Calculate the density of the unknown solid (remember that D=m/v) Density of the liquid unknown: 1. Weigh a clean dry 50-mL beaker to the nearest 0. 0001g. 2. Using a 10-mL pipet put exactly 10. 0 –mL of the unknown liquid in a clean, dry test tube. Transfer the liquid to the beaker and weigh immediately on the same scale used in step one. 3. Record the results. Remembering D=m/v.
Melting point of Solid unknown: 1. Retrieve a capillary tube and small rubber band. 2. On a clean watch glass pulverize a very small amount of the solid unknown with a pestal. 3. Fill the capillary tube by gentle tapping it into the solid. About a fingers height. 4. Tie the filled capillary tube to the bottom of a thermometer with the rubber band, making sure to get the capillary tube even with the red part of the thermometer. 5. Set up melting point apparatus. Hot plate or Bunsen burner. 6. Place the thermometer apparatus into a 250-mL beaker, half filled with water.
Make sure that the bottom of the capillary tube is covered with water. 7. Heat the water and pay attention to the solid in the tube as the water starts to heat. 8. Record the temperature at which the solid just started to melt and also record a melting point range by also recording the temperature at which all of the solid had melted. 9. Record the melting point and the melting point range. Boiling point of Liquid unknown: 1. Put about 3-mL of the material used to determine the density in a clean dry test tube. 2. Insert your thermometer into the test tube .
Position the thermometer about 1-cm above the surface of the liquid. Add a boiling stone. 3. Clamp the test tube to the Ring stand and place onto the hot plate. 4. Heat the water and watch for changes. 5. The temperature will become constant at the boiling point of the liquid. 6. Record the observed boiling point. Using the Refractometer 1. Remove the black cover exposing the sample well 2. Check that it is dry 3. Add the unknown sample to the well so that the entire surface of the well is covered 4. Replace the black cap and press Measure. 5.
The instrument will equilibrate the sample and run the refractive index. 6. Record the value Results A. Solubility | Water| Cyclohexane| Alcohol| Naphthalene| I| S| SP| Toluene| I| S| S| Liquid unknown| S| S| S| Solid unknown| S| SP | I| B. Density Solid Final volume of liquid in cylinder| 6. 6 mL| Initial volume of liquid in cylinder| 5 mL| Volume of solid| . 6 mL| Mass of solid| . 998 g| Density of solid 1. 6 g/mL Liquid Volume of liquid| 10 mL| | | Mass of 50-mL beaker plus 10-mL of unknown| 46. 669 g| Mass of 50-mL beaker| 38. 95 g| Mass of liquid| 7. 67 g| Density . 767 g/mL C. Melting point of solid unknown Observed melting point| 58 C| Corrected | | Observed melting-point range| 58-69 C| Corrected| | D. Boiling point of liquid Unknown Observed| 85 C| Corrected| | Estimated true b. p. | | E. Unknown Identification Solid unknown| Sodium acetate – 3H20| Liquid unknown| Isopropyl alcohol| | Substance| Density| Melting point| Boiling point| Water| Solubility cyclohexane| Alcohol| Refractive index| Table| | | | | | | | | Obsvd| | | | | | | | | Table| | | | | | | | |
Obsvd| | | | | | | | | Discussion Determining the solubility of the Naphthalene and the Toulene was interesting, I assumed that Naphthalene would not dissolve in alcohol, yet it partially did. Toulene, I was not familiar with but was surprised that it was not soluble in water. What I did enjoy doing was trying to determine the unknown substances by applying physical properties to them. To be able to find the density, find a melting point or a boiling point, and use the refractive index to first record the observed results and then be able to ompare them to Table 1, the Physical Properties of Pure Substances Chart and actually be able to determine what the unknown substance is based on a comparison of the observed data compared to the chart. One problem we encountered was trying to decide the melting point of our unknown solid, we waited to long to record the initial melting state I believe. I also believe that we did not let the liquid unknown reach it’s full potential on boiling point. Sources and Citations Lab handout- Identification of Substances by Physical Properties. Report sheet- Identification of Substances by Physical Properties.
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