Idea Transcript
Determination of the Identity of an unknown liquid
Group #
My Name The date My period Partner #1 name Partner #2 name Purpose: The purpose of this lab is to determine the identity of an unknown liquid by measuring its density, melting point, boiling point, and solubility in both water and alcohol, and then comparing the results to the values for known substances. Procedure: 1) Density determination Obtain a 10mL sample of the unknown liquid using a graduated cylinder Determine the mass of the 10mL sample Save the sample for further use 2) Melting point determination Set up an ice bath using a 600mL beaker Obtain a ~5mL sample of the unknown liquid in a clean dry test tube Place a thermometer in the test tube with the sample Place the test tube in the ice water bath Watch for signs of crystallization, noting the temperature of the sample when it occurs Save the sample for further use 3) Boiling point determination Set up a hot water bath using a 250mL beaker Begin heating the water in the beaker Obtain a ~10mL sample of the unknown in a clean, dry test tube Add a boiling stone to the test tube with the unknown Open the computer interface software, using a graph and digit display Place the temperature sensor in the test tube so it is in the unknown liquid Record the temperature of the sample in the test tube using the computer interface Watch for signs of boiling, noting the temperature of the unknown Dispose of the sample in the assigned waste container
4) Solubility determination Obtain two small (~1mL) samples of the unknown in two small test tubes Add an equal amount of deionized into one of the samples Add an equal amount of ethanol into the other Mix both samples thoroughly Compare the samples for solubility Dispose of the samples in the assigned waste container
Observations: The unknown is a clear, colorless liquid. There is a detectable odor, but it does not lend itself to identifying the unknown. No crystallization was ever detected in the melting point determination. The unknown started as and remained a clear, colorless liquid. During the boiling point determination, bubbles started to form around the boiling chip at first, and then the entire sample began to bubble vigorously at a steady temperature. There was no observable difference in the solubility test for the unknown either with water or ethanol. Data: Density determination Mass of sample = 7.86 g Volume of sample = 10.0mL Melting point determination Lowest temperature reached for unknown in ice bath = 4.0oC. Boiling point determination Bubbles first formed at 72oC; a steady boil was reached at 78oC. Please see the graph for further details. Solubility test Upon mixing the unknown with ethanol, there are no distinct phases. Upon mixing the unknown with water, there are no distinct phases. Data Analysis: Calculations Density determination density =
=
= 0.786 g/mL
Figure 1.1 Boiling point determination
Conclusion: After careful consideration of all the test results and all the possible identities of the unknown, the experimenters determined the unknown was, in fact, ethanol. Figure 1.2 Properties of potential identities of the unknown liquid Substance Acetone t-butanol Cyclohexane Ethanol 2-propanol unknown V = very soluble
Density (g/ml) 0.79 0.79 0.78 0.79 0.79 0.79
Freezing Point (oC) -95.0 25.5 6.5 -117.0 -89.5 undetermined
S = slightly soluble
Boiling Point (oC) 56.4 82.8 80.7 78.8 82.4 79.0
Solubility* Water Ethanol V V V V I V V V V V V V
I = insoluble
The experimenters considered the density test and the boiling test the most reliable due to the procedures used and the sensitivity of the equipment involved. After carefully calculating the density of the unknown, all the potential substances with similar or identical identities were considered (figure 1.2). As the unknown sample showed no sign of crystallization at temperatures as low as 4oC, t-butanol and cyclohexane were eliminated as possible identities, as they would have both frozen at temperatures above 4oC. In addition, cyclohexane would give different solubility test results than the unknown did. Acetone would reach its boiling point at a temperature far lower than that at which the unknown boiled at, so it too was eliminated from consideration. And as 2-propanol would have to reach 82.4oC to reach a steady boil, while the unknown sample never reached 80oC (figure 1.1), it was eliminated as well. Thus, the experimenters determined the identity of the unknown must be ethanol.