Physical Properties of a Pure Substance, Water

Transcription

1 Physical Properties of a Pure Substance, Water The chemical and physical properties of a substance characterize it as a unique substance, and the determination of these properties can often allow one to determine the identity of an unknown substance. Physical properties such as color, density, freezing point, normal boiling point and refractive index are those properties which can be determined without altering the chemical properties of the substance. That is, after the property has been determined, the material can be recovered in its original state by physical methods. The determination of chemical properties usually involves a chemical process, which changes the properties of the material under study by transforming it into a new substance(s). Thus the identity of the substance is altered in a positive test of a chemical property and the material cannot be recovered in its original state by physical methods. Later you will learn more about chemical properties, but the focus of this experiment is to determine some common physical properties of a known substance, water. This will give you experience at making some routine laboratory measurements such as measuring mass, volume, temperature and refractive index. This experience will be practice for a future experiment in which you will attempt to identify an unknown substance by measuring these four properties of the unknown, comparing your values to those reported in a list of the possible unknown substances and determining the best match for these four properties. Property 1: Freezing Point/Melting Point 1A. Introduction: The freezing point of a pure liquid is defined as the temperature at which the solid and liquid states of the substance can coexist in contact with each other. (The melting point of the solid is similarly defined.) This temperature is an intensive physical property, which is useful in identifying the material. During the freezing process, many pure liquids will first supercool and then crystallize very rapidly with a slight rise in temperature, making it very difficult to determine the temperature at which crystals began to form. In this experiment, you will use a graphical method to avoid this problem, making a cooling curve of time vs. temperature. 1B. Experimental Procedure for Determination of Freezing / Melting Point Prepare a clean, 6-inch test tube by washing it with soap and water, and rinsing it with tap water and then distilled water. Transfer enough water to fill the test tube ~1/3 full. Fill your 250 ml beaker with a mixture of ice and salt (~50 ml). Stir the mixture to make a salt ice bath. Record the temperature of this mixture and check to ensure it drops to at least -5 C. If this temperature is not reached, add more salt and stir until the bath is -5 C or lower. Record the initial temperature of the water in the test tube (to the nearest 0.1 C) before you place the test tube into the ice bath. You may need to warm this water to ~40-45 C to ensure you have enough data points during the initial phase of the experiment. [This is the temperature at time = 0.] Gently stir the water with the thermometer, and the ice bath with the test tube. Read the thermometer temperature every 10 seconds. Keep a written record of temperature vs. elapsed time. Do not remove the tube from the ice bath during this procedure. Note if you can the temperature at which you first observe crystals forming in the

2 liquid. Continue recording the time and temperature until the temperature remains constant (within 0.5 C) for five to six readings. If the thermometer reading will not remain constant for this long you may have too much salt in the ice. Add some more ice to the mixture and try again. In order to make a good graph, you need at least three (3) data points in the initial range where temperature is changing rapidly and 8-10 points in the temperature range near the freezing point where it is changing much more slowly or is giving constant readings. If you do not have this number of data points, repeat the process until you have sufficient data points to create two (2) appropriate cooling curve plots. Sometimes a liquid will supercool and reach a temperature below its freezing point before any crystals are observed. If the liquid reaches -3 C and no solid has been observed, scratch the side of the tube gently with the thermometer to try to initiate crystal formation. If crystallization does begin, the temperature should increase to the true freezing point and stay at that temperature while the sample is freezing. If this does not happen, you may have contaminated the sample and it will not be possible to get a definite freezing point temperature. In this case, dispose of that sample, rinse your test tube again with fresh distilled water and start over with a fresh water sample. If the water is frozen completely, the temperature will begin to drop again, eventually reaching the temperature of the ice bath. It is not desirable to cool the sample until it is frozen solid. Once the thermometer reading is constant or is dropping very slowly for eight (8) or more readings, you may remove the tube from the ice bath and warm again to about C. Put the test tube back into the ice bath and record a second cooling curve. Repeat the whole process as necessary until you have two acceptable sets of data. 1C. Calculations for Freezing Point of water Make a cooling curve from the data in each of your freezing point trials by graphing with Excel the observed temperature on the y-axis and the elapsed time on the x-axis. (You can not plot two trials on the same graph unless you have exactly the same number of xy data points in each experiment and all of the x-data points are the same for both graphs. Therefore, it is necessary to plot separate graphs). On each graph, using a straight edge, draw the best straight line through the initial linear portion of the data and then extend (extrapolate) this line until it extends below the temperature of the constant temperature plateau section of the graph. With the straight edge, draw another straight line through the data points on the plateau (horizontal) portion of the graph. The temperature at which these straight lines intersect is the observed freezing point and should be recorded as the T fp(1) and T (fp,2). These two values should be averaged. Look up the freezing point of water in the handbook and compare it with the value obtained from this experiment.

3 Data Thermometer readings for the freezing point of Water (T fp (avg) ) (Record to nearest 0.1 C) Trial 1 Trial 2 Trial 3 Time Temp ( C) Time Temp ( C) Time Temp( C) Observed freezing point temperatures of water (From the Graphs) Trial 1 C Trial 2 C T fp,avg = C Handbook Value C

4 Property 2: Boiling Point/Condensation Point 2A. Introduction When a liquid boils, the vapor pressure of the liquid is equal to the pressure of the atmosphere over it. Therefore, a liquid can boil at various temperatures at different atmospheric pressures. If the surrounding atmosphere is at 760 mm/hg pressure (1 atm), then the temperature is called the normal boiling point. When the surrounding pressure is greater than one atmosphere, the liquid will boil at a temperature above its normal boiling point. At a lower pressure, such as occurs in the typical laboratory, it will boil below the normal temperature. Boiling temperatures measured at pressures other than one atmosphere should be reported with the pressure noted. The most useful temperature is the normal boiling point, since this value will be reported in the literature and can thus be used in the identification of a substance. Although it is not possible for us to measure the normal boiling point directly, we can apply a calculation to convert the experimental boiling point to the temperature at which boiling would have occurred at one atmosphere if we know the pressure at which the experimental value was measured. 2B. Experimental Procedure for Determination of Boiling Point In order to determine the boiling temperature you must use a distillation flask. Clean the flask with soap and water and rinse thoroughly with tap water and then with distilled water from the white squeeze bottles. Fill the bulb of the flask ~2/3 full with distilled water from the carboy and add one or two boiling chips. The latter do not change the boiling point, but makes the boiling go more smoothly. Obtain a cork that will fit your flask and insert your thermometer into the cork. Hold the thermometer near the end to be inserted in the cork and carefully push and twist the thermometer through the cork. Do not force the thermometer, as this could be dangerous. Either make the hole larger or apply a small amount of stopcock grease. If stopcock grease is used, be sure to wipe the thermometer clean with a paper towel before continuing the experiment. Now, set up the distillation apparatus on the hot plate. Your instructor will provide a sample apparatus for you to use as a model. Place the cork on your thermometer and insert it into the neck of the flask. Position the tip of the thermometer in the flask near the point where the flask begins to flare into the bulb portion of the flask. Do not place the thermometer directly into the liquid. You will need to collect the distilled water so it is necessary to put a receiving test tube over the side arm of the distilling flask. Then, insert this test tube into a beaker of cold water (water with a small amount of ice in it) to condense the distilling vapor. Before you begin to heat the flask for the first time, have your instructor inspect your apparatus for safety. Carefully heat the water on the hot plate until it begins to boil and adjust the heat to maintain a gentle boil. Begin to record the temperature to the nearest 0.1 C every 15 seconds beginning when the temperature nears 90 C. By then, water should be condensing on the thermometer and dripping back into the flask. Continue boiling the liquid until you have recorded eight to ten (8-10) consecutive temperatures that do not vary by more than 0.5 C. Record this thermometer reading as the observed boiling point of water. Replace about ½ of the water in the flask, add two more boiling chips and repeat the determination of the boiling point. Repeat as necessary until two trials agree to within ±0.5 C. Average the values for the two trials and record this average as the observed boiling point of water.

5 Thermometer readings for the boiling point of water (T bp observed ) Time Temp ( C) Time Temp ( C) Time Temp( C) Trial 1 C Trial 2 C Trial 3 C Average C Barometer Reading in mm Hg mm Hg Boiling Point of Water at this Barometric Pressure C (See handbook)

6 2C. Calculations for Boiling Point of Water Average the observed boiling points from your two trials to give Tbp,avg. Boiling points are usually listed as normal Boiling Points, i.e., the boiling point at 760 mmhg pressure. Therefore, you will need to convert your observed boiling point to its corresponding value at 760 mmhg by using the following empirical equation. T normal,bp = T bp observed C/mm Hg x (760 - Barometric Pressure) T normal,bp = C C/mm Hg x (760 mm Hg - mm Hg) = C Handbook value of boiling point (normal) of water C Property 3: Density 3A. Introduction Density, mass per unit volume, is very useful for identification of a substance. Comparison of an observed value with a literature value should show excellent agreement. 3B. Experimental Procedure for Determination of the Density of Water Weigh a clean, dry, 10 ml graduated cylinder to the nearest g on the balance in your work area. Add about 2.5 ml distilled water to your graduated cylinder. Measure the volume of the water as closely as possible and re-weigh the cylinder plus the liquid. Repeat the volume measurement and weighing for ~5 ml, ~7.5 ml, and ~9.5 ml of the liquid. Do a second trial. Data for the Density of Water Trial 1 Trial 2 Mass Volume Mass Volume Empty g Empty g ¼ full g ml ¼ full g ml ½ full g ml ½ full g ml ¾ full g ml ¾ full g ml full g ml full g ml Temperature of the water C Handbook Density for Water at this Temperature g/ml

7 3C. Calculations for Determination of the Density of Water An approximate value of the density of water could be obtained by subtracting the mass of the empty graduated cylinder from the mass of the cylinder plus water and then dividing by the volume of the water. However, a more accurate way of obtaining the density is by determining the slope of the mass vs. the volume of the liquid for different amounts of sample. For a graph with the mass of the cylinder and water as the y-axis and the volume of the water as the x-axis, the slope of the trend line will be the density of the water. Make an Excel graph for each trial. Plot the mass of the cylinder plus water vs. the volume of water in the cylinder. Plot all of your mass-volume data points. Don t forget to include the mass of the empty cylinder and zero volume as a point. Add a trendline to determine the slope of the line produced. Plot each trial as a separate graph and determine a separate slope. Average the two slopes to obtain an average value for the density of water. Density, trial 1 g/ml Density trial 2 g/ml Average Density g/ml Property 4. Refractive Index 4A. Introduction When light passes from one medium into another, its speed changes, and this results in a change in the direction of the light ray. This bending of the light ray is termed refraction. The angle between the initial light ray and a line perpendicular to the refracting surface is called the angle of incidence Θ i. The angle between the refracted ray and this perpendicular line is called the angle of refraction Θ o. The refractive index, η, is defined as follows: η=sin Θ i /sin Θ o The refractive index is an intensive property of a pure compound and is useful in identifying materials and evaluating their purity. 4B. Experimental Procedure for Determination of the Refractive Index of Water See handout on student refractometers. Refractive Index Data Trial 1 Trial 2 Trial 3 Average Refractive Index for water Handbook Value of Refractive Index for Water

8 Summary of Results for Water Freezing Point Density Boiling Point Refractive Index Lab Values Accepted Values