Physical Properties of Pure Substances and Mixtures Objectives &...

Physical Properties of Pure Substances and Mixtures Objectives • To become familiar with common laboratory tools and techniques by measuring physical properties such as density. • To determine the densities of several pure substances and mixtures. Introduction Chemical substances can be described by their chemical properties and their physical properties. Determining the physical properties often helps one identify a pure sample of an unknown substance, or possibly identify the components of a mixture. Physical properties are properties that can be observed without altering the chemical composition of the substance. Physical state – whether a substance is a solid, liquid, or gas at ambient conditions – is a physical property of a substance. For the solid state, physical properties of color, hardness, density, solubility, melting point, tensile strength, electrical conductivity, and thermal conductivity are important properties to assess. Substances in the liquid state can be characterized by color, odor, density, specific gravity, boiling point, viscosity, surface tension, refractive index, vapor pressure, dielectric constant dipole moment, and miscibility (solubility) in another liquid. Viscosity and thermal conductivity are also important physical properties of gases. Physical properties can be described as extensive properties or intensive properties. Extensive properties (such as mass and volume), are properties dependent on the amount of substance present, whereas intensive properties (such as melting point, boiling point, and density) are properties that do not depend of the amount of substance present. Substances also exhibit chemical properties, which are properties that are observed only when a chemical change occurs. Chemical changes produce new substances that have chemical compositions and properties that are different than the properties of the original substances. Density The density of a substance is defined as the mass of an object or substance compared to its own volume. It is therefore the measured mass of an object divided by the measured volume of that same object. Although the correct symbol for density is the Greek letter rho, ρ, and the SI units of density are kg/m3, density continues to be represented in many textbooks using the older metric system of symbols and units (d = density in g/cm3 or g/mL for solids and liquids): 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑚𝑚𝑣𝑣 The density of a pure material in the solid state is usually greater than its density in the liquid state. Water is a well- known exception to this statement. The density of ice at 0 ̊C is 0.9167 g/cm3, while the density of liquid water at 0 ̊C is 0.9998 g/mL. The volume of a liquid (or gas) usually increases as temperature increases, so the density of a liquid (or gas) should decrease as temperature increases. Again, water is an exception: its density actually increases in the range 0 ̊C to 3.97 ̊C, although it demonstrates the expected variation at temperatures above 3.97 ̊C. The density of an aqueous solution routinely increases as the mass percent of solute in the solution increases, as mass is directly proportional to density. ρ= Specific Gravity The specific gravity of a substance is defined as the ratio of the density of that substance to the density of a standard substance. For liquids and solids, that standard substance is water at 4 ̊C, the density of which is 1.0000 g/mL. Specific gravity is dimensionless (no units), but it does change with temperature. Thus, for a pure solid, liquid, or aqueous solution at a given temperature, specific gravity is calculated from: 𝜌𝜌𝑚𝑚𝑣𝑣𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚𝑠𝑠𝑠𝑠𝑣𝑣 𝜌𝜌𝑤𝑤𝑚𝑚𝑠𝑠𝑣𝑣𝑤𝑤 where ρsubstance is the density of substance ρwater is the density of water at 4 ̊C sp gr = For equal volumes: sp gr = 𝑚𝑚𝑚𝑚𝑣𝑣𝑠𝑠𝑚𝑚𝑠𝑠𝑚𝑚𝑠𝑠𝑠𝑠𝑣𝑣 𝑚𝑚𝑤𝑤𝑚𝑚𝑠𝑠𝑣𝑣𝑤𝑤 where msubstance is the mass of substance mwater is the mass of water Density Determination of an Unknown Solid by Water Displacement When a solid is completely submerged in a liquid, it displaces a volume of liquid that is equal to the volume of the solid (consistent with Archimedes’ Principle). Consequently, the densities of a variety of irregularly-shaped solids (especially non-reactive metals) may be determined by submerging them in a known volume of liquid water, and recording the change in volume. This technique has several requirements and limitations: 1) The solid must be fully submerged in the water 2) The solid may not dissolve in the water 3) The solid may not react with the water Assuming all the above requirements are satisfied, one may determine the mass of an unidentified dry solid object, determine the volume of the same object by measuring the volume of displaced water, and calculate a density. The calculated density can be compared to published densities of pure substances, as an aid to identifying the object. In Part A of this experiment, students will determine the densities of 1) pure water and 2) calcium chloride solutions of known but differing compositions. Students will use that information to graphically determine the composition (mass %) of an unknown calcium chloride solution. In Part B, students will identify a sample of unknown metal by determining its density using the water displacement method. In Part C, students will determine the thickness of a piece of metal foil using the density from Part B and the other dimensions of the foil. Physical Properties of Pure Substances and Mixtures Experiment Procedure General Instructions: 1. All masses measured in this experiment should be recorded to four decimal places (0.0001 g). 2. The same balance should be used for all mass measurements in the experiment. 3. All volumes measured in this experiment should be recorded to the highest level of precision possible. Part A: Determination of Densities of Deionized Water and Calcium Chloride Solutions Materials: 1.00-mL volumetric flask with stopper, deionized water, six solutions of CaCl2 (aq) of known mass % (10%, 20%, 25%, 30%, 40%, 50%), one solution of CaCl2 (aq) of unknown mass %, disposable plastic transfer pipettes, small test tube, thermometer, beaker for waste solutions 1. Obtain several milliliters of distilled/deionized water in a clean small beaker, and record its temperature to the nearest 0.5 ̊C. (1) 2. Obtain a clean, dry 1.00-mL volumetric flask with stopper. Measure and record the mass of the empty volumetric flask with its stopper. (2) 3. Using a new disposable plastic transfer pipette, rinse the volumetric flask twice with deionized water, and pour the rinse water into the liquid waste beaker. 4. Using the same plastic transfer pipette used in Step 3, fill the volumetric flask with deionized water, so that the bottom of the meniscus rests exactly on the 1.00-mL calibration mark. 5. Stopper the volumetric flask, and use a Kimwipe to wipe off any traces of water on the exterior of the flask and stopper. 6. Record the mass of the stoppered volumetric flask and water. (3) 7. Calculate the mass of water in the 1.00-mL volumetric flask (4). Determine the density of water, assuming a volume of 1.00 mL (5). 8. Look up the density of water in the CRC Handbook of Chemistry and Physics, and record the density value on the Report Sheet. (6) Compare your experimental density with the literature value at the same temperature, and calculate the percent error for the measurement, using the following formula: 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑣𝑣𝐴𝐴𝐴𝐴𝐴𝐴𝑣𝑣 − 𝐸𝐸𝐸𝐸𝐸𝐸𝑣𝑣𝐸𝐸𝐸𝐸𝑚𝑚𝑣𝑣𝐸𝐸𝐴𝐴𝐴𝐴𝐴𝐴 𝑣𝑣𝐴𝐴𝐴𝐴𝐴𝐴𝑣𝑣 𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 𝑣𝑣𝐴𝐴𝐴𝐴𝐴𝐴𝑣𝑣 10. Using the same plastic transfer pipette used in Step 9, fill the volumetric flask with 10% by mass CaCl2 solution, so that the bottom of the meniscus rests exactly on the 1.00-mL calibration mark. 11. Stopper the volumetric flask, and use a Kimwipe to remove traces of solution on the exterior of the flask. 12. Measure and record the mass of the stoppered volumetric flask filled with the 10% solution. (7) 13. Repeat Steps 9 - 12 for each of the five other aqueous CaCl2 solutions (20%, 25%, 30%, 40%, 50%) and record your data on the Report Sheet. (7) Avoid contamination of the original solutions - Remember to use a new plastic transfer pipette for each new solution! 14. Repeat Steps 9 - 12 for an aqueous CaCl2 solution of unknown concentration, and record your data on the Report Sheet. Remember to record the unknown solution code on the Report Sheet. (8) 15. Calculate the masses and densities of each known CaCl2(aq) solution and the unknown CaCl2(aq) solution. (7) 16. Plot CaCl2 solution density (y-axis) vs. mass% CaCl2 solution (x-axis) for solutions of known concentration using a graphing program such as Excel. (9) Use the data points to produce a best-fit linear trendline (slope). The data point for 0% CaCl2 (deionized water (5)) should be included on the graph. 17. Use the graph and the density of the unknown CaCl2 solution to determine the mass % CaCl2 for the unknown solution (10). 18. Determine and record the specific gravity of the 50% CaCl2 solution. (11) � � 𝐸𝐸 100% 9. Using a new disposable plastic transfer pipette, rinse the volumetric flask twice with 10% by mass CaCl2 solution, and pour the rinse solution into the beaker set aside for waste liquids. Part B: Determination of the Density of an Unknown Metal Sample by Water Displacement Materials: Metal sample pieces, graduated cylinder (10- 1. Obtain approximately 20 g of dry unknown metal sample. Remember to record the code for the unknown metal sample on the Report Sheet. Measure and record its mass using any of the methods introduced in your instructor’s “balance orientation”. 2. Obtain the most precise graduated cylinder that is large enough to hold the metal pieces (10- or 25-mL). Add deionized water until the cylinder is one-third to one-half full. Record the water level to the highest level of precision allowed. 3. Carefully place the metal pieces in the graduated cylinder, making sure that all metal pieces are completely submerged. Do not let any water “splash out”. Record the water level to the highest level of precision allowed. 4. Calculate and record the volume of water displaced to the highest level of precision allowed. 5. Calculate and record the density of the metal pieces. or 25-mL), 6. 7. 8. deionized water Dry the metal pieces thoroughly, and repeat steps 1 – 5 with a similar quantity of the same dry metal. Determine the average density of the two trials. Identify the metal by comparing your density with the values in Table 1. Table 1. Densities of Common Unreactive Metals Metal Aluminum Zinc Tin Lead Density (g/mL) 2.70 7.14 7.31 11.3 Part C: Determination of the Thickness of Unknown Metal Foil Sheet Materials: Pre-cut piece of metal foil, ruler 1. Obtain a pre-cut piece of metal foil, being careful not to bend or fold it. 2. Using a ruler, measure and record the length and width of the foil sheet to the highest level of precision possible. Cleanup and Disposal 3. Measure and record the mass of the foil sheet to the highest level of precision possible. 4. Using the average metal density determined in Part B, as well as the mass, length, and width of the metal foil, calculate and record the thickness of the foil with appropriate units and significant figures. • Discard the CaCl2 waste solution (Part A) in the sink with plenty of running tap water. • Thoroughly rinse volumetric flask and stopper (Part A) with deionized water, dry, and return to instructor. • Return the Unknown Metal sample (Part B) to the correct, labeled “Drying Beaker” – do not place the wet metal sample back in the original container. • Return metal foil sheet (Part C) to instructor. • Rinse all other glassware with deionized water, dry, and return to original storage. • Dispose of used transfer pipettes in garbage. Physical Properties of Pure Substances and Mixtures Prelaboratory Assignment Date: __________ Name: ___________________________ Partner(s) Name:________________________ (1) Give an accurate word definition of the term “density”. (2) Is density a physical property or a chemical property? ___________________________ (3) Is density an intensive property or an extensive property? ___________________________ (4) What information would be needed to calculate the density of a spherical object? (5) Give an accurate word definition of “specific gravity”. (6) Toluene, a colorless organic liquid, has a density of 0.8669 g/mL at 20 ̊C. Determine the specific gravity of toluene at 20 ̊C. (Show your work) (7) A student placed a sample of an unknown metal weighing 23.7412 g in a graduated cylinder containing 13.3 mL of water, and the volume rose to 15.4 mL. Calculate the density of the metal, and identify the metal using Table 1. Physical Properties of Pure Substances and Mixtures Report Sheet Date: __________ Name: ___________________________ Partner(s) Name:________________________ Part A: Determination of Densities of Pure Water and Calcium Chloride Solutions (1) Temperature of water (to nearest 0.5 ̊C) (2) Mass of empty volumetric flask with stopper (3) Mass of volumetric flask, stopper, and water (4) Mass of water in the volumetric flask (5) Calculated density of water at temperature ((4) / 1.00 mL) (6) Density of water at temperature (from CRC Handbook) Show Percent Error Calculation: ________________ ̊C _________________ g _________________ g _________________ g ______________ g/mL ______________ g/mL 50% Unknown solution solution (7) Mass of stoppered flask containing Solution (g) Mass of Solution (g) Density of Solution (g/mL) 10% 20% solution solution 25% 30% 40% solution solution solution Volume of Solution 1.00 mL 1.00 mL 1.00 mL 1.00 mL 1.00 mL 1.00 mL 1.00 mL (8) Unknown solution code: _________ (9) Plot the data: a. Graph density (y-axis) vs. mass % (x-axis). The data point for deionized water (0% CaCl2) should also be included on the graph. b. Scale and label each axis properly, title the graph c. Include best-fit linear trend line and equation for slope (10) Mass% CaCl2 in unknown solution (from graph): (11) Specific gravity of 50% by mass solution: _________________ % _________________