INTRODUCTION
PART I: HONING OUR LABORATORY SKILLS
Every day we encounter a wide variety of materials. When we come across a new substance, we attempt to describe and classify it according to its physical and chemical properties. Measurements are involved in determining many properties and play an important part in chemistry and medicine. The metric system of measurement is used in all scientific work. Although the United States is only slowly converting to this system, we shall use it exclusively in this course.
There are two advantages to the metric system. First, it resembles our monetary system in that multiples and fractions based on the number 10 are used. The important prefixes are listed in the table below and you should become familiar with them. Second, there are relatively few basic units needed. The unit of length is the meter; the unit of volume is the liter; the unit of weight is the gram.
Prefix micro milli centi
Abbreviation
μ m c
Decimal Value
0.000001 or 1 X 10 -6
0.001
0.01 or 1 X 10 or 1 X 10
-3
-2 deci kilo d k
0.1
1000. or 1 X 10 -1 or 1 X 10 3
Sometimes we combine measurements to express a relationship between them. Familiar examples include miles per hour (mi/hr), dollars per gallon ($/gal), and miles per gallon (mi/gal). One important physical property of a substance, density, is defined in terms of two types of units: weight (mass) and volume. That is, density may be defined as weight per unit volume and is generally expressed mathematically as
Density

Weight
Volume
 g mL
In the medical field, specific gravity rather than density is often used to describe a substance (i.e. urine analysis). The specific gravity of a liquid compared to the density of water and, as shown below, it is a unitless or dimensionless value.
Density of Substance g / mL
Specific Gravity =

Density of Water @ 4º C g / mL
It is usually assumed that the density of water is 1 g/mL therefore the specific gravity of a substance is numerically equal to the density of the substance, but lacks any units.
PART II: IDENTIFICATION OF AN UNKNOWN SUBSTANCE USING PHYSICAL PROPERTIES
Properties are groups of characteristics that enable us to tell one substance from another. There are two main types of properties: Physical and Chemical. Physical properties are characteristics that usually do not involve a change from one substance into another. Such properties include color, solubility, density, odor, melting point, freezing point and electrical conductivity. A given pure substance (element or compound) has a unique set of physical properties that differentiates it from all other pure substances.
Determination of chemical properties usually results in a new substance being formed. Chemical properties include combustability, acidic or basic character and reaction with oxygen. In general, chemical properties are more difficult to measure than physical properties.
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The physical appearance of a substance may change while determining a physical property but not the substance's identity. For example, measuring the boiling point of a liquid cannot be accomplished without boiling it, which changes some of it from a liquid to a gas. Although the gas' appearance is much different from that of the liquid, the substance is still the same. Its chemical identity has not changed. Hence, boiling point is a physical property.
The physical properties density, boiling point and solubility are easily determined in the laboratory. In this experiment you will learn techniques for determining each of these properties and then use these properties to identify an unknown liquid.
PROCEDURE - Record all data on the Report Sheet
PART I:
A. Length
1. Measure and record the length of a test tube in inches using an English ruler.
2. Measure and record the length of the same test tube in centimeters using a metric ruler.
3. Showing your calculation, divide the centimeter reading by the inch reading and record your value.
4. Find the actual value from a conversion table.
B. Weight
1. Obtain metal cylinders of two different types and record their identifying marks.
2. Weigh each metal cylinder to the nearest 0.01 grams and record the value in the appropriate location.
3. Use these same metal cylinders for Part C.
C. Volume
1. Fill a 25 mL graduated cylinder with about 15 mL of deionized water. Determine the exact volume of the water in the cylinder by reading the bottom of the meniscus to the nearest 0.1 mL and record this value as the initial volume.
2. Tilt the graduated cylinder and slowly slide one of the preweighed metal cylinders from Part B into it. Be careful not to spill any water. Return the graduated cylinder to an upright position and record the new volume as the final volume. The difference in the volumes is the volume of the metal cylinder. (Note the metal cylinder must be completely submerged.)
3. Repeat this same procedure with the second metal cylinder.
D. Density
1. From the information obtained in Parts B and C you can determine the density of the metal cylinder. Showing your work, calculate the density for each metal cylinder.
E. Specific Gravity
1. Obtain a urinometer and about 75 mL of the "urine" specimen.
2. With the float in the urinometer, slowly add the urine until the float no longer touches the bottom of the urinometer.
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3. The float marked from 1.000 to 1.060 in .001 divisions. Making sure that the float is not touching the bottom or sides of the urinometer, read the value where the top of the liquid touches the scale and record it.
PART II:
CAUTION: The unknowns for this experiment are to be considered hazardous. Avoid contact with the skin and/or breathing their vapors unnecessarily. Wash your hands after the experiment to avoid contamination of food.
CAUTION: Many of the unknowns, both in the liquid state and as vapor, are highly flammable. Do not expose the liquids or their vapors to a direct flame.
A. DENSITY
Density is the ratio of the mass of a substance to the volume of that substance. For liquids, density is expressed in the units grams per milliliter (g/mL). Every pure liquid has a constant density at a given temperature.
Although two or more compounds may have similar numerical values of density, it is highly unlikely that these compounds will also have the same solubility properties and boiling point.
1. Carefully weigh your 10 mL graduated cylinder to the nearest 0.01g and record the weight.
2. Add about 5 mL of your unknown to the graduated cylinder and again weigh the cylinder. Record the weight on the Report sheet.
3. Immediately determine the exact volume of unknown in the graduated cylinder to the nearest 0.2 mL and record it on the Report sheet. Keep this unknown as it will be used for the next part of the experiment.
B. SOLUBILITY
Solubilities of substances in pure solvent are experimentally measured quantities that are usually expressed as grams of substance that will dissolve in a given amount of solvent at a given temperature. In this experiment you will qualitatively use the solubility properties of your unknown to aid in its identification.
1. Obtain 6 clean, dry test tubes and label them 1 through 6.
2. Add 2-3 cm of the solvent indicated in the chart below to the correct test tube:
Test Tube
1
2
3
Solvent
Water
Ethanol
Cyclohexane
Solute
Xylene
Xylene
Xylene
4
5
Water
Ethanol
Unknown
Unknown
6 Cyclohexane Unknown
3. Finally add 2 drops of the indicated solute to the test tube and mix as demonstrated in the lecture.
4. Record your results on the Report Sheet.
C. BOILING POINT
The boiling point depends upon the atmospheric pressure. When the vapor pressure of a liquid (the pressure of the gas formed by a liquid evaporating) is equal to the atmospheric pressure, the liquid is said to boil.
The temperature at which the vapor pressure of a liquid is equal to 1 atmosphere (atm) pressure is called the normal boiling point.
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1. Find another group who is using the same unknown as you and assemble the apparatus shown in Figure 1
(page 7) in one of the manual.
2. Combine the two samples of the same unknown from part A in the large test tube and position the thermometer so that it is about 1 cm above the liquid level.
3. Add a few boiling chips to the substance (obtain from instructor) and begin heating the water bath.
4. As the water bath heats up, watch the liquid inside the test tube for signs of boiling. When the liquid is continuously boiling monitor the temperature reading of the thermometer until a constant maximum temperature (about 2 minutes) has been reached. Record this value as the boiling point of your unknown.
Do not boil the contents of the test tube to dryness!
D. IDENTIFICATION OF UNKNOWN
Now that you have collected three different pieces of physical data you can identify your unknown liquid.
The Table below lists various liquids and their physical properties. Identify your unknown liquid by matching its physical properties to those listed in the Table.
Solubility
Substance Density (g/mL) Boiling Pt (ºC) Water Ethanol Cyclohexane
Benzene
Chloroform
0.88
1.49
80
61
I
I
CM
CM
S
CM
Ethyl acetate
Hexane
Heptane
Isopropyl alcohol
0.90
0.66
0.68
0.79
77
69
98
83
I
I
I
CM
CM
CM
CM
CM
S
S
S
S
Methyl (wood) alcohol
Methylene chloride
0.79
1.33
65
40
CM
I
CM
CM
I = Insoluble (2 layers); S = Soluble (bubbles); CM = Completely Miscible (no difference)
S
CM
POST-LAB QUESTIONS:
PART I:
1.
How does your experimentally determined value of cm/in compare with the value from the conversion table? Why might they be different?
2.
What is the normal range for urine’s specific gravity? Is the “laboratory urine” within this range?
3.
How can the specific gravity of urine be used as a diagnostic tool? At the chemical level, what causes the specific gravity of urine to change?
PART II:
1.
Explain the effect of atmospheric pressure on boiling point.
2.
Why are size and weight by themselves not useful properties for the identification of an unknown?
3.
Which of the properties you determined in this experiment do you consider to be the most useful in the identification of your unknown? Why?
4.
Why are boiling chips useful when boiling a liquid?
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Name_______________________________
REPORT SHEET
PHYSICAL MEASUREMENTS & PROPERTIES: Essential Tools for Lab
PART I:
A. Length
1. Length of test tube in inches
2. Length of test tube in centimeters
3. cm/in. for test tube readings:
4. cm/in. from conversion table
B. Weight
1. Identity of metal cylinder
2. Weight of metal cylinder
________
________
C. Volume
__________
__________
__________
__________
Identity of metal cylinder
Weight of metal cylinder
________
________
1. Initial Volume
2. Final Volume
________
________
3. Volume of cylinder
D. Density
________
1. Density of metal cylinder ________
Initial Volume
Final Volume
________
________
Volume of cylinder ________
Density of metal cylinder ________
F. Specific Gravity
1. Specific gravity of the urine specimen ________
PART II:
Unknown number ____________
A. Density
Weight of graduated cylinder and unknown
Weight of graduated cylinder
Weight of unknown sample
Volume of unknown determined from graduated cylinder
Density of unknown
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B. Solubility
Xylene
Unknown
C. Boiling Point
Water
_____
_____
Boiling point of unknown
D. Identification Of Unknown
Identity of Unknown
Ethanol
_____
_____
Cyclohexane
_____
_____
__________
__________
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Figure 1: Apparatus for the determination of boiling points
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