Density and Specific Gravity
Floating an Egg and Urine Analysis: Skill Building
Lab
 2013, Sharmaine S. Cady
East Stroudsburg University
Skills to build:
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Using a burette
Using a Mohr pipette
Using a micropipette
Using a hydrometer
Using a digital balance
Using Excel graphs to determine density
Introduction
The proper choice and correct use of laboratory equipment is a fundamental skill
required for success as a scientist. Digital balances for measuring mass and volumetric
glassware for measuring liquid volume are frequently encountered in many laboratory
situations. This experiment is designed to provide an introduction to the digital balance and
three types of volumetric tools: burette, Mohr pipette, and the micropipette. Measurements
recorded are used to determine the density of an egg. The linearity of mass vs. volume Excel
plots are used to compare the accuracy of the three volumetric tools. In addition, a urinometer,
a specialized type of hydrometer, is used to directly measure the specific gravity of urine
samples. Go to the web site and click on the Measurements and Significant Digits link. Read
how to use and properly record your quantitative data for each piece of equipment mentioned
in the experimental methods section of this laboratory.
The following sections provide background for understanding density and specific
gravity and their applications to the experiment.
Archimedes's Principle
Body weight is decreased by 90% when in water. The water acts like a natural support
for all the limbs easing the pressure on joints. This means that it is possible to move and
exercise in water with little risk of injury. Hence, water therapy helps in the treatment of
arthritis and other debilitating conditions. The loss of weight when an object is submerged in
water is a direct consequence of a net upward force exerted on the object by the pressure of
the water acting on the object (Figure 1). This force is known as buoyancy and is directly
Eggs and Urine
proportional to the weight of the water displaced by the object. Archimedes's principle states
that any object partially or completely submerged in a fluid experiences an upward force that is
equal to the weight of the fluid displaced.
Figure 1. Forces acting on a submerged object.
(Source:http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/Pressure/BouyantForce.html)
Weight is the downward force caused by gravity acting on an object. If force (F) is mass
(m) times acceleration (a), the weight (W) of the fluid displaced is given by its mass (mfluid) times
the acceleration due to gravity (g).
The density of the fluid (fluid) is its mass per unit volume (V). This definition can be used to
obtain an expression for mfluid.
Since the buoyant force(FB) is equivalent to the weight of the fluid displaced, the following
equation can be written:
If the density expression for mfluid substituted into the above equation, it can be rewritten as
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A second force acting on the object is its own weight:
The density of the object (obj) is its mass (mobj) per unit volume (V). This definition can be used
to obtain an expression for mobj.
If the density expression for mobj is substituted into the expression for Wobj, the equation
becomes
A body partially or completely submerged in a fluid experiences a net force that is equal
to the sum of the buoyant force and the object's weight (Figure 2). The net force acting on an
object can be written as
Figure 2. Forces acting on an object. For the left-hand object, its weight (black arrow) is greater
than the buoyant force (gray arrow) and it sinks. For the right-hand object, its weight is less
than the buoyant force and it floats.
(Source: http://www.spacegrant.montana.edu/msiproject/blimp.html)
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Substituting for Wobj and FB, the equation becomes
An object that sinks experiences a net downward force where its weight is greater than the
buoyant force. Hence, an object whose density is greater than that of the fluid sinks.
For an object to float, the net force acting on it must be zero; therefore,
Rearranging the equation yields
If part of the object floats above the surface of the fluid, the above equation can be rearranged to
determine the fraction of the object’s volume that is submerged.
where Vsub is the volume of the object that is submerged , which displaces an equal volume of fluid
(Vfluid) (Figure 3). From the above equation, an object that is partially submerged has a density
that is less than that of the fluid. An object that floats totally submerged has a density equal to
that of the fluid.
Figure 3. Forces acting on a partially submerged object.
(Source:http://www.ac.wwu.edu/~vawte/PhysicsNet/Topics/Pressure/BouyantForce.html)
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The above equation allows for the calculation of the percent volume of an iceberg that is
submerged. If seawater has a density of 1025 kg/m3 and ice a density of 917 kg/m3, then
Hence, 89.5% of the volume of an iceberg is submerged. One really sees only the "tip of the
iceberg."
Density and Specific Gravity
Density and specific gravity are physical properties of matter that measure mass per unit
volume. The equation for density is
Common units for density are g/mL (liquids), g/cm3 (solids), and kg/m3 (SI standard unit).
Note that specific gravity is a ratio of two densities and therefore has no units. Water is usually
the reference liquid used in specific gravity calculations because its density value is 1.0000 g/mL
at 4 C; as a result, specific gravity and density conveniently have the same numerical value
when a substance's density is compared to water at 4C.
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Eggs and Urine
Density is temperature-dependent. As the temperature increases, volume increases.
The denominator in the density equation becomes larger, and the calculated density value
decreases. As the temperature decreases, volume decreases. The denominator in the density
equation becomes smaller, and the calculated density value increases. The temperature at
which a density or specific gravity measurement is taken should always be indicated on a report.
The Hydrometer
The specific gravity of a liquid can be measured using a hydrometer. Hydrometers work
on Archimedes principle that a submerged or floating body experiences a buoyant force equal to
the mass of the liquid volume it displaces. Since this force depends on the mass and volume of
liquid displaced, it is directly proportional to the specific gravity of the liquid. If a hydrometer is
floated in a liquid of known temperature, the specific gravity of the liquid can be read from the
scale on the stem of the hydrometer. The density of the liquid can then be determined from the
equation for specific gravity. Figure 4 shows the use of a hydrometer to measure specific
gravity. If attractions exist between the hydrometer surface and the liquid being measured, a
meniscus occurs as shown in Figure 5. The reading is taken at the lowest level of the meniscus;
the value is read as 1.00023 in the figure. This is a common occurrence when the solution
contains water and the hydrometer is made of glass.
Figure 4. A hydrometer.
Figure 5. Reading a floating in a liquid.
Volumetric Glassware
Volumetric glassware is used to provide a more accurate measure of volume during
quantitative analysis. The three types of volumetric glassware used in this experiment are the
buret, Mohr pipet, and micropipette. This glassware is calibrated to deliver a specific volume of
liquid to a container. The burette and Mohr pipet usually give the accuracy at 20C near the top
of the glassware. Typically, their volumes are reported to 2 decimal places. Figure 6 shows
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Eggs and Urine
examples of the three types of glassware. Like the hydrometer, if a meniscus forms, the bottom
is used to determine the reading. In filling the glassware, it is important to remove all air
bubbles from the tip to the top of the meniscus before dispensing the liquid.
Figure 6. From left to right: burette, Mohr pipette, and micropipette.
Urine
Urine is waste material secreted by the kidneys. It is rich in end products of protein
metabolism (such as urea, uric acid, and creatinine) together with salts and pigments, and forms
a clear, amber, usually slightly acidic fluid. Specific gravity measures the ability of the kidney to
concentrate or dilute urine with respect to plasma. Normal values for urine specific gravity are
in the range 1.010-1.025. A hydrometer specifically made for measuring the specific gravity of
urine is known as a urinometer. The more concentrated the urine, the higher the urine specific
gravity value. Specific gravity values outside the normal range may be indicators of underlying
physiological problems that may require treatment to correct. Table 1 on the following page
lists some physiological conditions that may cause an abnormal value of urine specific gravity.
Antidiuretic hormone (ADH) is a hormone stored in the posterior pituitary gland in the brain. It is
the primary regulator of body water. ADH acts on the kidneys to increase total body water. This
has the effect of decreasing the plasma concentration,increasing blood volume, and increasing
blood pressure. The release of ADH is controlled by cells,called osmoreceptors and
baroreceptors. Osmoreceptors are specialized areas in the hypothalamus (an area in the
brain). These cells sense the concentration of particles in the blood. When the concentration is
high, the pituitary releases more ADH. This stimulates retention of water to dilute the body
fluids and simultaneously decreases urine volume, which leads to a higher than normal specific
gravity. When the concentration is low, the pituitary releases less ADH. Baroreceptors are
specialized areas in the heart that sense blood volume and blood pressure. The heart signals
the pituitary to release more ADH when blood volume or blood pressure is low and less when
they are high.
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Table 1. Abnormal Specific Gravity Values and Potential Causes
BELOW NORMAL (<1.010)
ABOVE NORMAL (>1.025)
excessive fluid intake
diabetes insipidus
renal (kidney) failure
glomerulonephritis
dehydration
diarrhea
excessive sweating
vomiting
glucosuria (excess glucose in urine)
(occurs in diabetes mellitus)
pyelonephritis
proteinuria (excess protein in urine)
increased secretion of ADH
(occurs in trauma, stress, surgery, and with
some drugs)
In this experiment, Archimedes principle is used to determine the density of an egg.
Sodium chloride is added to water to just float the egg below the surface. Volumetric glassware
is used to determine the mass vs. volume relationship of the salt solution. The mass vs. volume
data is then graphed using Excel. The density is determined from the slope of the line. The R2
factors are analyzed to determine the accuracy for the volumetric glassware. The closer the R2
factor is to 1, the more linear the relationship between mass and volume and the more accurate
the density of the egg.
In addition, three separate urine samples collected from an immediate post-operative
patient, a patient suffering from excessive fluid intake, and a patient with normal urine will be
measured for their specific gravity values with a urinometer. The samples will then be assigned
to the proper patient.
3D Molecules
Go to the web site and click on the NaCl and Water link. Use the radio buttons to show
different views and properties of each molecule. Answer the questions based on the
manipulation of the molecules.
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Experimental Methods and Materials
Safety considerations
Wear suitable protective clothing, gloves, and eye/face protection!
You should read the online MSDS for:
Sodium Chloride
Urine
Density of an Egg
Obtain a 1000-mL beaker and place an egg inside. Fill the beaker with tap water
until the egg is covered by an inch of water. Add NaCl (sodium chloride) to the water
until the egg is just below the top surface. Stir to make sure the NaCl is dissolved before
adding more. CAREFUL! Do not break the egg while stirring. Remove the egg and
record the temperature of the solution. Use your saline solution for the next part of the
experiment.
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Use of volumetric glassware
Burette: Rinse a 50-mL burette with some of your solution. Fill the buret with your
solution and drain to the 0.00 mL mark. Check for air bubbles in the tip. If there are air
bubbles present, continue to drain until they are removed. You may need to open and
close the stopcock quickly several times to remove the air bubbles. Ask the instructor
for help if you are not successful. Tare a dry 25-mL Erlenmeyer flask on the digital
balance. Add 1.00 mL of the solution to the flask and record your mass. Add another
1.00 mL to the flask for a total volume of 2.00 mL. Record the mass. Repeat three more
times until you have a total of 5.00 mL in the flask.
Mohr pipette: Rinse a 1-mL Mohr pipette with your solution. Tare a dry 25-mL
Erlenmeyer flask. Pipet 1.000 mL of your solution into the flask and record the
mass.Repeat this step 4 more times until you have a total volume of 5.00 mL in the
flask.
Micropipette: Practice using the 1000-L micropipette with distilled water first.Tare a
dry 25-mL Erlenmeyer flask. Pipet 1.0000 mL of your solution into the flask and record
the mass.Repeat this step 4 more times until you have a total volume of 5.0000 mL in
the flask.
Determination of density with Excel
Access the online Excel link for instructions on how to graph with Excel. Note
that you must place the independent variable (volume) on the x-axis and the dependent
variable on the y-axis (mass).
Determination of urine specific gravity
Fill the urine cylinder about 2/3 full with a patient sample. Using a urinometer,
measure the specific gravity to three decimal places for patients A, B, and C. Indicate
what the urine specific gravity implies about each patient: normal, excessive fluid
intake, or immediate post-operative.
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Laboratory Report
Answers to the following questions should appear in the discussion section of your report:
1. Which volumetric glassware provided the best linear graph? Explain using the R2 value
for your trendline. The closer the R2 value is to 1, the more reliable the experimental
data and the more the line represents a linear relationship between mass and volume.
When R2 = 1, all experimental data points are on the line.
2. Which volumetric glassware provided the worst linear graph? Explain.
3. Using the information from the graphs, what is the best density value for your egg?
Does the value make sense given your observations of the egg in pure water (Did it float
or sink in pure water?)? Assume the density of pure water is 1.00 g/ml.
4. Explain why you diagnosed each patient as normal, post-operative, or diabetic.
5. Explain how and why hydrometers can be used to monitor changes in the fermentation
process that occurs during wine and beer making. You may want to check the following
web site: http://www.cellar-homebrew.com/store/
catalog/Triple-Scale-Hydrometer-p-505.html
References
Hobble, M.Urine Specific Gravity.http://www.rnceus.com/ua/uasg.html (accessed August 2005)
MedlinePlusEncylcopedia Page. http://www.nlm.nih.gov/medlineplus/encyclopedia.html
(accessed August 2005)
Wink, D. J.;Gislason, S. F.; Kuehn, J. E.Working with Chemistry, 2e; W. H. Freeman & Company:
New York, 2000; pp B-1-B-8.
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