Specific Gravity of Bone
Introduction
Doctors and veterinarians have to deal with fractures on an almost daily basis.
Therefore, it is important that they know where fractures are likely to happen. In this lab
we will look at the specific gravity of sections of bones to determine what sections have
the least density and are therefore more likely to fracture.
Specific gravity is a ratio of densities. It compares the density of the substance that is in
question to the density of water which is 1 g/ml. Since it is a ratio of densities, specific
gravity has no units. An object with a specific gravity of 1 or greater will sink. When
measuring specific gravity of an object that sinks we can convert the specific gravity
equation into an equation that is more manageable. Our original equation below can be
simplified if we deconstruct it to the equations for density. Then we can simplify it by
making our volume of water equal to our bone volume. This would also be known as the
volume of water displaced by the bone.
With both volumes equal to each other we can cancel them out of the equation and be
left with the equation above.
Archimedes’ principle states that the buoyant force is equal to the weight of the water
displaced. Therefore we can further reduce the equation as seen below.
This last equation will allow us to quickly and easily assess the specific gravity of each
section of bone in this lab.
Materials
● Electronic scale
● Beaker - able to accommodate each section of bone without letting them touch
the beaker
● Long bone cut into transverse sections
● Thread
● Cabinet, ledge, or other item to hang the bone from
● Tape
Instructions
1. Weigh and record a bone section.
2. Fill the beaker with enough water to allow the bone section
to be submerged without touching the bottom of the
beaker.
3. Place the beaker with water on the scale and tare the
scale to 0.
4. Tie the thread to the bone section; use as little thread as
possible around the bone section.
5. Submerge the bone section into the water in the beaker
without letting it touch the beaker. Tape the free edge of the thread to the
cabinet. Record the weight. This will be you buoyant force.
6. Repeat steps 1-5 for each bone section.
7. Use the equation that was mentioned in the introduction to find the specific
gravity for each bone section. Record the specific gravity on the graph.
Bone
section
1
2
3
4
5
6
Dry weight (g)
Buoyant Force (g)
Specific Gravity
Activity Questions
1. After examining the graph determine which section of the bone has the:
a) Greatest specific gravity:
b) Least specific gravity:
2. How does the specific gravity at the ends of the long bones compare to that of
the middle of the bone?
3. What does this experiment tell us about the density of the ends of the long bones
compared to the middle?
4. Take a close look at the type of bone found at the ends of the bone compared to
the middle. What difference do you see?
5. What type of bone tissue makes up the majority of the ends of the bone?
6. What type of bone tissue makes up the majority of the middle sections of the
bone?
7. What can be concluded about where your long bone is likely to fracture when
taking into account density only?