Below are some general points to take from the density lab. These are things that apply to many
of the labs that were turned in:
* Don’t copy introduction from lab sheet
* Make sure reader understands how the lab worked by reading procedure. Make it clear how the
work was split up between the groups and data was compiled.
* Elaborate and use data in the conclusion section. Post-lab questions usually give good hints as to
some good points to include in the conclusion section. Use and reference the graph we made to help
make points or conclusions. We didn’t make the graph for nothing!
* Real sources of error, not “calculations could be messed up”. For instance, in this lab the method
used for finding circumference wasn’t exactly a 100% precise method to use. The variation in these
measurements most likely led to some error in the data. This is a good example of a source of error,
not that calculations were wrong or the balance was read wrong.
* Include units! Any time you are talking about a number, make sure a unit is with it!
* If there is a piece of data that doesn’t match your conclusion, be sure to mention it! Otherwise, it is
BAD SCIENCE!! (Example: if baseball floated yet had a density over 1 g/mL)
* Pay close attention to detail. Read the guidelines for each section of a lab report.
* Get reports in on time!! You have multiple ways to communicate with me. Don’t tell me the day of
or even a day afterwards that google docs was acting weird. Take initiative! You have plenty of
time to work out the bugs. Any Joe-schmo could make up an excuse that google docs wasn’t
working. No more excuses!
* Grading will get tougher and tougher as you do more and more reports. Learn from the comments
and suggestions.
*On the following two pages are an example of a conclusion and post-lab question sections.
These are examples of good work that has come from your classmates. Compare it with you own
work.
Conclusion/Discussion
During this lab, the densities of 13 sports balls were recorded, as were their reactions to being placed
in a tub of water. Some of the balls floated and others sank - of the balls that floated, some had a
greater percentage submerged than others, and of the balls that sank, some did so more quickly than
others. All of this had to do with their densities. Balls with higher densities like the small gray mouse
ball (3.805 g/cm³) sank very quickly, whereas balls with lower densities sank more slowly, like that
golf ball (1.076 g/cm³). Balls with very low densities, such as the ping pong ball (0.07705 g/cm³), were
barely submerged in the water at all, and balls with higher densities, like the bowling ball (0.5455
g/cm³) floated with almost equal parts above and under the water.
Throughout this experimentation, a direct correlation between density and reaction to water became
clear. Balls with densities higher than 1 g/cm³ sank in the water (except for the baseball, which floated
- this is a probable mistake), while balls with densities lower than 1 g/cm³ floated. This shows the
density of water to be 1 g/cm³. This measurement makes sense for water - it is a small whole number
that is easy to use in scientific equations. The density of the balls also determined not only if they sank
or floated, but how they did so. The most dense balls sank faster than the less dense ones. When the
small gray mouse ball (3.805 g/cm³) and the mini ivory cue ball (1.44 g/cm³) were both dropped into
the water at the same time, they both sank, but the mouse ball hit the bottom much faster. The golf ball
(1.076 g/cm³) sank even slower still. The least dense balls (like the ping pong ball) seemed to be
almost floating on the water, barely touching it at all. Balls with densities higher than that of the ping
pong ball but lower than 1 g/cm³ floated partially submerged in the water.
Another point the lab made clear was the connection between the volume, mass, and density of
objects, and what density truly means. The bowling ball was expected to sink in the water because it
was heavy - but just because something is heavy, does not mean it is necessarily dense. Mass and
volume both need to be considered when figuring out the density of an object. There were several
balls, such as the field hockey ball and the lacrosse ball, with very similar masses, but one sank and
one floated. This is because of their volumes. The field hockey ball had a volume that was much
higher, so when the mass by divided by it, its density came to only 0.75 g/cm³ while the lacrosse ball’s
was a compact 3.16 g/cm³. The same results could be seen when 2 balls had similar volumes - their
differing masses made their densities quite different.
There was a large potential for error in this lab, considering all the mathematical calculations that
had to be made. It is entirely possible that certain mass, volume, or density measurements could have
been off throughout. One example of this was the baseball’s density calculation. It came to 1.1975
g/cm³, but floated in water. Logically, its density should have been less that 1 g/cm³. Other errors could
have occurred in copying the data from other groups off the board and onto the density data sheet, as
the method used to do so was mildly chaotic.
The overall purpose of this lab was to examine the meaning of density, how different densities cause
objects (in this case sports balls) to sink or float in water, and the connection between mass, volume,
and density. These objectives were achieved and the lab was completed successfully.
Post-Lab Questions:
1. Examine the average density values of the balls that sink. Do the same for the balls that
float. By comparing the densities of these two groups (sinkers and floaters), what would
you estimate the density of water to be and why?
The average density values of the sinkers are all greater than 1 g/cm 3. The average
density values of the floaters are all less than 1 g/cm 3. According to the data, the density of water
would be somewhere between 0.89515 g/cm 3 and 1.133 g/cm3. It would have to be between
these two numbers because 0.89515 g/cm 3 is the greatest density calculated for a floater.
Floaters have to have less density than water to float in it. It would be less than 1.133 g/cm 3
because it is the smallest density calculated for a sinker, meaning even though its density is
small, it still sinks and water has a lesser density.
2. Find two balls that have a similar average mass, but one floats and one sinks. Compare
the average volume of both balls. Thoroughly explain why one sinks and one floats.
Two balls that fit into this category are the small gray mouse ball with an average mass of
26.55 g and the small green basket ball with mass of 25.48 g. The average volume for the small
gray mouse ball is 6.05 cm3 while the volume of the small green basketball is 28.61cm 3. Just
because they have a similar mass, it does not mean they will have the same density. It all
depends on the volume of the ball too. The equation for density is D=m/V. If the volume is a
smaller number than the mass, the density will most likely be greater and vice versa. In this case,
the volume of the small gray mouse ball was much smaller than that of the small green basketball
and was a lesser number compared to its mass, and when it was plugged into the equation the
density happened to be greater than the density of water.
3. Find two balls used that have a similar average volume, but one floats and one sinks.
Compare the average mass of both balls. Thoroughly explain why one sinks and one
floats.
Two balls that fit this are the clear marble with an average volume of 9.2 cm 3 and the ping
pong ball with an average volume of 9.3cm3. The average mass of the clear marble is 20.595 g
and the average mass of the ping pong ball is 2.72 g. Going back to the answer of question two,
this kind of works out in the same way. If the mass is less than the volume, the density will be
smaller (ping pong ball), but if the mass is greater than the volume, the density will be greater
(clear marble). The clear marble had a greater density that happened to be greater than the
density of water, allowing it to sink while the ping pong ball had a density less than that of water,
allowing it to float. It does not matter if the volumes of two of the balls are the same or similar
because the mass will be different, and because of the equation to find density (D=m/V) relies on
both measurements, the densities of different things will be different.
4. Other than the method used in this lab, describe a method that we could use in our lab
to experimentally figure out the density of water. Include equipment, steps, etc. Be
detailed.
The equipment needed for this would be a graduated cylinder, an electronic scale, and 40
mL of water. To find the mass, find the mass of the cylinder first. Once the mass has been
recorded, fill the cylinder up to the 40 mL line (this is the volume, so record it). Put the cylinder
with the water on the scale and record it. Subtract the mass of just the cylinder from the mass of
the cylinder with water in order to get the mass of just the water. Plug the numbers into the
equation for density (D=m/V). Note that 40 mL doesn’t have to be used in order to get the
answer. Depending on the amount of water put in the cylinder, the mass will match up to it, giving
the same value for density.
5. Assuming that it would have the same volume as the bowling ball we tested, how much
mass would a new bowling ball need in order to sink in the water? Think about the density
equation, and your estimation in question 3.
The mass of a new bowling ball would need to have a mass less than it’s volume. So, it’s
mass would have to be less than 5,010.8 in order for the density to be less than that of water.
6. Using the average density values of the two balls, give an explanation for what you saw
in numbers 2 and 4 of the observation section.
In number 2, the golf ball took more time to sink compared to the small gray mouse ball
because their density values are different. Despite the fact that both of them have a density
greater than water, the golf ball’s density is lesser compared to that of the small gray mouse ball.
In number 4, the small green basketball is less above water than the ping pong ball
because their density values are different. They both float, therefore their density values are
lesser than that of water, however one has a greater density compared to the other, meaning it
had a lesser ability to float.
7. What was the benefit of having 2 different groups make measurements for each ball and
then use an average value for making graphs and comparisons?
Depending on the equipment used, the calculations for each ball could differ slightly and
confuse the whole class if each group were to have used their own measurements. The
averaging of the data gets rid of conflict and confusion when trying to find the final answer.
8. What are some likely sources of error that could have occurred during this lab? How
would they have affected the data?
A group could have measured something wrong. Maybe they didn’t include enough
significant digits or maybe they calculated something wrong without realizing it (possibly
multiplying numbers instead of dividing them). Since averages were being used, one of the
measurements used to create the average could have been wrong, then making the average
wrong. Also, depending on where the mistake was made, the volume or mass could be wrong
and affect the group’s or class’s answers in the post lab questions.