Caryn Asherson
SED 690
Dr. Foley
Conceptual Interview
Prior to the interview, students (6th grade) were asked to layer four colored
liquids. In the first experiment, students layered four unknown liquids – glycerin,
rubbing alcohol, salt water, and water. In the second experiment the four colored
solutions were made using water, salt, and food coloring. The only difference
was the amount of salt dissolved in the four cups of water.
Student #1
Why do you think the liquids layered the way they did?
Different kinds of chemicals were stronger than the others so they built up. You
have to stack it in the right way or they will mix. Different chemicals are strong
enough to hold the other kinds.
What do you mean when you say the chemicals were stronger?
Because, like, there’s pressure. No…not pressure, the weight of them were all
different. The heaviest one went on the bottom and the lightest ones went on the
top.
What may have been responsible for keeping the layers separate?
Because maybe that some of them were syrup and sticky. Some were watery.
The sticky ones would go to the bottom and the watery ones would go to the top.
Why do you think the liquids with more salt layered at the bottom, while
those with less salt layered at the top?
Because the salt is like heavier than nothing. Like in the ocean it pulls you down
sometimes, but in plain water it doesn’t. Well it does sometimes.
Student #2
Why do you think the liquids layered the way they did?
Well, um, when it usually…when it’s two dark colors, they’ll mix in. In a dark,
they won’t mix into each other…when the dark ones they go together…they
wouldn’t stay on top of each other. When you put a light and a dark, sometimes
they wouldn’t mix.
Do you think that the color of the liquids had something to do with the way
the liquids layered?
Because some of them just don’t go together. Blue and green make turquoise.
Red and yellow always goes orange so it can’t mix together.
What may have been responsible for keeping the layers separate?
I would say that because of the different stuff that was inside the liquids. It could
have been stuff inside the liquids.
Why do you think the liquids with more salt layered at the bottom, while
those with less salt layered at the top?
The one with more salt…it would have kept the color above it stabilized.
Because it was more thick. The salt was more thick. Without the salt it was less
thick, more liquidy.
Student #3
Why do you think the liquids layered the way they did?
First of all…the liquids layered the way…The other liquids may have been
heavier and some are lighter. Orange sank and blue rised. They didn’t mix. It is
not to do with heat or cold. It has…if you have more of something it will sink to
the bottom. If you have less density, it will go to the top.
What may have been responsible for keeping the layers separate?
The reflection of the light hitting the different colors and the density. The darker
colors are usually more denser. Certain types are more denser than other types.
How could we know if something has a low or high density?
You could test to see if it floats.
Why do you think the liquids with more salt layered at the bottom, while
those with less salt layered at the top?
The ones were heavier…not as heavy…so they wouldn’t be able to go through
them. The heavy ones would go through the less heavier ones. Because the
salty ones were heavier.
Analysis
The answers to the questions asked here are similar to the answers I
receive every year when teaching about density. Density remains one of the
most difficult concepts for students to grasp, partly because it is often confused
with weight. The trick question, “Which is heavier, a ton of bricks or a ton of
feathers?” is funny because of this common difficulty in understanding density. I
have found that students often enter my class having participated in science
activities involving objects that sink or float in water. This approach seems to
add to the confusion because most students focus on an object’s weight, and
have trouble recognizing that the substance the object is made from has a
certain density.
Student #1
This student uses several adjectives to describe the way the liquids are
acting…strong, pressure, heavy, light. She seems to equate pressure with force
and assumes that fluid pressure only acts downward. However, she corrects
herself and refers to the liquids as being heavy and light. She believes that the
heavy liquids sink.
Students #1 and #2 both have the common misconception that liquids of
high viscosity are also liquids with high density. Student #1 refers to the liquids
as syrup, sticky, and watery; whereas Student #2 describes them as thick versus
liquidy.
Additionally, Student #1 refers to the liquids as chemicals. This may be
because I introduced the activity as an experiment. Students, especially the
younger ones, tend to refer to the liquids in any experiment as chemicals. A
good follow-up question would seek the student’s understanding on whether
everyday substances (orange juice, vinegar, oil, water) would layer on top of
each other.
Student #2
Student #2 seems to be relying primarily on past experiences to make
sense of why the liquids layered the way they did. This student gives the
impression that he has had experience with colors mixing. He is applying this
prior knowledge to create meaning in regard to the floating liquids. Yet, the
student’s answers remain inconsistent. The student first states that the reason
the liquids layered on top of each other is a result of the colors of the liquids.
However, eventually the student says that the thickness of the liquid is what
causes one liquid to layer on top of the other.
Student #3
This student must have some prior experience with density because he
uses the word to describe the way the liquids are acting. He also seems to know
that temperature can affect density, but clarifies that this is not the case in this
particular situation.
It appears that although the student has some understanding of density,
he needs more exposure to the concept because he also describes the liquids
using the words heavy and light. Moreover, it is possible that he thinks that the
amount of liquid has something to do with the density of the liquid. But this is
unclear because he starts to say, “If you have more of something it will sink to
the bottom.” This led me to believe that he thought the more liquid you have, the
denser it will be. Yet, in the following statement he says, “If you have less
density, it will float to the top.” In his first statement, he might have meant to say,
“If something has more density, it will sink to the bottom.”
The student also alludes to the reflection of light, as well as the color of
the liquids having something to do with why the liquids layered on top of each
other. This shows that his understanding of density is still incomplete.
Conclusion
In conclusion, to reduce the common misconceptions about density, I will
begin by having students layer several different liquids. After that, students will
learn that water containing different amounts of salt will also form layers.
Drawing on these two experiences, I will define density and present a model to
help the students visualize the difference between liquids of different densities at
a molecular level. These activities can be followed by observations of how heat
affects density. Following this, students can be exposed to floating and sinking
activities.
In hindsight I thought of a couple of questions I could have asked the
students which would have provided me with a little bit more insight regarding
their prior knowledge. These questions included, “Would gases separate into
layers just like the liquids did? Would a large quantity of a less dense liquid float
on a small quantity of a more dense liquid?” These will have to wait until the next
time.