Problem Statements and Variables

During the next two weeks, we will be working on a big project.  We will be designing and performing an experiment involving density.  The final product will be a written lab report.  This report will be our first really big summative assessment, bigger than both of our quizzes so far combined.  It is really important that you put some effort into this and use your class time to ask questions and make sure you really understand everything.

Today, we talked about writing proper problem statements and identifying the variables in an experiment.  A problem statement is just the question we are trying to answer.  In this and in all future experiments, we will use a specific format for our problem statements:

What is the effect of 'experimental variable' on 'dependent variable'?

All of the problem statements we will ever write in this class will follow this format - only the variables will change.  We also identify each of the variables by circling the experimental variable and underlining the dependent variable.  The experimental variable is the thing we change in an experiment.  There should only be one experimental variable in an experiment - everything else should stay the same.  Recall the t-charts we made back when we talked about the requirements of science.  The experimental variable was on one side of the chart, and we listed all of our constants on the other side.  The dependent variable is the thing we measure in an experiment.  The dependent variable 'depends' on the experimental variable.  Measuring the dependent variable is how we will answer our question.  We spent most of the class period today writing problem statements in the correct format and identifying the experimental and dependent variables in each one.

Density Columns

Today, we had a density column competition.  Table groups worked together to apply the methods we learned yesterday to find the mass and volume of different types of liquid.  With those values, we could easily find the density for each liquid.  Then, we layered the liquids in the large graduated cylinder with the most dense liquid on the bottom.

A lot of groups had trouble with this activity.  Here are some things that could have given you trouble:

1.  Not making accurate measurements
     The students that measured the mass most accurately zeroed out the balance with a completely empty, dry graduated cylinder.
     The groups that measured the volume most accurately had each member of their group read the measurement, and they estimated to at least the nearest tenth of a milliliter.
     If either the mass or the volume measurement was off by even just a little bit, you had no chance at calculating an accurate density.

2.  Not doing the math correctly
     If you numbers weren't coming out they way you thought they should, my first suggestion was to make sure that you did mass divided by volume and not the other way around.

3.  Being careless when making the column
     The groups that had the most success stacking their liquids in the graduated cylinder used the pipettes (eye droppers) I supplied to drop the lighter liquids very carefully onto the more dense layers below them.


I know that this wasn't a very forgiving activity.  It only worked out if you did everything perfectly which frustrated many of you.  However, the groups that took their time and followed the procedures correctly were very successful.  Keep this in mind for future labs - I give you everything you need to succeed.  All you have to do is listen, follow instructions, and take your time.

Introduction to Density

Today, we started our two-week study on density. Density is the ratio of a material's mass to its volume. It can be calculated using the equation:

density = mass / volume

Density is not a new quantity that we are going to have to figure out how to measure. It is a relationship between two quantities we've already learned about; mass and volume. We practiced calculating the density of various liquids today. We did this by following these basic steps:

1) Place a graduated cylinder on a balance and hit the zero button.
2) Remove the graduated cylinder from the balance.
3) Transfer some of the unknown liquid into the graduated cylinder. (We are not concerned about getting a specific amount. We just need to have an accurate measurement.)
4) Record the volume of the liquid.
5) Place the graduated cylinder on the balance.
6) Record the mass of the liquid.
7) Use our density equation to calculate the density.
8) You're all done!

Once we had calculated densities for all of the liquids, we stacked them up in a large graduated cylinder. This is called a density column and I think it looks really neat!

Tomorrow, we will be working in small groups to make more density columns on your own.

Don't forget, if you are retaking the measurement quiz for next week, that needs to get done tomorrow. I'll be here before and after school as well as during advisory.

Here are the two videos I used to start class today. Enjoy!


Archimedes Lego Video http://www.youtube.com/watch?v=wEvtahSn_ms

Archimedes Explained http://www.youtube.com/watch?v=KMNwXUCXLdk