My First Lab of the Year

measuring volume and mass of water

This was my first week of classes. It is the beginning of the 30th year that I have been a high school teacher and the 28th straight year I have been in the same physical classroom. Shockingly, all of those in the same school (yes I meant it to sound that way). In a previous post I wrote about the demonstration that I use on my first day of class called the Ira Remsen Demonstration. This time I would like to describe the first lab that I do every year with my chemistry students. It is a short and simple lab and maybe that is why it works so well for me. We seem to always get stuck with short periods our first week and I need to plan shorter than usual lessons. I also like that this lab gives the students a chance to go around the room and meet each other.

My first experiment involves measuring the density of water. Each group of two kids is assigned a specific volume of water from 10 to 100 mLs on the tens. They simply measure the mass of an empty graduated cylinder and then add the water and find the mass again. Once they have their data they go around the room and find another group that has one of the volumes that they need and get the data from them and record their names. Once complete they generate a graph of the data and answer a few simple questions. The whole procedure can be completed in about 20 minutes.

Now while this seems very simple there are several facets I want to point out. One is that we have an influx of new 9th and 10th grade students each year. We are a magnet and do not have a specific feeder school and it allows for the newer students to meet new people. It also does not require any specific previous skills. We introduce the balance, graduated cylinder, units for measurements, density calculations, and graphing in a very short period of time. Finally it is a relatively easy lab for students to make up if they check in late.

I do not believe that this is an original idea in any way but I am very happy with the way I have worked it up and how it serves me as an early lab.

Concepts: 

This activity serves to introduce the balance, graduated cylinder, units for measurements, density calculations, and graphing

Time required: 

10 minutes prep

20 minutes classtime

Materials: 

assorted sizes of graduated cylinders 10mL to 100mL (1 per group)

balance

pipettes

beakers of water

 

Procedure: 
  1. Find the mass of an empty and dry graduated cylinder using a balance. Never try to dry the inside with a paper towel. It will just get stuck inside of it and leave small pieces of paper behind. Only dry the outside if it is wet.
  2. Fill it with the volume of water assigned to your group. Make sure the bottom of the meniscus is touching the correct line on the cylinder.
  3. Find the mass of the cylinder and water using a balance.
  4. Calculate the mass of the water alone. It will be the difference between the mass of the empty cylinder and the mass of the cylinder plus water.
  5. Clean up. Walk around the room and get data from other groups to fill in table 2 below.
  6. Answer the questions and make the graph. 
Questions: 

1. Construct a graph of mass (y axis) versus volume (x axis) for the data in table 2. Label the axes(mass and volume not x and y), title the graph, include units, and draw the line as smoothly as possible. Use a computer to generate the graph. It should fill the entire page. Staple it to this page.

2. What is the accepted value for the density of water at room temperature?

3. What physical quantity does the slope of the line on your graph represent?

4. What are the units for the slope of your line?

5. Select one possible source of error and tell how it would change your data. Would your value for the density be higher or lower? Explain with a sample calculation 

Preparation: 

Set out balances, graduated cylinders, beakers of water and pipettes

Attribution: 

I do not believe that this is an original idea in any way but I am very happy with the way I have worked it up and how it serves me as an early lab.

Join the conversation.

Comments 2

Eric Sullenberger | Wed, 09/06/2017 - 08:48

I've been dissatisfied with how I start the year in my general science (8th grade) and physical science (9th grade) classes and have been trying to incorperate more short, simple labs into my curriculum.  I decided to try this activity paired with another on a double-period lab day, and ended up having the most productive discussion I've ever had with freshmen about measurements, rounding, precision, accuracy, graphing, line of best fit, etc. that I have ever had.  The activity took only about 20-30 minutes as you said, but with one of my two classes we had a LONG 40 minute dicussion afterwards and ended up not doing the second activity because this conversation was so productive.  I love how simple it was and how well it engaged a thoughtful discussion.  Thanks again.

Bill Jorden | Fri, 09/08/2017 - 10:19

We do something similar to this when discussing precision and accuracy.  Students are asked to measure 50 mL of water in a beaker, in a graduated cylinder, and dispensed from a buret.  They find the masses of these volumes of water and calculate the density. They repeat their work 3 times with each container.

Using these results, students can discuss the precision of their measurements as well as the accuracy and precision of each container.