While it is possible to present numerical information in tabular form, it is frequently far more effective to present the data in a graph. There are many different ways to do this, and you are probably already familiar with some of them. Here is a pie chart showing the ages of businesses in the U.S. The data were taken from a Newsweek article (August 4, 1997) where the point was that relatively few businesses have a long history (the longevity champion is probably The Longfellow Wayside Inn in Sudbury MA). In this representation, the sizes of the various pie slices represent the percentage of all businesses that fail in a particular age range.

 

Another common graph type, made famous by the Wall Street Journal (and the SAT's), is the bar graph. Here is a bar graph showing the federal deficit as a percentage of expenditures for the period 1901-1995, the data coming from the President's 1996 Budget.

  

One particular type of bar graph, the histogram, is relatively common in the experimental sciences and you will work with a histogram in a later lesson.

In this lesson we will focus on the most common methods for graphing experimental data in the physical sciences, the x-y (or scatter in Excelese) graph. You will probably prepare most of your graphs using Excel or EasyPlot, or some other computer graphing system, but let's begin by preparing a graph by hand. Clicking here will take you to a page of graph paper. Print this page, and on it prepare a graph of the following data, obtained by measuring the temperature necessary to bring a fixed amount of gas to various volumes (at constant pressure). In preparing this graph, keep the following guidelines in mind:

 

the graph should occupy as much of the graph paper as possible  the choice of the numerical range of each axis should be informed by the range of the data  in this data set, the volume is the independent variable (x-axis data) and the temperature is the dependent variable (y-axis data)

Here are the data:

Submit your graph to your Laboratory Instructor. What do you conclude about the relationship between temperature and volume? Is this what you expected to find? Why? (The answers to these questions can be submitted on the email form.)

Once you have completed this hand drawn graph, open Excel and prepare a graph of the same data. Did Excel make the same choices you did for the axes? If you are not familiar with Excel, press the button here to go to a brief tutorial.

Many of you will be using Excel frequently, so there is one point about graphs in Excel that needs to be clarified (beyond the fact that Excel refuses to use the word graph). When you begin making a graph using the Chart Wizard, there are two similar looking choices - line and scatter (xy). These are not the same, and it is important that you appreciate the difference. If Excel is open, close it (close means to quit the program, not just minimize it). Pressing the button below will open an Excel spreadsheet containing some data for the hydrolysis of sucrose to glucose and fructose in acidic solution.

Prepare two graphs of concentration versus time (note that the phrase concentration versus time means that concentration is the dependent variable and time the independent variable), one a line graph and the other an xy graph. In the dialog for preparing the line graph you will have to tell Excel that you want to use the first column of data as the category (Excelese for x-axis) data. Once you have both graphs on screen, print them to turn in to your Laboratory Instructor. How do the two differ? What is the problem with using the line graph option in Excel? (Again, email the answers to these questions using the form.)

You will gain more experience with graphing in future lessons, and will continue to use these skills throughout all of your science courses, and in many courses in other disciplines.

• Flick Coleman wcoleman@wellesley.edu
• Dept. of Chemistry
• Date Created: Aug 12, 1997
• Last Modified: Aug 2, 1998
• Expires: Aug 1, 2000
• copyright by W.F. Coleman - 1997