Guidelines for Graphing

Guidelines for Graphing

Often the goal of an experiment is to find the relationship between two variables (pressure and volume, time and temperature, etc.). As one variable changes, so does the other. Graphing is a useful way to visualize and describe these relationships. Because the use of graphs is so common in the sciences, it is important that you know how to construct and interpret graphs. When preparing graphs as part of chemistry lab reports keep the following guidelines in mind:

  1. If drawn by hand, graphs must be neatly drawn, using a straight edge, French curve, or flexible curve.

  2. Tabulate all data to be graphed before beginning.

  3. Use the x-axis for the independent variable (that which is experimentally varied; also known as the manipulated variable, it will generally vary by a whole number amount) and the y-axis for the dependent variable (that which is a function of the independent variable; also known as the responding variable, it will generally vary by a non-whole number amount).

  4. Decide on the limits of the graph (maximum and minimum values) and select the axis. The lower left corner of the graph does not have to represent zero on either axis unless you have data in this region.

  5. Select divisions on the axes which are easy to read. Common graph paper is divided in units of tens. Hence one square may equal 1, 2, 5, 10, or 10, 20, 50, 100, but never 3.75 or some other "odd" number.

  6. For greatest accuracy, select scales so that the graph nearly fills the page.

  7. Grid lines should be shown on the graph. (If drawn by hand, all graphs are to be drawn on graph paper with at least 10 squares per inch. Adequate graph paper is provided for you in the back of your lab notebook)

  8. Label both axes with both quantity and units. For example: "Pressure (torr)"

  9. Mark the data points with a small dot. (Pencil may be used before being inked for permanency.) Draw a small circle around the point or darken to make more visible.

  10. A smooth curve should be drawn through the points. The curve should pass as close as possible to each of the points but should not be connected point-to-point. (The data may be irregular but nature rarely is.) If the relationship appears to be linear, the smooth curve should be a straight line. If the line is extended past the range of the measured values, this extension should be indicated by a dashed rather than a solid line.

  11. Title the graph in a descriptive manner.