Pre-IB Chemistry
Rules of Graphing
All graphs must be drawn on graph paper. If you do not have any graph paper, you can
download and print some from my web site.
Bar Graphs
Bar graphs are used for discontinuous (or discrete) data. Usually the independent
variable is not quantitative but the dependent variable is.
The independent variable is usually placed along the horizontal axis. The separation
should be even and the axis should be well labeled. The dependent variable goes on the
vertical axis. This axis should also be marked with a scale and the axes neatly labeled.
Example:
Table 1: Autoworkers in the US and Worldwide
Automaker
Chrysler
Ford
GM
US
Employment
94000
190000
391000
Worldwide
Employment
114000
327000
647000
Autoworkers in the US and Worldwide
700000
600000
Employment
500000
400000
US Employment
Worldwide Employment
300000
200000
100000
0
Chrysler
Ford
Automaker
GM
Scatter Plots
A scatter plot is used when both the independent and dependent variables are continuous
and numerical.
1. Construct and label your axes with both the variable names and the units. The
independent variable goes on the horizontal or “x” axis, the dependent variable
goes on the vertical or “y” axis.
2. Make the graph large enough. You should number the axes in such a way that
your graph takes up at least half a page. Remember that a graph is a picture of
your data. If you were asked to draw a picture in art class, you would not just use
one small corner of your paper!
3. Choose the origin. This does not have to be (0,0); it depends on your data. If you
know that (0,0) should be a data point or if 0 should have come first in your data,
then it should be the origin of your graph. If your graph does not start at (0,0),
you should mark a break on the axis. You may not break the axis within the
range of your data.
4. Choose a scale. Look at the range of your data before you begin to determine
how to fit your data to the graph. Be sure that the spacing of the numbers if
uniform and appropriate. Also readable. There should be as much space between
2 and 3 as between 6 and 7. Having a box represent 7 units is difficult to plot and
read. Also, if your axes take up half the page, but all your data fits in a little
corner of the graph, you have missed the point. Each axis may have a different
scale.
5. Plot your points. If you know (0,0) should be a point, plot it.
6. Put in a best fit line or curve. Your points will almost never fall precisely on a
line or curve because of experimental error. NEVER JOIN THE DOTS WITH A
ZIG ZAG LINE! Take a straight edge and see if your points all fall close to a
line. If they do, draw it in with the straight edge getting as close to as many
points as possible. They may instead indicate a curve, in which case, draw a
smooth curve getting as close to as many points as possible. Perhaps the graph
will plateau. There may also be no relationship. You must decide what is
appropriate.
7. Title your graph. Indicate simply what is being graphed in the title.
Example:
Temperature of Water Over Time
Temperature
(degrees C)
0
0
0
0
2
10
15
22
27
Time (minutes)
0
2
4
6
8
10
12
14
16
Temperature of Water Over Time
30
Temperature (degrees C)
25
20
15
10
5
0
0
2
4
6
8
10
Time (min)
12
14
16
18
Now it’s time to practice:
Graph the following data appropriately.
1. Effect of Pressure on the Volume of a Balloon
Pressure (torr)
100
150
200
250
300
350
400
Volume (L)
10.0
6.7
5.0
4.0
3.3
2.9
2.5
2. Effect of Fertilizer on Seed Mass
Fertilizer per gallon of water (g)
0.05
1.75
2.00
3.00
4.00
Average Seed mass per 1000 seeds (g)
1.00
1.75
2.35
3.18
4.07
3. Heron Sightings in Different Seasons
Season
Spring
Summer
Fall
Winter
Number of Herons
30
39
18
4
4. Mass of Water Absorbed (in grams) by Different Brands of Paper Towels
Time (seconds)
10
15
20
A
.5
1.5
2.3
Brand of Towel
B
1.2
1.7
2.5
C
1.2
2.3
2.5