Graphing In Chemistry - slider-dpchemistry-11

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WHY GRAPH?
In Chemistry, we make frequent use of graphs to provide a visual
representation of data.
This will often make the data easier to interpret.
We can easily see trends or relationships.
We can predict unknown values.
Line graphs for continuous data.
Bar graphs for non-continuous data.
Fig1. Favourite Subject
WHAT IS A GRAPH?
A graph is a visual representation of
numerical systems and
relationships.
For example, a straight line can be
represented by the equation to
the right where:
y = the value on the vertical axis
x = the value on the horizontal axis
m = the slope of the line (Δy/Δx)
b = the y intercept
What is the slope of this line and
what would the slope represent?
COMPONENTS OF A LINE GRAPH
Title
Error Bars (not
shown here)
Reasonable
scale
Key (if needed)
Labels
(units)
Smooth lines
Big enough:
(takes up
more than
½ the graph
For detailed rules on
constructing a graph click
HERE
PLOTTING A LINE GRAPH
there are two axes - horizontal (called the x-axis) and vertical (called
the y-axis),
a point on the graph is denoted by an ordered pair (or coordinates
(e.g., (3,8)) where:
 the first number refers to horizontal position on the x-axis,
 the second number refers to vertical position on the y-axis,
 sometimes the ordered pairs are listed in tabular format with
headings that correspond to the labels on the axis
the two axes intersect at point called the origin with coordinates (0,0),
the reason that we plot data is so that we can more easily observe
trends or behaviour of the data
(modified from Anderson and Swanson, 2005)
TYPES OF LINEAR LINE GRAPHS
Fig. 1 Directly proportional
Fig. 2 Inversely proportional
Directly proportional graphs show variables that are directly related to one
another while inversely proportional graphs show an inverse relationship (e.g. P =
1/V as in Boyle’s Law.)
NON-LINEAR LINE GRAPHS
These non-linear graphs show exponential relationships – growth (right) and decay (left)
LINE OF BEST FIT
When presenting data, it is most likely that the point-to-point connection of the data
points will not result in a smooth curve or straight line.
Often you will need to draw a “best fit line” to show the overall relationship that is
present. This will allow you to make predictions about other data points or to
measure gradients.
Look at the graph below. How would you draw a best fit line?
•
The line should represent the general trend as close as possible
•
There should be as many points above as there are below the line
•
Excel and Logger Pro will draw best fit lines for you.
BAR/COLUMN GRAPHS
Bar graphs can also be used in scientific
reporting.
We choose to use a bar graph when we
have non-continuous data. That is,
there is no reason to show data
between two points on the graph.
Notice that it would make no sense to
draw a line between testosterone and
prolactin as there is no data between
them (i.e. non-continuous)
If there is only one set of data, we use a
bar graph (i.e. no x and y data)
You must still fully label your bar graph.
WHAT’S WRONG WITH THESE GRAPHS?
A
B
C
D

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