# Graphing Straight Lines

Two variables may be related to one another in such a way that when plotted on a graph, the points form a straight line. The two properties are then said to have a linear relationship. There are many applications in the laboratory that involve linear relationships.

For example, let's take a look at a hypothetical experiment – the amount of reactant used by a chemical reaction over a seven hour period.

 Time (hours) Reactant (g) 1 2.5 2 5 3 7.5 4 10 5 12.5 6 15 7 17.5

From the data above, we can draw the following graph:

However, during your studies you will be asked to draw a 'best fit line' as opposed to simply connecting the dots. Scientists generally draw best fit lines when they want to use them for predictive purposes or to determine the general trend of the data.

A best fit line is meant to describe the trend of the data and as such, the line may not pass through many of the plotted points. Instead, the idea is to get a line that has equal numbers of points on either side. Such graphs can be used to predict what is happening between points.  For instance we can see that the plant would have a height of 6 cm at 2.5 days.  We don't have data about height at 2.5 days but we can determine how high the plant would be from the graph.

There is a video on the next page that demonstrates how to answer a typical graphing question you may come across in Chemistry and also how to draw a best fit line.