Often, when an investigation has been completed, you will need to draw a graph to represent your results. There are a few simple rules to help you decide what sort of graph to draw and a few other useful guidelines to make sure that your graph is as clear as possible.
The first thing to decide is whether you are drawing a bar graph or a line graph and this depends on the type of data you are using. Two common types are categoric variables (discrete variables) and continuous variables. Categoric variables are ones where there is no direct mathematical connection between one and another. For example, if you were plotting a graph of the heights of boys in your form, the names of the boys would be categoric variables. There is no way of being half way between one boy and the next. In this case you would draw a bar chart.
More commonly your independent variable will be a continuous variable such as time, mass, temperature etc. In this case you will want to draw a line graph as it will be possible to predict what would happen half way between two data points on the graph. The independent variable always goes on the x-axis unless you are specifically told to plot it on the y-axis.
Some things to think about when drawing a graph include…
- Use a sharp pencil to produce neat, precise points and lines
- Colour code the lines and points and supply a key so that they can be identified
- The graph should be as big as possible in both width and height
- Each axis must be labelled with the variable name and the unit
- The line of best fit should be neatly and smoothly drawn
- Your graph should have a title that includes a reference to both the x and y axes
When drawing the line of best fit, don’t just join the dots. Decide what the trend of the data is. Should it be a curve or a straight line? Does the line go through the origin? The following pictures illustrate a few common graph lines.
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This line passes through the origin, the value y increases at a constant rate and is directly proportional to x. This could be a graph of load v. extension for a spring before reaching the elastic limit.
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The value y increases at an increasing rate. This might be something like a solubility curve where a substance becomes more soluble as the temperature of the solvent increases. I am going to write more about solubility tomorrow.
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The value y increases at a decreasing rate. A good example might be a reaction where one of the reactants is being used up. The graph eventually flattens out when the reaction stops.
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Questions…
Select from Examples A, B or C as the graph most likely to be produced by the following scenarios.
- The extension of a spring.
- The total mass of an open-topped flask containing calcium carbonate and hydrochloric acid.
- The densities of several different sized objects made from the same material.
- The way that the solubility of potassium nitrate changes with increasing temperature.
- The total volume of hydrogen released over time as zinc reacts with sulphuric acid.
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