Independent And Dependent Variables

Independent and dependent variables are identified before an experiment begins and results from an experiment are usually plotted in a graph. They are needed as part of GCSE and A Level science courses. So what are they?

• Independent variable – the variable that is altered by the chemist during a scientific experiment.

• Dependent variable – the variable being tested or measured by the chemist during a scientific experiment.

• Controlled variable – a variable that is kept the same during a scientific experiment, otherwise results are invalidated.

In the language of maths, the independent variable is ALWAYS plotted on the x-axis (horizontal scale) and the dependent variable plotted on the y-axis (verticle scale). This is why time is typically found along the horizontal.
[For straight line graphs with the relationship y = mx + c y is the dependent variable and x the independent variable].

It`s best to examine this further with a simple experiment: Reacion of marble chips with excess dilute hydrochloric acid and measurement of CO₂ evolved in given time periods (results would be plotted leter).



CaCO₃(s) + 2HCl(aq) -> CaCl₂(aq) + CO₂(g) + H₂O(l)

1. What is kept the same (controlled variable)? Mass of marble chips and concentration of hydrochloric acid (e.g. 2.0 moldm^-3). Temperature must also be kept the same at the start (otherwise the rate changes).

2. What changes during the experiment (independent variable)? Time.

3. What is measured during the experiment? Volume of CO₂ given off.

As these variables are numerical, they are quantitative.

The volume of gas produced is noted at specific time periods and written down in a table the results can be plotted in a graph and a line of BEST FIT drawn between the points to produce the rate curve.

Why can we simply NOT just joing up the points? The line of best fit represents the best estimate of the underlying relationship between the variables.



As can be seen in the graphs, the rate of reaction slows down, until the reaction stops (no further effervescence). The MEAN rate of reaction = total volume of gas produced / reaction time (units: cm³/s or cm³s^-1). The gradient (change in y / change in x) represents the rate at specific points along the graph.

As with all experiments that take numerical readings, there are sources of error. For example, random errors (human error such as misreading a scale) can occur and these can be reduced (but not eliminated) by repeating the experiment again and taking an avarage of the results (if time permits, but in most practicals this is not asked for). This also improves overall precision. There is also some uncertainty, such as the precision of the pipette (e.g. 0.01 cm³) and timing equipment (e.g. 0.1s). A measuring cylinder under water is not recommended as CO₂ is slightly soluble in water.

Can you think of other experiments where similar rate results are observed?

For example, reaction of magnesium ribbon with dilute hydrochloric acid. Hydrogen gas is produced and results plotted in a similar way. Or the catalytic decomposition of hydrogen peroxide (H₂O₂) to water and oxygen gas using MnO₂.