Why do you need to know the Independent and Dependent Variables before you make your hypothesis?

Answer 1

Actually, I believe that you set out your hypothesis first. If your teacher has perhaps told you to set out your variables first, you may wish to direct her towards a reliable source of information like a textbook - all of mine explicitly say that you must first create a hypothesis to know what the variables are to be:

1. Hypothesis
2. Prediction
3. Investigation
4. Analyse the data
5. Refine the hypothesis, if necessary

Your teacher is not wrong, but in all truth, the variables are implied by the hypothesis e.g.

You notice that the salmon have not returned to your local river this year. Your hypothesis:

The salmon have not returned due to abnormally low levels of oxygen in the water, caused by hot water ejected into the river by factories.

This implies that the variables should be:

• Independent: the location of a sample (as you will sample the oxygen levels at different locations)
• Dependant: the concentration of oxygen in the water at these locations
• Controlled variables: Time of day, time of year.

Therefore, I do not understand how you can possibly define the variables without first defining the hypothesis. Surely it is only possible this way.

The only exception would be if you are blindly going about a random experiment. For example, you have been given 20 cm2 of HCl in concentrations of 0.1, 0.2, 0.3, 0.4 and 0.5 mol/dm3 and an excess of NaOH, 0.1 mol/dm3. You have a conical flask, a funnel and a burette (like a long thin cylinder of glass, with a controllable output, whereby you can determine exactly how much of the solution you are adding; similar, but more accurate, than a pipette). What do you do?

You say, I am going to titrate this acid and this alkali by measuring the amount of the alkali (the NaOH) which I add to the acid (the HCl) to neutralise it. You do the experiment, collect the data, and present it perhaps in a graph. I won't explain the experiment to you, as it may only confuse you.

In this case, you have some meaningless results. You attempt to solve this problem by making a conclusion and saying 'By increasing the concentration of the acid, more alkali is needed to neutralise it.'. Having said this, you will then have conducted an experiment without a hypothesis at all. However, from here, you may create a hypothesis to further the meaning of the experiment so that in future, the experiment can be carried out again, but with different parameters. Your hypothesis may be 'The greater the concentration of any acid, the greater the volume of an alkali needed to neutralise it.' Whereby you would carry the same experiment out, but with different acids etc.. The ultimate effect of the experiment is that you have blindly done one without a hypothesis.

You cannot base the hypothesis on anything other than an observation you have made (like the fact that light is reflected by shiny objects, for example). From the hypothesis, you can derive what the variables, including the control variables will be for the experiment.

I hope this answers your question.