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**Using preliminary work to choose the values of control variables**

Consider a simple experiment – like measuring the temperature rise in a reaction between Copper Sulphate and Magnesium powder. We will change the mass of magnesium.

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**Get it clear in your mind what the following variables are:**

In this investigation, you will need to control some of the variables. Write down one variable that will need to be controlled. Describe briefly how a preliminary investigation could help you to find a suitable value for the control variable. You should explain how the results of this preliminary investigation will help you to decide on the best value for the control variable. The rules: Get it clear in your mind what the following variables are: Independent Dependent Mass of magnesium Temperature rise

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**Now identify the control variables:**

Volume of Copper Sulphate Concentration of Copper Sulphate Starting Temperature Pick one that is easy to understand Eg Volume of copper sulphate To get the marks for this question: Choose two values for the volume – a big one and a small one would be good. I will try a volume of 10cm3 of copper sulphate and a volume of 100cm3 of copper sulphate

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**Say what you will do with these two volumes: **

I will measure the starting temperature for both volumes. I will add one spatula of magnesium powder to each volume and measure the temperature after. It is important to say what you are going to measure To get the final mark – say how you will decide which is the best to use. Whichever volume gives the bigger rise in temperature will be the best one to choose

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