6. Experimental Design

Lesson 6

Question that test students on writing the experimental plan

A student wanted to find out what temperature is the best for making the biggest crystals. (The aim)

The independent variable is temperature.

The dependent variable is the mass of copper sulphate crystals.

The constants are:

(a)  The concentration of the copper sulphate solution used

(b) the total volume of the copper sulphate solution used

(c)  the humidity of the environment above the beaker

(d)  the mass of the seeding crystal

(e)  the shape of the seeding crystal     

Her hypothesis is that the lower the temperature, the bigger is the crystal grown.

Design an experiment to help her achieve this aim. You should include the following in your answers:

(a)  Equipment list

(b)  Diagram of your experimental set up

(c)  Procedure of your experiment

(d)  Describe how she should analyse her data. 

Equipment list:  

-       clean beaker (250 ml) x 10

-       copper sulphate in powder form

-       stirrer

-       Bunsen burner

-       Lighter

-       Weighing machine

-       Magnifying glass

-       Silica gel

-       refrigerator (-10 degree celcius to room temperature)

-       incubator oven ( room temperature to 50 degree celcius)

-       chopsticks x 100

Procedure:

1.    Set up the experiment as shown in the diagram above.

2.    Pour 50 grams of copper sulphate crystals into a beaker of 200 ml of distilled water and dissolve it using a stirrer.

3.    To ensure that the concentration of copper sulphate is saturated, heat the beaker using a Bunsen flame so that we can dissolve the maximum amount of copper sulphate.

4.    When we observe that the crystals cannot be dissolve any further due to heating and stirring, stop the heating process.

5.    Pour equal amounts of the copper sulphate solution into 6 different beakers of different amounts. (e.g. 25 ml each)

6.    Make sure that the seeding crystals used to make the big crystals are of the same shape by using a magnifying glass to inspect.

7.    Make sure that the seeding crystals used to make the big crystals are of the same mass by using a weighing machine.

8.    Place the seeding crystals into the 6 different beakers.

9.    Place the 6 different beakers into refrigerator and the incubator ovens set according to the temperatures specified.

10. To ensure that the humidity of the space above the beakers remains the same, place a packet of moisture absorber (silica gel) in each of the refrigerator and the oven so as to make sure that the humidity is low. This is to make sure that there is no condensation of water vapour in the refrigerator and oven into the beaker of copper sulphate.

11. Wait patiently for a period of 5 days without disturbing the 6 beakers of copper suphate solution.

12. After 5 days, remove the beakers from the refrigerators or ovens and remove the seeded crystal using a pair of chopsticks carefully.

13. Measure the final mass of the grown crystal using a weighing machine.

14. Repeat the experiment again using a similar setup and measure the average mass of the grown crystal again.

Analysis of data:

15. Tabulate the data and calculate the average mass of the grown crystal. 

16. Plot a graph of the average mass of the crystal against the temperature in which it is grown.

17. From the graph, we can find out what temperature is the best for making the biggest crystals. (The aim)