You can achieve 6 marks with Foundation level knowledge. You must carefully read your syllabus, notes and textbooks to find the information that will help you with the investigation.
You are expected to identify all the key factors (i.e. all the important variables.) You only need to investigate the effect of changing one of the key variables. If you investigate two key variables you will not gain any more marks than if you investigate one variable well.
Always make a prediction. Go into as much detail as you can. Pictures and diagrams often make a big difference.
This means amount of evidence (observations/measurements).
You must plan to take at least 5 results. For quick experiments this should be more like 8 or 10 results. The measurements must be repeated, 3 repeats is a reasonable number for most experiments.
It is important that your plan shows how you are going to measure and record all the other key variables you have identified. These are variables you must keep the same throughout all the experiments to ensure a fair test. You should be able to give the value of these control variables in the plan.
e.g. The temperature will be kept at 20 degrees Celsius (room temperature).
Decide on the smallest and biggest values you can sensibly cope with. You then need to split this into (at least 5) equal steps from the smallest value to the biggest value.
HINT: draw up a blank results table showing the values you intend to use.