First I would say avoid trying to predicit it since mocks don't have anything to do with it, you should learn 3 topics to be almost guaranteed 2. I did heat & optics & electicity.
Experiments that came up in this years mock for higher level are:
Mech: investigate the relationship between the acceleration of a body and the force applied to it
Sound: measure speed of sound (legth of a column of air)
Light: refractive index of a substance
Electicity: risistivity of nichrome