how to change concentration of a salt in a physics experiment using a short procedure?

Question

concentration has to be changed every time the experiment is conducted (at least 6 times)

Answer 1

If your original volume is, say, 10 cm3, then add 2 cm3 of water, do the measurement, add another 2cm3 of water and so on, up to a total of 10 cm3 being added. This will give you 6 readings and will eventually halve the concentration.

Answer 2

Add more water (solvent) to decrease concentration or add salt (solute) to increase the concentration.