The experiment described involves heating water, which causes some of it to evaporate. The reduction in the volume of water from 10 mL to 6 mL indicates that the water has changed from a liquid to a vapor (steam), which is a phase change.
Since this process does not involve any change in the chemical composition of the water (H2O remains H2O), it is classified as a physical change.
Thus, the correct answer is: B. physical change involving phase changes.