Final answer:
A 20-volume hydrogen peroxide solution will generally lift hair one to two levels, although individual results can vary based on hair characteristics.
Step-by-step explanation:
A 20-volume hydrogen peroxide solution is typically used in the process of hair lightening and can generally lift the hair colour one to two levels. It is a common salon treatment for changing hair color or for achieving a 'peroxide blonde' look. The term 'volume' refers to the amount of oxygen gas that the hydrogen peroxide is capable of producing; therefore, a 20-volume solution will release a certain amount of oxygen gas sufficient to lighten hair by one to two levels. It should be noted that hair composition, current color, and condition can influence the final result of bleaching with hydrogen peroxide.