Final answer:
Chemical depilation is the hair removal method that uses a caustic alkali preparation, specifically depilatory creams containing active ingredients such as thioglycolic acid, to break down and dissolve hair.
Step-by-step explanation:
The type of hair removal that uses a caustic alkali preparation is known as chemical depilation. These preparations often contain active ingredients like salts of thioglycolic acid and thiolactic acids that are effective at breaking the disulfide bonds in keratin, which helps to weaken and dissolve the hair structure. The key reaction involves thioglycolic acid reacting with cystine in hair to form dithiodiglycolic acid and cysteine, ultimately leading to the hair becoming fragile enough to be wiped away. The process is completed by applying these preparations for about 10 minutes, after which the skin is rinsed to remove the dissolved hair and the skin's pH is restored.