Final answer:
The ingredient that lightens the skin when using a mask is hydroquinone.
Step-by-step explanation:
The ingredient that lightens the skin when using a mask is hydroquinone.
Hydroquinone is a chemical compound that is commonly used in skincare products, including masks, to lighten dark spots and hyperpigmentation. It works by inhibiting the production of melanin, which is responsible for giving color to the skin.
However, it is important to note that the use of hydroquinone should be done under the supervision of a dermatologist, as it can have side effects and should not be used in excessive amounts.