Which product is commonly used to lighten hair during highlighting?

Bleach is commonly used to lighten hair during the highlighting process because it contains chemical agents, specifically ammonia and hydrogen peroxide, that penetrate the hair shaft and remove natural pigments. This allows for a lighter appearance by creating contrast against the darker base color of the hair.

When bleach is applied, it generates a chemical reaction that oxidizes the melanin in the hair, effectively lifting the color. This process is essential for achieving the desired level of lift and brightness, especially for techniques like balayage or traditional highlights where a dramatic color change is often sought.

The other products listed, such as hair conditioner, moisturizing cream, and hair serum, are designed for hair care and maintenance rather than altering hair color. They can help with hydration, shine, and manageability but do not have the chemical properties necessary for lightening hair. Therefore, bleach is the only effective choice listed for the purpose of highlighting hair.