Distinguish true acid waves from endothermic waves.

• A. True acid waves require external heat; endothermic waves require no heat.
• B. Both require external heat and produce identical curl strength.
• C. True acid waves use low pH acids; endothermic waves require external heat to speed processing, often providing stronger curl with heat.
• D. True acid waves are alkaline; endothermic waves are neutral.

Answer: (C)

The key idea is how pH and heat affect how the waving solution works on the hair. True acid waves are formulated with acids at a relatively low pH, which gently softens and reshapes the hair’s disulfide bonds in a mildly acidic environment—processing can happen at room temperature and may even generate some heat on its own, but they don’t require external heat to activate. Endothermic waves, on the other hand, need external heat to proceed; applying heat speeds the diffusion of the waving solution into the hair and accelerates the chemical changes, often resulting in a stronger curl when heat is used.

So the statement that best fits is that true acid waves use low pH acids, while endothermic waves require external heat to speed processing and often provide a stronger curl with heat. The other descriptions misstate the role of heat or the pH, so they don’t match how these two types differ.