Having a pH ("potential hydrogen") less than 7. The skin’s barrier, or acid mantle, is naturally slightly acidic, with a pH hovering around 4.5 to 5.5. When it drops out of range, skin becomes prone to breakouts and irritation. An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen ion, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid.
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