An acid is a molecule or ion capable of either donating a proton, known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the proton donors, or Brønsted–Lowry acids.
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Sulfuric acid, Phosphoric Acid & Citric Acid
Sulfuric acid is a strong acid, whereas phosphoric acid is a weak acid