Acids are substances that can donate protons (hydrogen ions, H⁺) or accept electron pairs, which defines their chemical properties. They can be inorganic, like hydrochloric acid (HCl) or sulfuric acid (H₂SO₄), or organic, such as acetic acid (found in vinegar) and citric acid (in citrus fruits).

Acids are known for their ability to lower the pH of a solution, making it more acidic. A solution with a pH below 7 is considered acidic, with stronger acids having lower pH values. Strong acids, like hydrochloric acid, dissociate completely in water, while weak acids, such as acetic acid, only partially dissociate.

One key reaction involving acids is neutralization, where acids react with bases to form water and salts. Acids also react with metals to produce hydrogen gas and with carbonates to release carbon dioxide.

In everyday life, acids are vital for digestion, as stomach acid breaks down food. They are also used in cleaning products to remove mineral deposits and are essential in various industries, like manufacturing fertilizers and pharmaceuticals.

Although acids are useful, they can be corrosive and dangerous, so handling them with care is important to avoid harm.