An acid is a substance that releases hydrogen ions (H⁺) when dissolved in water, which increases the reactivity of the solution. The greater the concentration of hydrogen ions, the stronger the acid. Acids act by donating these ions during chemical reactions, allowing them to break down materials, change the color of indicators, or react with bases to produce water and salts. Everyday examples include vinegar (which contains acetic acid) and stomach acid (hydrochloric acid)! #history #science #s