Base (chemistry)
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In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century.
Soaps are weak bases formed by the reaction of fatty acids with sodium hydroxide or potassium hydroxide.
Ammonia fumes from aqueous ammonium hydroxide (in test tube) reacting with hydrochloric acid (in beaker) to produce ammonium chloride (white smoke).
Sodium hydroxide
PH
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In chemistry, pH, also referred to as acidity or basicity, historically denotes "potential of hydrogen". It is a logarithmic scale used to specify the acidity or basicity of aqueous solutions. Acidic solutions are measured to have lower pH values than basic or alkaline solutions.
Test tubes containing solutions of pH 1–10 colored with an indicator
Lemon juice tastes sour because it contains 5% to 6% citric acid and has a pH of 2.2 (high acidity).