An acid (often represented by the generic formula HA [H+A−]) is traditionally considered any chemical compound
that, when dissolved in water, gives a solution with a hydrogen ion activity greater than in pure water, i.e. a pH
less than 7.0. That approximates the modern definition of Johannes Nicolaus Brønsted and Martin Lowry, who
independently defined an acid as a compound which donates a hydrogen ion (H+) to another compound (called
a base). Common examples include acetic acid (in vinegar) and sulfuric acid (used in car batteries). Acid/base
systems are different from redox reactions in that there is no change in oxidation state.
In chemistry, a base is most commonly thought of as an aqueous substance that can accept protons. A base is
also often referred to as an alkali (only if OH− ions are involved). This refers to the Brønsted-Lowry theory of
acids and bases. Alternate definitions of bases include electron pair donors (Lewis), as sources of hydroxide
anions (Arrhenius) and can be (commonly) thought of as any chemical compound that, when dissolved in water,
gives a solution with a pH higher than 7.0. Examples of simple bases are sodium hydroxide and ammonia.
Bases can be thought of as the chemical opposite of acids. A reaction between an acid and base is called
neutralization. Bases and acids are seen as opposites because the effect of an acid is to increase the
hydronium ion (H3O+) concentration in water, whereas bases reduce this concentration. Bases react with acids
to produce water and salts (or their solutions).
pH is a measure of the acidity or basicity of a solution. It is defined as the cologarithm of the activity of dissolved
hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on
theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of standard solutions whose
pH is established by international agreement.
The concept of pH was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg
Laboratory in 1909. Sørensen suggested the notation "PH" for convenience, standing for "power of hydrogen",
using the cologarithm of the concentration of hydrogen ions in solution, p[H] Although this definition has been
superseded p[H] can be measured if an electrode is calibrated with solution of known hydrogen ion
Pure water is said to be neutral. The pH for pure water at 25 °C is close to 7.0. Solutions with a pH less than 7
are said to be acidic and solutions with a pH greater than 7 are said to be basic or alkaline. pH measurements
are important for medicine, biology, chemistry, food science, environmental science, oceanography and many
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