React H-ions with water

Bases (Greek βάση, basé - the starting point, the foundation, the foundation ) are in the narrower sense all connections in aqueous solution are able to generate hydroxide ions (OH), i.e. to increase the pH value of a solution. Hydroxide ions are chemical compounds that protons can take over from an acid to form a water molecule. A base is thus the counterpart to an acid and is able to neutralize it.

In a broader sense, various acid-base concepts describe a much broader range of chemical reactions that can go beyond the properties of hydroxide ions in water. The concepts according to are particularly significant Lewis (Lewis base and Lewis acid) and that after Pearsonthat of hard and soft acids and bases speaks (HSAB concept).

Historical development of the term "base"

In alchemy, some alkalis, such as lime (CaCO3, CaO and Ca (OH)2), Baking soda, soda, potash and salmiak. Until the beginning of the 18th century, however, no correct distinction was made between soda and potash. The term “alkalis” was rarely used and no precise connection between these substances was recognized. The base (alkali) as the opposite pole of the acid was postulated by O. Tachenius in the medical-theoretical teaching building, the chemiatry. There was a close connection between alkalis and fire or fire matter until the 18th century, also because of known exothermic reactions. The term "base" was introduced in the 17th century by chemists such as G. E. Stahl, R. Boyle and G. F. Rouelle because "basic" substances the non-volatile Basis for Fixation of volatile acids and can neutralize the (caustic) effect of acids. Basic steps in chemistry are taken by A. L. Lavoisier. He thought that acids were always made from non-metal oxides and water, and bases were made from metal oxides and water. Sir H. Davy and J. von Liebig saw acids as hydrogen compounds that can be converted into salts by metals. In 1887 S. Arrhenius defined bases as substances which, when dissolved in water, give off hydroxide ions and acids, as substances which dissociate and give off protons. Acids and bases neutralize each other. However, the theory was still insufficient, as compounds without oxygen were not included: ammonia also neutralizes an acid.

In 1923 J. N. Brønsted presented his very important point of view. It has become widely accepted and has proven its worth particularly in analytical chemistry. According to his theory, the base and acid interact in a proton transfer reaction. Bases take up protons from acids. The model presented by G. N. Lewis in 1923 is helpful when considering reaction processes in organic chemistry and in coordination chemistry and goes beyond the usual definitions. Therefore one speaks preferentially of Lewis base and Lewis acid. Many compounds normally referred to as acids are not acids according to this model. The Concept of hard and soft acids and bases (HSAB concept) developed by Ralph G. Pearson in 1963, thereby expanding the view of reactions in organic and coordination chemistry.

Properties of bases


  • Many bases are soluble in water (e.g. sodium hydroxide, ammonia), but not all (e.g. aluminum hydroxide)
  • They are corrosive and have a destructive effect on plant and organic substances.
  • They form soaps and glycerine from oils and fats.
  • There are strong and weak bases.
  • Bases can be diluted with water; depending on the dilution, their effect is significantly weaker.
  • The basic solutions cause phenolphthalein to redden or turn red litmus paper blue.
  • The "opponents of the bases" (base solution = lye) are the acids (see illustration). You can neutralize bases. Bases are also corrosive and attack many other substances that do not necessarily react with acids.
  • Clothing, skin and eyes are at risk on contact. Care must be taken to wear protective goggles, as chemical burns can always occur.

What are bases?

Are closely related to bases usually and often without explicit mention the presence and certain properties of the water. Pure water is subject to what is known as autoprotolysis. Oxonium ions (H.3O+) and hydroxide ions (OH):

In this reaction equation for water, the property of a base is shown by the formation of OHIons in water. At the same time, H are formed3O+-Ions in water, a property that an acid can have. However, water is neither called a base nor an acid and its behavior is named neutral. This refers to the pH, which is the concentration of H3O+Indicating ions in water. Pure water has a pH of 7, a very small concentration. This reaction is - like all the reactions described in this section - an equilibrium reaction: The formation of the ions and their combination to form water takes place continuously and with the same frequency.

Many compounds called bases have hydroxide ions (OH) and dissociate in water into metal and hydroxide ions. The solution is often called alkaline solution or Lye designated. The solid sodium hydroxide (NaOH) in water forms the so-called sodium hydroxide solution and potassium hydroxide (KOH) forms the potassium hydroxide solution.

Other connections have no OH themselvesIons, but form them in a reaction with water. They react alkaline by taking over a proton H+ from an H2O molecule and thus leave an OH-Ion back. For example, the salt forms trisodium phosphate (Na3PO4) or the salt sodium carbonate (Na2CO3) in aqueous solution oxonium ions. Organic compounds such as salts of carboxylic acids and amines as derivatives of ammonia also react in the same way. The corrosive effects All of these bases are largely due to the formation of OHIons.

Basic reactions

At weak and medium strength In the equilibrium reactions, all the components involved in the reaction are present in the solution. Any two of the reactants differ only in the presence or absence of a proton (H.+). You make one corresponding acid-base pair. Particles that have a suitable proton are called proton donors and particles that have the ability to absorb a proton are called proton acceptors. The entire reaction is called protolysis.

At strong and very strong Bases, the equilibrium reactions are (almost) completely on the side of the OH-Ions. This is for example with the reaction

the case. NaOH transfers all of its basic properties to the water. The acid-base pairs here consist of H.2O itself and particles formed from water. The cation Na+ does not matter.

Very strong and strong bases can no longer be distinguished in aqueous solution due to their alkalinity. Here one speaks of leveling effect (from French: niveler = equalize) of the water. In order to be able to differentiate even very strong bases with regard to their strength, equilibrium constants are determined in non-aqueous solutions and these are approximately transferred to the solvent water.

Water plays an important role in acid-base reactions. In addition to the protolysis described above, water is capable of what is known as autoprotolysis. It can donate protons and OH- form, or take up protons and H3O+ form. One is a reaction as a base and the other is a reaction as an acid. That is why water is called an ampholyte.


Compounds are called bases because of their special chemical properties. The wide range of these chemicals can be classified into groups according to various characteristics. Bases can be classified according to their ionic charge neutral, anionic or cationic bases organize. Ammonia (NH3) has no ionic charge and is therefore a neutral base. Sodium hydrogen carbonate can be designated as an anionic base, since the anion HCO3- is present. The hydroxide anion (OH-) itself can be called an anionic base.

Another way of dividing it is by dividing it into monovalent or divalent bases. Sodium hydroxide (NaOH), forms one OH per Na in solution and is monovalent, calcium hydroxide (Ca (OH)2) forms two OH per Ca and would then be two-valued.

As Base builder can be used to designate compounds in which another chemical reaction precedes the basic reaction. The metal oxides which form the corresponding hydroxides when dissolved in water can be described as base formers. Calcium oxide (CaO) forms the base Ca (OH) with water2. Base metals such as alkali metals can be oxidized beforehand by the action of water. In the vigorous reaction of sodium, hydrogen is also formed in addition to the sodium hydroxide solution.


The basis of neutralization is based on the fact that when mixed with a base, the effects of an acid do not add but cancel each other out. So can a base with a suitable amount of an acid neutralized become. Bases and acids react to form water.

Reaction of sodium hydroxide in and with water to form sodium hydroxide solution:

Reaction of hydrogen chloride in and with water to form hydrochloric acid:

Reaction of a sodium hydroxide solution with hydrochloric acid (neutralization):

Sodium hydroxide + hydrochloric acid reacts to form dissolved sodium chloride and water.

The crucial process is the reaction between the hydroxide and oxonium ions:

Acid-base balance

The strength of a base is called its basicity and is described by the base constant. The base constant (Kb) describes the position of equilibrium in the reaction between an acid-base pair in aqueous solutions. The negative decadic logarithm of Kb, the so-called pKbValue specified.

In the reaction

Is the base constant Kb defined as follows:

, with c (X) = concentration of X

The pKb-The value is accordingly:


Acid-base reactions without water

Analogous to the acid-base reactions that take place in aqueous solutions and with the participation of water, there are reactions in other media. In anhydrous ethanol, a reaction takes place with hydrogen chloride in which ethanol takes on the role of a base:

In the gas phase, the ammonia gas and the hydrogen chloride gas react to form the salt ammonium chloride.

In addition to water, other sufficiently polar solvents can also act as reactants in acid-base reactions. A good example is the autoprotolysis of liquid ammonia:

Categories: Fabric Group | Acid-base reaction | base