An acid in chemistry is a compound that releases hydrogens when it dissolves in water . The hydrogens are released as positive ions or H + protons , with a positive charge. The word “acid” derives from the Latin acere , which means sour.
Properties of acids
The acids share certain properties:
- Sour taste.
- Change the litmus paper from blue to pink.
- They are corrosive.
- Reacts with sodium bicarbonate (or other carbonates) and produces carbon dioxide.
- Strong acids burn biological tissues.
- Acids have the ability to conduct electricity when dissolved in water.
- The acids react with the bases to form salt and water. This process is called neutralization.
How to identify an acid?
Many of the properties of acids are difficult to detect by simple physical inspection. In this case, use is made of materials that change color when they come into contact with the acid . These compounds are known as indicators .
Among the best known indicators we can mention:
Many plants and flowers contain pigments that can act as indicators. For example, purple cabbage juice changes to purple in the presence of acids and yellowish green in the presence of bases.
Theories of definition of acids
The Swedish chemist Svante Arrhenius defined an acid as a substance that produces hydrogen ions or H + protons when dissolved in water. Hydrochloric acid HCl can be considered a typical Arrhenius acid:
In the Arrhenius theory, the H + ions from the acid react with the OH produced by the bases, forming water:
This theory does not explain the acidity of aqueous solutions of some salts or the behavior of acids in other solvents than water.
Johannes Brønsted and Thomas Lowry reinterpreted the concept of acid as a H + proton donor (molecule or ion) . These protons then interact with the water to form the hydronium ion H 3 O + :
In the reaction of ammonia with water, it behaves like acid, with ammonia being the proton receptor:
Theory of Lewis
The American chemist Gilbert Lewis formulated a general definition of acid as any compound that can accept a pair of electrons. In counterpart, the bases are compounds that can donate a pair of electrons.
Measurement of pH in acids
The pH is the measurement of free hydrogen ions in an aqueous solution. The pH scale ranges from 0 to 14; acid substances have pH values less than 7. Water and neutral substances have a pH equal to 7. Above 7 we are in the presence of basic substances.
In the following table we show you the pH of different acid substances:
|Hydrochloric acid (HCl)
|Sulfuric acid (H 2 SO 4 )
Strong acids and weak acids
There are some acids that we can consume while others can only be handled with strong safety measures. Strong acids are those that completely dissociate or ionize in aqueous solutions. On the other hand, weak acids release protons more restrictedly.
One way to visualize how to distinguish weak and strong acids is to make the analogy with rugby players:
- a strong acid is that player who releases the ball as soon as he receives it,
- a weak acid is like that player who stays with the ball and runs with it without letting go for more time.
Examples of strong acids
Among the known strong acids we have:
- Hydrochloric acid HCl,
- the sulfuric acid H 2 SO 4 ,
- Nitric acid HNO 3
- the perchloric acid HClO 4
- Fluoroantimonic acid H [SbF 6 ]
When acids and water are mixed, it is important to add the acid to the water, since this is a strong exothermic reaction, that is, heat is released. If the water is added to the acid, it heats up quickly and can cause splashing and burns. On the other hand, by adding the acid little by little over the water, the risk of splashing will be less.
Examples of weak acids
Normally the acids of organic compounds are known as weak:
- acetic acid H 3 CCOOH,
- butyric or butanoic acid H 7 C 3 COOH (the organic acid in butter),
- citric acid or 2-hydroxy acid 1, 2, 3-tricarboxylpropane C 6 H 8 O 7,
- lactic acid or 2-hydroxy propanoic H 6 C 3 O 3 .
Most common acids
In our day to day we are constantly in contact with acids: we consume them, participate in the metabolism or we need them to obtain energy. Let’s see some examples of everyday acids.
Hydrochloric acid HCl is a strong acid, which is produced in the stomach. It participates in the digestion of proteins and lipids, in the absorption of iron, calcium and other nutrients and in the destruction of infectious agents.
Citric acid is a tricarboxylic acid, that is, it has three carboxyl groups COOH, whose formula is C 6 H 8 O 7 . It is a typical ingredient of juices, which gives the acid taste. It is also part of the metabolism of living beings.
C 6 H 8 O 6 ascorbic acid is better known as vitamin C. This vitamin is essential for the functioning of some enzymes and its deficiency produces scurvy.
Vinegar is a liquid mixture that contains 5% acetic acid or ethanoic acid:
Vinegar is the product of the transformation of ethanol by bacteria of the Acetobacteraceae family :
Vinegar is a weak acid with a pH around 2.6. This low pH is what allows the preservation of food in pickles, for its bactericidal activity.
Aspirin or acetylsalicylic acid is used to reduce inflammation, lower fever and relieve pain. Originally it was obtained from the trunk of the tree Salix sp., Which was used to treat inflammatory problems.
Sulfuric acid H 2 SO 4 is a strong inorganic acid that is used in vehicle batteries. The reaction that occurs in the batteries is the following:
A lead battery contains lead plates Pb and lead oxide PbO 2 embedded in sulfuric> acid When plates Pb and PbO 2 are completely coated with lead sulfate PbSO 4 battery stops operating.
Folic acid C 19 H 19 N 7 O 6 is a B vitamin that participates in the synthesis of nucleic acids. This compound is found naturally in vegetables, legumes and eggs. Its deficiency produces anemia and defects in fetal growth.