An acid is a compound that releases hydrogens when dissolved in water . Hydrogens are released as positive ions or H + protons , with a positive charge. The word “acid” derives from the Latin acere , which means sour.
Acid properties
Acids share certain properties:
- Sour taste.
- Change 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.
- Acids react with 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 a 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, the juice of purple cabbage changes to purple in the presence of acids and yellowish green in the presence of bases.
Another way to determine the acidity of a substance is by measuring its pH using a device: the pH meter.
Acid definition theories
Arrhenius theory
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 Arrhenius theory, the H + ions from the acid react with the OH produced by the bases, forming water:
This theory does not allow explaining the acidity of aqueous solutions of some salts or the behavior of acids in solvents other than water.
Brønsted-Lowry theory
Johannes Brønsted and Thomas Lowry reinterpreted the concept of acid as an H + proton-donating species (molecule or ion) . These protons then interact with water to form the H 3 O + hydronium ion :
In the reaction of ammonia with water, it behaves like acid, ammonia being the receptor for the proton:
Lewis theory
American chemist Gilbert Lewis formulated a general definition of acid as any compound that can accept an electron pair. In counterpart, bases are compounds that can donate an electron pair.
PH measurement in acids
PH is the measure of free hydrogen ions in an aqueous solution. The pH scale ranges from 0 to 14; acidic 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 or alkaline substances.
In the following table we show you the pH of different acidic substances:
| Substance | pH |
| Hydrochloric acid (HCl) | 0 |
| Sulfuric acid (H 2 SO 4 ) | 0.8 |
| Lemon juice | 2.0 |
| Vinegar | 2.2 |
| Carbonated drinks | 3.5 |
| Came | 4.0 |
| Black coffee | 5.5 |
| Milk | 6.5 |
See also pH .
Strong acids and weak acids
There are some acids that we can consume while others can only be handled with strong security measures. Strong acids are those that fully dissociate or ionize in aqueous solutions. On the other hand, weak acids release protons in a more restricted way.
One way to visualize how to distinguish weak and strong acids is by making the analogy with rugby players:
- a strong acidis that player who releases the ball as soon as he receives it,
- a weak acidis like that player who stays with the ball and runs with it without releasing it for a longer amount of time.
Examples of strong acids
Among the known strong acids we have:
- hydrochloric acid HCl,
- sulfuric acid H 2SO 4 ,
- nitric acid HNO 3
- perchloric acid HClO 4
- fluoroantimonic acid H [SbF 6]
How to mix acids and water safely
When acids and water are mixed, it is important to add the acid to the water, as this is a strong exothermic reaction, i.e. heat is released. If the water is added to the acid, it heats up quickly and can cause splashes and burns. On the other hand, by gradually adding the acid to 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 3CCOOH,
- butyric or butanoic acid H 7C 3 COOH (the organic acid in butter),
- citric acid or 2-hydroxy acid 1, 2, 3-tricarboxylpropane C 6H 8 O 7,
- lactic acid or 2-hydroxy propanoic H 6C 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 need them for energy. Let’s see some examples of acids for everyday use.
Hydrochloric acid
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
Citrus fruits, such as lemon, orange, and tangerines, are rich in citric acid.
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 in juices, which gives it an acidic flavor. It is also part of the metabolism of living things.
Ascorbic acid
Vitamin C is an acid derived from glucose.
Ascorbic acid C 6 H 8 O 6 is better known as vitamin C. This vitamin is essential for the function of some enzymes and its deficiency produces scurvy.
Vinegar
A preservation technique is to dip vegetables in vinegar.
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, due to its bactericidal activity.
Aspirin
Aspirin or acetylsalicylic acid is the best known medicine in the world.
Aspirin or acetylsalicylic acid is used to reduce inflammation, lower fever, and relieve pain. Originally it was obtained from the trunk of the Salix sp. Tree, which was used to treat inflammatory problems.
Sulfuric acid
Vehicle batteries are filled with sulfuric acid H 2 SO 4 .
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 as follows:
A lead battery contains some lead Pb and lead oxide PbO 2 plates embedded in sulfuric acid> When the Pb and PbO 2 plates are completely coated with lead sulfate PbSO 4 the battery stops working.
Folic acid
Folic acid is an important vitamin in cellular metabolism.
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.
