one acid is a substance that is able to release ions from hydrogen (H+) in a solution. However, a substance that can receive a pair of electrons is also considered an acid.
Regarding the basethis is considered as a substance capable of dissociating ions from hydroxide (OH–) in a solution. In addition, those substances capable of donating a pair of electrons are also considered.
Both acids and bases can be identified according to their position on the pH scale. In the case of acids, these have a value lower than 7, while bases (alkalines) have a value higher than 7.
|definition||An acid is a substance that is able to release hydrogen ions H+ in solution||A base is a substance capable of dissociating OH hydroxide ions– in a solution.|
|Arrhenius theory||It is a substance that releases hydrogen ions H+ in aqueous solution.||It is a substance that dissociates an OH hydroxide anion– in an aqueous medium.|
|Brønsted-Lowry theory||They are substances with the ability to give or give up protons (hydrogen atoms without their negative electron: H+).||It is a substance capable of accepting protons (H+) in dissolution.|
|Lewis theory||It is a substance capable of accepting a pair of electrons.||It is a substance that has the ability to give or give up electrons.|
|pH level||Less than 7.||Greater than 7.|
What is an acid?
An acid is a substance capable of releasing hydrogen ions in a solution. In addition, a compound that can receive a pair of electrons is also considered an acid.
The word “acid” comes from Latin acidswhich means ‘sour’ or ‘sharp’, and refers to the unpleasant taste of certain substances (eg vinegar).
Strong and weak acids
Acids can be considered strong or weak according to how they dissociate in an aqueous medium, that is, according to the amount of hydrogen ions they release in a solution.
An acid is strong when it is easily ionized, that is to say, the great majority of its hydrogen ions or protons are given up in solution. These acids are highly corrosive and good electrical conductors.
Examples of strong acids are H sulfuric acid2SO4hydrobromic acid (HBr) and hydrochloric acid (HCl).
In contrast, acids weak they are those that do not release a large amount of hydrogen ions and are less corrosive than strong acids. Examples of weak acids are carbonic acid (H2Co3) and acetylsalicylic acid (C9H8O4).
Characteristics of acids
- They are highly soluble in water.
- They react with some metals.
- They work as conductors of electric current.
- They have a sour taste (lemon, for example).
- They change the color of the litmus paper from blue to red.
- They can destroy organic tissues.
- They react with bases, producing water and salt.
- Acid-base reactions are exothermic (release heat).
Examples of acids in everyday life
- Ascorbic acid (vitamin C).
- Citric acid, present some fruits.
- Acetic acid (vinegar and wine).
- Lactic acid, produced during anaerobic exercise.
- Acetylsalicylic acid (aspirin).
- Hydrochloric acid (gastric juice).
- Sulfuric acid.
Find out about other characteristics of acids and bases.
What is a base?
A base is one substance capable of dissociating hydroxide ions in a solution, having a pH greater than 7. A substance capable of donating a pair of electrons is also considered a base, and includes all alkaline solutions.
The word “base” comes from the Greek basis and means ‘to go’ or ‘to walk’, while “alkaline” comes from Latin alkaliwhich at the same time comes from Arabic Al-Qaliand means ‘ashes’, particularly those from burnt wood.
Strong and weak bases
Strong bases ionize completely, releasing hydroxide ions into the solution. Examples of strong bases are lithium hydroxide (LiOH), potassium hydroxide (KOH) and sodium hydroxide (NaOH).
As for weak bases, these are those that dissociate partially. Examples of weak bases are ammonia (NH3) and sodium bicarbonate (NaHCO3).
Characteristics of bases
- They do not react with metals.
- In dissolution, they conduct electric current.
- They have a bitter taste (soapy, like chlorine/bleach).
- They change the color of the litmus paper from red to blue.
- In dissolution, they are slippery to the touch.
- They react with acids, producing water and salt.
- Acid-base reactions are exothermic (release heat).
- The pH is greater than 7.
Examples of basics in everyday life
- Magnesium hydroxide (milk of magnesia).
- Sodium hypochlorite (bleach, chlorine).
- Baking soda (baking powder).
- Sodium tetraborate (borax).
- Sodium hydroxide (caustic soda).
Get more examples of acids and bases.
Theories of acids and bases
Historically, these substances have been studied based on their properties and interactions with other elements. There are several theories that explain these phenomena and that are still valid.
Some of the best known, and which will be presented below, are the acid-base theory of Arrhenius (after his theory of electrolytic dissociation) of 1887, the acid-base theory of Brønsted-Lowry ( which presents the notion of acid-base conjugate pairs) of 1923, and the Lewis theory (in which the reception and donation of electrons is fundamental).
Arrhenius acid and base theory
According to the Swedish chemist Svante August Arrhenius (1859-1927), an acid is a substance that releases hydrogen ions H+ in an aqueous solution (water).
To the theory of electrolytic dissociation from Arrhenius (1887), acids are compounds that possess hydrogen and that, when dissolved in a aqueous mediumrelease hydrogen ions (protons) or hydronium (H3O+ protons surrounded by water molecules). In this case, the electrolytes (anions or cations) are capable of conducting electrical charges.
On the other hand, a base is a substance that dissociates a negatively charged ion (anion) hydroxide (OH–) in an aqueous medium.
The Arrhenius definition has the limitation that it does not consider reactions in which there is no aqueous solution, nor basic compounds that do not release hydroxide.
Example of Arrhenius acid and base
Acid: hydrochloric acid or HCI → CI–(aq) + H+(here)
Base: sodium hydroxide or NaOH → Na+(aq) + OH–(here)
Brønsted-Lowry acid-base theory
The Danish scientist Johannes Nicolaus Brønsted (1879-1947) and the English scientist Thomas Martin Lowry (1874-1936) published studies (1923) in which acids are defined as substances with the ability to give or give in protons (hydrogen ions H+ without its negative electron) to another that must accept them. As for the base, this is a substance capable of accept protons (H+) in dissolution.
In this theory, acids are not restricted to dissolving in water, other solvents are also included.
Thus, this definition extends the one presented by Arrhenius, in which an acid was limited to a substance that releases hydrogen ions in an aqueous medium. That is, an acid is a substance that gives protons to another substance, while a base accepts them from another substance.
Acid-base conjugate pair
With the Brønsted-Lowry theory, the notion of acid-base conjugate pairs is introduced, by a transfer of protons, in which the acid gives them and the base accepts them. In this case, acid and base coexist, since an acid can only act in the presence of a base and vice versa.
When an acid gives up a proton, that acid is called conjugate base. The same thing happens, on the contrary, when a base receives a proton. This basis is known as conjugate acid.
This happens because the acid becomes a conjugate base by donating a proton, that is, a substance that is able to accept a proton. In the case of the base, when it accepts a proton, it becomes a substance capable of donating a proton.
A neutralization reaction occurs when an acid and a base produce water and a salt.
Example of Brønsted-Lowry acid-base reaction
Hydrochloric acid and ammonia:
HCl (is the acid) + NH3 (is the base) ⇋ NH4+ (is the conjugate acid) + Cl– (is the conjugate base)
Lewis acid-base theory
The American scientist Gilbert Lewis (1875-1946) proposed a theory at the same time (1923) in which the theory of Brønsted-Lowry was presented. For this scientist, an acid is a substance capable of to accept a pair of electrons.
This definition of acid includes all Brønsted-Lowry acids, since hydrogen ions (protons) are electron acceptors, and includes many other substances that do not contain hydrogen.
In Lewis’s theory, bases are substances that have the ability to to give a pair of electrons.
By including Brønsted-Lowry acids and bases (proton donors and acceptors, respectively), Lewis theory also incorporates Arrhenius acids and bases (hydrogen and hydroxide ions that react in an aqueous medium).
Example of a Lewis acid-base reaction
Ammonia and boron trifluoride:
BF3 (is the acid) + NH3 (is the base) → H3N – BF3
The pH is the hydrogen potential of a solution, devised by the Danish scientist Søren Peder Lauritz Sørensen (1868-1939) in 1909. It indicates the concentration of hydrogen ions in a substance. To represent this concentration, a scale is used that indicates the level of alkalinity or acidity of a solution.
This scale is quantified from 0 to 14. Substances that have a level less than 7 are considered acids, while substances that have a level greater than 7 are considered bases (alkaline).
pH scale: pH = -log10 [H+]
Each step up the scale is logarithmic, meaning that one step increases or decreases acidity/basicity by 10 times the step immediately below or above. In other words, if the acidity of vinegar is a pH of 3, the acidity of lemon juice is 10 times higher, with a pH of 2.
The water has a pH that goes from 6.5 to 8.5, where the pH of the pure water is 7 (which is considered neutral). When the water has a pH lower than 6.5, it can have toxic metals in the composition, being corrosive and acidic. When its pH is higher than 8.5, it is called hard water, more basic or alkaline, with more magnesium and carbonates.
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