The acids are chemical compounds that have a pH lower than 7. This means that in contact with water they produce a higher activity of the hydronium cation (H3OR+ or abbreviated, H+) than that of pure water.
There are different theories to define an acid:
- Arrhenius theory. It states that an acid is a chemical species that in aqueous solution contributes hydrogen ions (H+) to dissolution. This definition is similar to the one mentioned above.
- Brønsted-Lowry theory. It states that an acid is a chemical species that gives up a hydrogen ion to another chemical species (Brønsted-Lowry base).
- Lewis theory. It states that an acid is a chemical species that accepts a pair of electrons from another chemical species (a Lewis base).
Acids can exist in any state of aggregation, that is, solid, liquid or gaseous, and have certain properties common to all of them:
- Its taste is sour.
- In contact with litmus paper, they make it change from blue to pink.
- They are corrosive substances, both in contact with inorganic and organic substances. That is why they cause burns to the skin.
- They are good conductors of electricity by forming solutions in water, as they dissociate into their ions.
- When in contact with bases, they produce a neutralization reaction that produces salts and water.
- When in contact with metal oxides, they produce a reaction that forms salts and water.
Acids can be named using several types of nomenclature:
If they are oxoacids, they are named as follows:
- Traditional Nomenclature. To name them, the word ‘acid’ is written followed by the root of the name of the non-metal (explained later) accompanied by prefixes and suffixes that correspond to the oxidation number of the non-metal. Thus, we have the following options:
- If the non-metal has oxidation number 1 or 2, the prefix ‘hypo’ (root of the name of the non-metal) is written with the suffix ‘bear’. For instance: hyposulfurous acid (H2SW2), where sulfur has oxidation number 2+.
- If the non-metal has oxidation number 3 or 4, the suffix ‘bear’ is written (root of the name of the non-metal). For instance: bromous acid (HBrO2), where bromine has oxidation number 3+.
- If the nonmetal has oxidation number 5 or 6, the suffix ‘ico’ is written (root of the name of the nonmetal). For instance: sulfuric acid (H2SW4), where sulfur has oxidation number 6+.
- If the non-metal has oxidation number 7, the prefix ‘per’ (root of the name of the non-metal) is written with the suffix ‘ico’. For instance: perchloric acid (HClO4), where chlorine has oxidation number 7+.
- Stock Nomenclature. To name them, the word ‘acid’ is written followed by the root of the name of the non-metal accompanied by the prefix corresponding to the subscript number that oxygen has (di (2), tri (3), tetra (4) …), then it is written ‘oxo’. In addition, the suffix ‘ico’ is written to the root of the name of the nonmetal followed by its oxidation number in Roman numerals and between parentheses. For instance: tetraoxosulfuric acid (VI) (H2SW4), dioxosulfuric acid (II) (H2SW2), trioxonitric acid (V) (HNO3) and trioxychloric acid (V) (HClO3).
- Systematic Nomenclature. To name them, the numerical prefix that corresponds to the subscript number that oxygen has is written, then the word ‘oxo’ is written, followed in turn by the root of the name of the nonmetal with the suffix ‘ato’ and then the number is written nonmetal oxidation in Roman numerals in parentheses. Then it is written ‘hydrogen’. For instance: hydrogen tetraoxosulfate (VI) (H2SW4), hydrogen dioxosulfate (II) (H2SW2), hydrogen trioxonitrate (V) (HNO3).
Oxo acids also have other prefixes:
- Goal. When they are formed from a water molecule.
- Ortho. When they are formed from three water molecules.
- Pyro. When they are formed from two molecules of water.
A tool that can be used in many cases to know when an acid is meta, ortho or pyro is:
- ‘Pyro’ acids have more than one atom of the non-metallic element. For instance: pyrophosphorous acid (H4P2OR5).
- The ‘ortho’ acids have more than two hydrogen atoms. For instance: orthophosphorous acid (H6P2OR6).
- The ‘meta’ acids have 1 or 2 hydrogen atoms. For instance: metaphosphorous acid (HPO2).
You can call root of the acid nomenclatureor to the word that is put to indicate the name of the non-metal. For instance:
- Sulfur. Root: sulfur. Symbol: S. Examples: sulfurous acid (H2SW3), sulfuric acid (H2SW4)
- Bromine. Root: brom. Symbol: Br. Examples: hypobromous acid (HBrO), bromous acid (HBrO2), bromic acid (HBrO3), perbromic acid (HBrO4)
- Carbon. Root: carbon. Symbol: C. Examples: carbonic acid (H2CO3)
- Chlorine. Chlor root. Symbol: Cl. Example: Hypochlorous acid (HClO) chlorous acid (HClO2), chloric acid (HClO3), perchloric acid (HClO4)
- Chrome. Root: chrom. Symbol: Cr. Examples: chromic acid (H2CrO4), dichromic acid (H2Cr2OR7)
- Match. Root: phosphor / phosphon. Symbol: P. Example: phosphonic acid (H3PO3), phosphoric acid (H3PO4)
- Manganese. Root: mangan. Symbol: Mn. Examples: manganic acid (H2MnO4), permanganic acid (HMnO4)
- Nitrogen. Root: nitr. Symbol: N. Examples: nitrous acid (HNO2), nitric acid (HNO3), hyponitrous acid (H2N2OR2)
- Selenium. Root: seleni. Symbol: Se. Examples: selenium acid (H2SeO3), selenic acid (H2SeO4)
- Silicon. Root: silic. Symbol: Yes. Examples: metasilicic acid (H2Yes3), orthosilicic acid (H4Yes4), disilicic acid (H2Yes2OR5), pyrosilicic acid (H6Yes2OR7)
- Tellurium. Root: tellur. Symbol: Te. Examples: tellurous acid (H2TeO3), telluric acid (H2TeO4)
- Iodine. Root: yod. Symbol: I. Examples: iodic acid (HIO3), periodic acid (HIO4)
If they are binary acids (composed of hydrogen and a non-metallic element) they are named:
The word ‘acid’ is written followed by the root of the name of the nonmetal with the ending ‘hydric’. For instance: hydrochloric acid (HCl(ac)), hydrobromic acid (HBr(ac)).
In these cases it is important to clarify that (ac) must be put (which means that it is in aqueous solution) in the acid formula to specify that we are formulating the acid and not a volatile hydride, for example, hydrogen chloride (HCl).