Naming Bases and Inorganic Salts
A base is a compound that produces hydroxide ions in water (or accepts hydrogen ions), and a salt is an ionic compound formed when the hydrogen ion of an acid is replaced by a metal or ammonium ion; bases are named like ionic compounds ending in “hydroxide”, and salts are named with the cation first and the anion from the parent acid second. In brief: NaOH is sodium hydroxide, Ca(OH)2 is calcium hydroxide, and the reaction of sulfuric acid with sodium hydroxide gives the salt sodium sulfate, Na2SO4.
Bases and salts are everywhere: soap, antacids, fertilizers, table salt and baking soda. Many have common names that are still used in shops and industry, which differ from systematic names. This article explains how to name bases, how salts form and are classified, how to derive a salt’s name from its parent acid and base, includes a table of common names, works through neutralization calculations and gives some history.
How are bases named?
Most common bases are ionic compounds with the hydroxide ion (OH−). They are named like any ternary ionic compound: the name of the metal followed by “hydroxide”. If the metal has variable charge, a Roman numeral is added: iron(II) hydroxide, Fe(OH)2, and iron(III) hydroxide, Fe(OH)3. Ammonia, NH3, is a molecular base that accepts a hydrogen ion to form ammonium, and its aqueous solution is often called ammonia water or, less accurately, ammonium hydroxide.
In the Arrhenius definition, a base releases OH− in water. Soluble metal hydroxides are called alkalis. Metal oxides such as CaO and Na2O are called basic oxides, because they react with water to give hydroxides.
Common bases
| Formula | Systematic name | Common name | Solubility in water |
|---|---|---|---|
| NaOH | sodium hydroxide | caustic soda, lye | Very soluble |
| KOH | potassium hydroxide | caustic potash | Very soluble |
| Ca(OH)2 | calcium hydroxide | slaked lime | Slightly soluble (limewater) |
| Ba(OH)2 | barium hydroxide | baryta | Soluble |
| Mg(OH)2 | magnesium hydroxide | milk of magnesia | Very slightly soluble |
| Al(OH)3 | aluminum hydroxide | antacid ingredient | Insoluble |
| NH3 | ammonia | household ammonia (in solution) | Very soluble |
What is a salt and how is it named?
A salt is an ionic compound made of a cation (other than H+) and an anion (other than OH− or O2−). Salts are the typical products of the neutralization of an acid by a base: acid + base → salt + water. The name of a salt has two parts: the cation, which comes from the base, and the anion, which comes from the acid. The anion name is obtained from the acid:
- hydro-…-ic acid gives an anion ending in -ide (hydrochloric acid gives chloride).
- -ic acid gives -ate (sulfuric acid gives sulfate, nitric acid gives nitrate).
- -ous acid gives -ite (sulfurous acid gives sulfite, nitrous acid gives nitrite).
For example, hydrochloric acid and sodium hydroxide give sodium chloride: HCl + NaOH → NaCl + H2O. Nitric acid and calcium hydroxide give calcium nitrate: 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O.
What types of salts are there?
| Type | Description | Example |
|---|---|---|
| Normal salt | All replaceable H+ and OH− are replaced | Na2SO4, sodium sulfate |
| Acid salt | Contains still-replaceable hydrogen | NaHSO4, sodium hydrogen sulfate; NaHCO3 |
| Basic salt | Contains hydroxide or oxide in addition | Mg(OH)Cl, magnesium hydroxide chloride |
| Double salt | Two cations in one crystal | KAl(SO4)2·12H2O (alum) |
| Hydrated salt | Contains water of crystallization | CuSO4·5H2O |
Common names of salts and other compounds
| Common name | Formula | Systematic name |
|---|---|---|
| Table salt | NaCl | sodium chloride |
| Baking soda | NaHCO3 | sodium hydrogen carbonate |
| Washing soda | Na2CO3·10H2O | sodium carbonate decahydrate |
| Limestone, chalk | CaCO3 | calcium carbonate |
| Quicklime | CaO | calcium oxide |
| Gypsum | CaSO4·2H2O | calcium sulfate dihydrate |
| Epsom salt | MgSO4·7H2O | magnesium sulfate heptahydrate |
| Saltpeter (niter) | KNO3 | potassium nitrate |
| Blue vitriol | CuSO4·5H2O | copper(II) sulfate pentahydrate |
| Borax | Na2B4O7·10H2O | sodium tetraborate decahydrate |
Worked examples
- Name the salt. The reaction H2SO4 + 2NaOH → Na2SO4 + 2H2O gives sodium sulfate. The acid is sulfuric (-ic), so the anion is sulfate.
- Acid salt from partial neutralization. H3PO4 + KOH → KH2PO4 + H2O gives potassium dihydrogen phosphate, as only one of the three acidic hydrogens is replaced. With 2 mol of KOH the product is K2HPO4 (dipotassium hydrogen phosphate, or potassium hydrogen phosphate), and with 3 mol the product is K3PO4.
- Formula from the name. Ammonium chloride: NH4+ and Cl−, so NH4Cl. Iron(III) hydroxide: Fe3+ and OH−, so Fe(OH)3.
- Antacid calculation. Aluminum hydroxide neutralizes stomach acid: Al(OH)3 + 3HCl → AlCl3 + 3H2O. To neutralize 0.100 mol of HCl, 0.100 ÷ 3 = 0.0333 mol of Al(OH)3 is needed. The molar mass is 26.98 + 3 × 17.01 = 78.01 g/mol, so the mass is 0.0333 × 78.01 = 2.60 g.
- Strength of limewater. A saturated solution of calcium hydroxide contains about 1.7 g/L. The molar mass is 74.09 g/mol, so the concentration is 1.7 ÷ 74.09 = 0.023 mol/L. Since each formula unit gives two OH−, [OH−] = 0.046 mol/L and pOH = 1.34, so pH = 14.00 − 1.34 = 12.66.
- Identify the type. NaHCO3 contains replaceable hydrogen, so it is an acid salt, even though its solution is slightly basic.
How did these names develop?
The word “salt” originally meant common salt, sodium chloride, and was later extended to all compounds with similar properties. The word “alkali” comes from the Arabic al-qali, which referred to the ashes of plants. The old names such as “caustic soda” (sodium hydroxide) and “saltpeter” (potassium nitrate) come from traditional trades. The 1787 system of chemical nomenclature organized salt names by their parent acid, so that “sulfate” comes from sulfuric acid. Many traditional names remain in the food, building and medical industries.
Which mistakes are common?
Mistake 1: Calling every base a hydroxide
Ammonia is a base, but contains no hydroxide ion in the pure state. Oxides, carbonates and amines can also act as bases.
Mistake 2: Assuming that every salt is neutral
A salt of a weak acid and a strong base gives a basic solution, such as sodium carbonate, and a salt of a strong acid and a weak base gives an acidic one, such as ammonium chloride.
Mistake 3: Using the wrong ending for the anion
-ic acid gives -ate, -ous acid gives -ite and hydro-…-ic acid gives -ide.
Mistake 4: Confusing table salt with the class of salts
Sodium chloride is one example of a salt. Chemists call many compounds salts that do not taste like table salt.