Salts: Names, Formulae and Physical Properties

1Why this matters

"Salt" in the kitchen means sodium chloride. To a chemist, sodium chloride is just one member of a huge family. The ammonium nitrate and potassium chloride in a bag of fertiliser, the copper(II) sulfate in a fungicide spray, the calcium carbonate in agricultural lime and the sodium hydrogencarbonate that makes mandazi dough rise are all salts.

Farmers, pharmacists, builders and water engineers all handle salts, and they all need to read their names and formulae correctly: iron(II) sulfate and iron(III) sulfate are different chemicals with different uses. By the end of this lesson you will be able to define a salt, name salts and write their formulae, and describe their physical properties.

2What is a salt?

A salt is an ionic compound formed when the hydrogen ions (H⁺) of an acid are replaced by metal ions or by ammonium ions (NH₄⁺). For example, when sodium hydroxide neutralises hydrochloric acid, sodium ions take the place of the hydrogen ions:

3Naming salts

1. Name the cation first: the metal, or ammonium. If the metal can form ions with different charges, give the charge in Roman numerals: iron(II) for Fe²⁺, iron(III) for Fe³⁺, copper(II) for Cu²⁺, lead(II) for Pb²⁺.
2. Then name the anion. An anion made of one element ends in -ide (chloride, oxide, sulfide). A polyatomic anion that contains oxygen usually ends in -ate (sulfate, nitrate, carbonate, phosphate).

4Common ions

5Writing formulae

The total positive charge must equal the total negative charge. When you need more than one polyatomic ion, put it in brackets.

Worked example: formulae of salts

• Calcium nitrate: Ca²⁺ needs two NO₃⁻, so Ca(NO₃)₂. Check: +2 + 2 × ( − 1) = 0.
• Ammonium sulfate: SO₄²⁻ needs two NH₄⁺, so (NH₄)₂SO₄.
• Iron(III) sulfate: Fe³⁺ and SO₄²⁻; the lowest common multiple of 3 and 2 is 6, so two Fe³⁺ (+6) and three SO₄²⁻ (−6): Fe₂(SO₄)₃.
• Potassium carbonate: two K⁺ balance one CO₃²⁻: K₂CO₃.

6Normal salts and acid salts

If all the replaceable hydrogen of the acid has been replaced, the product is a normal salt, such as sodium sulfate, Na₂SO₄. If only some has been replaced, it is an acid salt, such as sodium hydrogensulfate, NaHSO₄, or sodium hydrogencarbonate, NaHCO₃. The fertiliser DAP, diammonium hydrogenphosphate, (NH₄)₂HPO₄, is an acid salt.

7Physical properties of salts

• Salts are crystalline solids at room temperature with fairly high melting points, because they are giant ionic lattices.
• Most salts are white. Salts of transition metals are often coloured: copper(II) salts are blue or green, iron(II) salts pale green, iron(III) salts yellow-brown.
• Many salts crystallise from water with water of crystallisation: water molecules built into the crystal in a fixed ratio. Such a salt is hydrated; without the water it is anhydrous. Blue hydrated copper(II) sulfate, CuSO₄·5H₂O, loses its water on heating and becomes white anhydrous copper(II) sulfate: CuSO₄·5H₂O(s) → CuSO₄(s) + 5H₂O(g). Adding water turns the white powder blue again, and this is the usual test for water: CuSO₄(s) + 5H₂O(l) → CuSO₄·5H₂O(s)
• A deliquescent substance absorbs so much water from the air that it dissolves in it and forms a solution (for example calcium chloride and iron(III) chloride). Magnesium chloride impurity is the reason cheap table salt becomes damp in the rainy season.
• A hygroscopic substance absorbs water from the air but does not dissolve in it (for example anhydrous copper(II) sulfate).
• An efflorescent substance loses its water of crystallisation to the air. Crystals of washing soda, Na₂CO₃·10H₂O, slowly crumble to a white powder when left open.

Practice

Try each item before you read the solution.