What are salts?

Authors: Hans-Jürgen Schwarz
back to Fundamentals

Abstract

Salts are compounds made up of cations (e.g., K⁺, Ca²⁺, NH₄⁺) and anions, such as chloride (Cl^-), nitrate (NO₃^-, that are held together by ionic bonds. They are crystalline materials.

The formation of salts

Salts normally consist of positively charged ions, cations, and negatively charged ions, anions, that form a crystal lattice. In addition, some salts may include the water molecule (H₂O) in the lattice. there are referred to as hydrated salts.

Fig. 1:The crystal lattice of NaCl

Salts, by definition, are the result of a neutralization reaction, i.e., the mixture of an acid with a base.

$Na^{+}+OH^{-}+H_{3}O^{+}+Cl^{-}\rightarrow NaCl+2H_{2}O$

base    +     acid       →       salt   +   water

While some salts, such as NaCl are neutral, that is, their solution does not change the normal pH 7 of water, other salts may be alkaline or acidic, depending on the strength of the participating acids and bases:

1. strong acid + strong base → neutral salt + H₂O

2. strong acid + weak base → acidic salt + H₂O

3. weak acid + strong base → alkaline salt + H₂O

e.g.

1. $H_{3}O^{+}+Cl^{-}+Na^{+}+OH^{-}\rightarrow NaCl+H_{2}O$

2. $H_{3}O^{+}+Cl^{-}+NH_{3}\rightarrow NH_{4}Cl+H_{2}O$

3. $H_{3}O^{+}HCO_{3}^{-}+2Na^{+}+2OH^{-}\rightarrow Na_{2}CO_{3}+3H_{2}O$

Another important point is that salts may have different solubilites. While some salts are very soluble, others may be practically insoluble. In general, soluble salts are more "damaging" to inorganic porous materials than the less soluble ones.

The chemical notation of salts is written like this:

(positive ion (cation))_x (negative ion (anion))_y • nH₂O; e.g. CaCl₂ • 6 H₂O ,

x,y,n being the number of the ions or water molecules

Salts consist of cations and anions that are bound together by ionic bonds. They are crystalline materials that may have a wide range of solubilities.

Composition of salts

The most frequently found salts consist of these
anions:

• Sulfate SO₄^2-
• Nitrate NO₃-
• Chloride Cl^-
• Carbonate CO₃^2-

And the cations

• Sodium Na⁺
• Potassium K⁺
• Calcium Ca²⁺
• Magnesium Mg²⁺

other anions are:

• Acetate CH₃COO^-
  
• Formiate HCOO^-
• Oxalate C₂O₄^2-
• Phosphate PO₄^3-

further cations are:

• Ammonium NH₄⁺
  

Salt properties

Salts have certain properties that help to explain their behaviour in solution and their "damaging" effect on objects. The most important are:

• solubility
• deliquescence humidity

Other properties of salts, such as density and crystallographic data, are described elsewhere.

A molecule is an electrically neutral group of at least two atoms.

an atom or molecule with a positive electrical charge

an atom or molecule with a negative electrical charge.