Salts

Chemical Structure and Differences from Acids

All salts possess an ionic crystal lattice consisting of regularly alternating cations (metal atoms or polyatomic groups, such as ammonium) and anions (monatomic or polyatomic acid residues). This fundamentally distinguishes them from acids, which in most cases are liquids or gases.

Replacing the metal in a salt structure with another metal leads to the formation of a new salt. If the metal is replaced by hydrogen instead, a substance of an entirely different class is formed — an acid.

To write the complete structural formula of a salt accurately, one must know the coordination number of the metal. If this data is not available, only the structure of the acid residue is depicted. For example, in potassium phosphate, potassium ions are surrounded by oxygen atoms from different groups, and without knowing the exact structure, it is simply written as the $PO_4$ group.

The diversity of salts is due to the fact that a single acid can correspond to a vast number of salts from various bases. There are also salts of unstable acids. A classic example is zinc dithionite ($ZnS_2O_4$), formed by the reaction of zinc with sulfur dioxide, whereas dithionic acid itself ($H_2S_2O_4$) decomposes instantly upon an attempt to obtain it.

Physical Properties, Solutions, and Hydrates

Salts have relatively high melting points. Most salts are white or colorless (like baking soda or sodium chloride), but many possess their own characteristic color (like potassium permanganate $KMnO_4$). Solubility varies; reference data can always be found in specialized tables.

A key property of aqueous solutions is high electrical conductivity, which directly depends on the concentration of ions. If a beaker with a sodium chloride solution is connected to a circuit with a lamp, the brightness of the glow will increase as new portions of salt dissolve.

Upon crystallization, salts are isolated in two forms:

• Anhydrous crystals: characteristic of insoluble precipitates and certain potassium, sodium, and ammonium salts.
• Crystal hydrates: solid chemical compounds in which the salt crystallizes with the entrapment of a specific number of water molecules (characteristic of most soluble salts).

Medium Reaction and Chemical Properties

The dual nature of salts (presence of an acid residue and a base cation) leads to the fact that their aqueous solutions can alter pH:

1. Acidic reaction (red litmus) — characteristic of salts of a strong acid and a weak base (aluminum, zinc, iron).
2. Alkaline reaction (blue litmus) — characteristic of salts of a strong base and a weak acid. For instance, sodium carbonate ($Na_2CO_3$) is used in practice to create an alkaline medium as a safer alternative to caustic soda.
3. Neutral reaction — characteristic of salts of strong acids and strong bases.

Salts enter into exchange reactions that proceed to completion if a precipitate forms, a gas is released, or a weak substance is produced (such as water or a weak acid). In displacement reactions, an active metal displaces a less active one from a salt solution. Thus, an iron paperclip in a copper sulfate ($CuSO_4$) solution becomes coated with copper, and a zinc pellet in lead nitrate ($Pb(NO_3)_2$) becomes overgrown with lead crystals (a "lead tree").

Classification of Salts and Thermal Stability

Salts can be divided into several major groups:

• Binary salts (compounds with halogens and sulfur). They differ in thermal stability. An interesting fact: among silver halides, fluoride is highly soluble, whereas chloride, bromide, and iodide are not. For calcium salts, it is the reverse: fluoride is insoluble, while the other halides are soluble.
• Oxygen-containing salts (sulfates, carbonates, phosphates). The metal in them is bonded to oxygen.
• Salts of organic acids. Simple ones (acetates) resemble inorganic ones. Salts of higher acids with a long carbon chain (soaps) do not crystallize, form a film on water, and do not dissolve like ordinary salts.
• Salts of ammonia and amines. They contain polyatomic cations. Upon reaction with alkalis, they release ammonia, and upon heating, they completely decompose into gaseous products.

Upon heating, binary salts and alkali metal salts simply melt (e.g., $NaCl$ at 801 °C). However, most oxygen-acid salts of other metals decompose, releasing oxygen and nonmetal oxides (e.g., the decomposition of copper(II) nitrate or zinc sulfate).

Biological Significance and Major Salts

In medicine, the class of salts is studied with particular attention, as these substances are invariably present in living organisms and ensure homeostasis.

Among the most important salts in the human body:

• Sodium chloride ($NaCl$) — table salt;
• Potassium chloride ($KCl$) and magnesium chloride ($MgCl_2$);
• Sodium bicarbonate ($NaHCO_3$) — baking soda;
• Calcium phosphate ($Ca_3(PO_4)_2$) — the main mineral component of bones and teeth.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Rules for drawing structural formulas of salts and coordination numbers
• Specific solubility features of fluorides compared to other halides
• Thermal decomposition of oxoacid salts
• Chemical properties of higher carboxylic acid salts (soaps)
• Characteristics of organic amine salts and the ammonium cation

Other topics in section «Chapter 3. Classification of Substances»

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.