Salts are chemical compounds made by the reaction between acids and bases. When an acid reacts with a base, a salt is formed. Salts have characteristic and consistent properties different from their ingredient elements. They are ionic compounds, made of metal cations and organic or inorganic anions. Hence, they are generally soluble in water. They are classified based on their nature as acidic salts, basic salts, and neutral salts.
Preparation of salts
Most acids when reacting with metals, hydroxides, carbonates, and bicarbonates produce salts.
1. Reaction of Metals with acids
Metal + Acid → Salt + H2 (g)
HCl + Mg → MgCl2 +H2
H2SO4 + Zn → ZnSO4 +H2
CH3COOH + Na → CH3COONa + H2 (g)
Notice from the above reactions that the choice of the acid in the reaction is dependent on the salt required. When magnesium chloride is required, hydrochloric acid is used. If magnesium sulfate is required, sulfuric acid is used, and similarly, if nitric acid is used, magnesium nitrate is prepared.
2. By the reaction of acid and base
As a general rule, when an acid reacts with a base, salt is formed. The reaction is called the neutralization reaction. The salt produced in this reaction has cation from the base and anion from the acid reagent.
Acid + Base → Salt + H2O
2HCl + Mg(OH)2 → MgCl2 + 2H2O
H2SO4 + NaOH → Na2SO4 + H2O
3. By the reaction of acids with Carbonates/ Bicarbonates
When acids are treated with carbonates and bicarbonates, salts are formed. The salts are in an aqueous medium, and thus, can be separated by physical techniques.
Acid + Carbonates or Bicarbonates → Salt + CO2 + H2O
2HCl + MgCO3 → MgCl2 + CO2 + H2O
HCl + NaHCO3 → NaCl + CO2 + H2O
4. Laboratory synthesis of salts
In the laboratory, salts are synthesized by applying various methods. The summarized flow sheet diagrams of salt synthesis in the lab are given below:
1. Preparation of soluble salts
2. Preparation of insoluble salts
Naming Salts in chemistry
Initially, salts were named on the basis of their properties and uses but as with any other trivial naming system, with time, it became difficult to use those names for all the salts. Today, the IUPAC system of naming is being used globally.
This system says to name the cation in the salt first, followed by anion. The charge or oxidation state on cation is shown in braces in roman numerals.
Some common salts with their trade name and IUPAC names are given in the table below:
Types of salts
Salts are ionic compounds usually obtained by neutralization reactions. They are generally neutral compounds but some of them are acidic and basic in their solution form. This concept was explained by Bronsted-Lowry. It states that ions can act either as an acid or a base. So, the acidity or basicity of salts depends upon the type of cations and anions produced in the solution.
The common types of salts are acid, basic, neutral, double, and complex salts.
Ammonium chloride solution is acidic in nature because of the acidic nature of ammonium (NH4+) ions.
NH4+ + H2O → NH3 + H3O+
Here ammonium ions act as an acid and donate a proton to H2O resulting in H3O+.
Sodium cyanide is a basic salt. Upon its addition to water, it hydrolyzes into its ions.
NaCN → Na+ + CN–
CN– + H2O → HCN + OH–
Sodium ions are not reactive while CN– ions react with H2O to form OH– ions. By the Lowry-Bronsted concept, cyanide ion is a base as it accepts a proton. This is why NaCN is a basic salt.
Some salts produce ions on hydrolysis which do not react with water to produce hydronium ions or hydroxyl ions. For example, NaCl is soluble in water. It gives Na+ and Cl– ions. Both these ions are unreactive to species present inside water.
Cl– + H2O → no reaction
Na+ + H2O → no reaction
Double salts contain more than one cation. They occur naturally and are found in the earth’s mantle. For example:
- Mohr’s salt, (NH4)2 Fe(SO4)2.6H2O
- Tutton’s salt, K2Cd (SO4)2.6H2O, etc.
The coordination compounds are sometimes referred to as complex salts. This is because they contain transition metal atoms as cations. The “complex” word describes the varying valencies of the transition metals. In such complexes, cations are in the center surrounded by anions and ligands to form complexes.
Some properties of salts
Salts in chemistry are studied according to their properties. Among these properties are solubility, brittleness, high melting and boiling points, and conductivity of salts.
Salts are usually ionic in nature, therefore, most of them are soluble in water. For sparingly soluble, a concept of solubility product (Ksp) is used. The solubility of some salts is given in the illustration below:
Salts are ionic compounds. Positive and negative charges are arranged to form unit cells. When hammered, the layers displace from above causing opposite charges in front of each other, and thus a crystal breaks apart. This brittle nature of salts also explains why these compounds are hard and exist in the form of crystalline solids.
Melting and boiling points
Salts are giant ionic lattices which is why they have high melting and boiling points. For example:
- Sodium chloride (NaCl) melts at 801 °C and boils at 1413°C.
- Magnesium oxide (MgO) melts at 2852 °C and boils at 3600 °C.
- Barium chloride (BaCl2) melts at 961 °C and boils at 1560 °C.
Salts are good conductors of electricity in molten and aqueous states. Their conductivity is due to the presence of electrostatic charges and electrons that can flow upon application of external force. So, the conductivity of salts depends upon their solubility or ionic nature.
Applications of Salts
- Hot and cold packs
Sodium nitrate dissolution is an endothermic process. Enthalpy change of this reaction is positive which is used in cold packs to produce a cold effect to relieve pain. Similarly, hot packs are made up of those salts which undergo dissolution with an exothermic reaction.
- Clearing of ice on roadways
When salt, for example, sodium chloride is sprinkled over the ice on road, it depresses the melting point of ice. This helps in clearing ice from roadways during winters. It is one of the colligative properties of solutions, which says that when a solute is added to a solvent, its melting point is decreased.
- Food flavor
Since the beginning of life, man has been in touch with salts. It makes the meal tasty and provides the necessary nutrients to the body.
- Food preservative
Salts are used since ancient times to preserve food. When a pinch of salt is present around the microbes on meat or food, it increases the osmotic pressure inside the microbe which causes the movement of liquid or solvent outwards and these pathogens die.
Mineral and leather tanning use mineral salts.
Salts allow a dye to penetrate deep into the cloth, the dye is being put on.
Bleaches are salts of hydrochloric acid and hypochlorous acid.
- Production of soap
Saponification or, the soap-making process uses caustic soda and fatty acids to produce salts that are later called soaps.
- Production of pottery
Salt-glazed ware or clay surfaces are industrially produced and applied. Sometimes, this application produces waterproofing materials.
- Source of chlorine
Salts have been considered a cheap and easily accessible source of chlorine.
What are the different types of salt in chemistry?
Salts are classified on the basis of the pH of their solution is acidic, basic, and neutral salts. Furthermore, mixed salts and double salts are the type of salts based on the number of cations present in them. Complex salts are the salts having transition metals cations and ligands in coordination compounds.
What are the properties of salts?
Salts are chemical substances that are ionic in nature. They are brittle and have high melting and boiling point. They are usually soluble in water.
What exactly is a “salt”?
Salt is the neutralized product of an acid and a base.
From which elements or compounds are salts made?
Metals are the elements when treated with acids, form salts. Acid and bases react with each other to produce salt as well.
What is the chemical structure of salt?
Salts are made up of cations from the bases and anions from the acids.
What are bath salts?
Epsom salt MgSO4.7H20 is also called bath salt.
What is the chemistry behind saltwater?
Saltwater refers to seawater. It contains chlorides, sulfates, carbonates, and bicarbonates of sodium potassium, magnesium, and calcium.
Is ester a salt?
Ester is prepared by the chemical reaction of alkanoic acids with alcohols. Although esters are covalent compounds, they are named according to the IUPAC naming system of salts. For example, acetic acid reacts with ethanol to form ethyl ethanoate, which is a salt.
Is salt a pure substance?
Not all salts are pure salts. Their impurities can be removed by purification techniques, for example, recrystallization. So, pure salts are single, compound-like substances.
- Oxford IB-Diploma Programme Chemistry (second edition) By Sergey Bylikin, Gary Horner, Brian Murphy, and David Tarcy
- Cambridge International AS and A Level By Ryan and Norris Cambridge International AS and A Level Chemistry Coursebook
- Salts (britannica.com)
- The naming of salts (stolaf.edu)