Salts are ionic compounds with a net charge of zero. They are produced when cations and anions form electrostatic forces of attraction.
Salts are most commonly defined as products of neutralization reactions when cations from bases and anions from acids combine. Other reactions that produce salts are:
Acid and metal,
2HCl + Zn → ZnCl2 + H2
A metal and a non-metal,
Mg + F2 → MgF2
A base and an acid-anhydride,
NaOH + SO3 → NaHSO4
An acid and a base-anhydride,
H2SO4 + CaO → CaSO4 + H2O
Physical properties of Salts
Salts have some common physical properties:
- Salts usually have hard and brittle crystalline structures.
- They are usually soluble in water and other polar solvents.
- They can act as electrolytes.
- Salts can be acidic, basic, amphoteric, or neutral.
- They usually have high melting points.
- Strong electrostatic interactions between oppositely charged ions bring the properties explained above.
Some most common properties of salts have been explained below:
The reaction of cations and anions of salts with water is called hydrolysis. Salts can be acidic or basic or neutral depending on the nature of the constituent ions that form them.
1. Acidic, Basic, or Neutral behavior
Cations of weak bases (e.g NH3) are strong conjugate acids (e.g NH4+) and vice versa. A strong conjugate acid (NH4+) can displace H+ ions from water, forming an acidic solution, whereas a weak conjugate acid (from a strong base) cannot. So, acidic salts will have a cation (NH4+) from a weak base (NH3), and an anion (Cl–) from a strong acid (HCl). Hence the salt NH4Cl is acidic and turns blue litmus red.
NH4+ + H2O → NH4OH + H+
Similarly, anions of weak acids (e.g CH3COOH) are strong conjugate bases (e.g CH3COO–) and vice versa. A strong conjugate base (CH3COO–) can displace OH– ions from water, forming an alkaline solution, while a weak conjugate base (from a strong acid) cannot. So, basic salts possess an anion (CH3COO–) from a weak acid (CH3COOH) and a cation (Na+) from a strong base (NaOH). Hence, the salt CH3COONa is alkaline and turns red litmus blue.
CH3COO– + H2O → CH3COOH + OH–
Salts of strong acids and strong bases do not hydrolyze whereas salts of weak acids and weak bases do hydrolyze, but the solution formed can be acidic, basic, or neutral.
2. Solubility
Ionic compounds, owing to the charged ions, are soluble in water and other polar solvents. When salts are dissolved in water, the crystal lattice is overcome and the salt is dissociated into its constituent ions. The solute-solvent interactions (ion-dipole) to be formed must be releasing more energy than is required to break the solute-solute interactions (electrostatic forces) and solvent-solvent interactions (dipole-dipole).
If the process of dissolution is exothermic or even endothermic to a small extent, the salt will dissolve in water. However, if the enthalpy of dissolution has a high endothermic value, i.e more energy is required to break the solute-solute and solvent-solvent interactions than would be produced by the formation of solute-solvent interactions, the salt will not dissolve by itself.
Therefore, the solubility of salts having a high endothermic value for the enthalpy of dissolution can be increased by increasing the temperature.
In general, all ammonium and group-1 metal salts are soluble in water with the exceptions of potassium cobalt nitrite (K3[Co(NO2)6]) and ammonium hexachloroplatinate ((NH4)2PtCl6) among others. Sodium bismuthate (NaBiO3) is another example of water-insoluble group-1 salt.
Almost all inorganic nitrates are water-soluble except bismuth oxynitrate (BiONO3.H2O).
Most sulfates and chlorides are water-soluble except lead sulfate, calcium sulfate, lead chloride, and silver chloride because of higher lattice energies and sparingly soluble barium sulfate. This is also the reason why most metal carbonates are insoluble in water.
Additional Concepts
3. Electrical Conductivity
Ionic compounds do not conduct electricity in the solid state because the ions are fixed in a lattice. However, in the liquid state (molten or aqueous), the ions are free to move about and can conduct electricity. Therefore, salts in aqueous and molten states can act as electrolytes.
Strong salts are made of strong electrolytes that dissolve completely in water. They are non-volatile and odorless.
Weak salts are composed of weak electrolytes such as sodium acetate. They do not ionize as much in water and are hence termed weak electrolytic salts. They are more volatile and possess odors similar to the weak acid or weak base they are derived from.
4. Melting points
Salts are ionic compounds having strong electrostatic forces of attraction (ionic bonds) and high lattice energies, and so possess high melting points. However, some salts containing organic cations have low lattice energies and exist as liquids at room temperature (ionic liquids).
Molten salts, which are usually a mixture of salts, also exist in the liquid state around room temperature. These ionic liquids and molten salts show unusual properties as solvents.
5. Color of Salts
Solid salts are generally transparent, like NaCl, but can exist in different colors, owing to anions, cations, or solvates. For example, CuSO4 has a blue color due to the Cu2+ ion, KMnO4 is violet by virtue of the MnO4– ion, Co(NO3)2.xH2O is red because of hydrated [Co(H2O)6]2+ cation.
6. Taste
Salts have their specific tastes, e.g sodium chloride (NaCl) is salty, lead diacetate (Pb(CH3COO)2) is sweet, magnesium sulfate (MgSO4) tastes bitter, potassium bitartrate (KC4H5O6) is sour, and monosodium glutamate (C5H8NO4Na) tastes umami/savory.
7. Odor
Strong salts (product of strong acids and strong bases) are non-volatile and mostly odorless while salts of weak acids and weak bases may smell like conjugate acids (e.g cyanides have an almond-like smell) or conjugate bases (e.g ammonium salts have an ammonia-like odor) they are derived from.
8. Water of Crystallization
Salts usually exist in their hydrated forms. This means that there is a certain number of water molecules associated with their lattice structures. This number varies from salt to salt owing to the type of crystals and spatial orientation.
Not only does this water of crystallization change the structure of the anhydrous crystal, but it also affects the colors of the salts. The change in the crystal structure is accompanied by a change in properties such as densities and refractive index etc.
When a hydrated salt is heated, it drives off the water of crystallization, resulting in the formation of anhydrous salt. Adding water to the anhydrous salt reverses this process.
An example is the hydrated copper sulfate CuSO4.5H2O (copper sulfate pentahydrate), which has a triclinic crystal structure and is blue in color. Upon heating the hydrated salt, anhydrous CuSO4 is formed which has an orthorhombic crystal structure and is a greyish-white powder.
Chemical properties of Salts
When a more reactive metal is placed in an aqueous salt solution of a less reactive metal, a displacement reaction occurs. The more reactive metal is oxidized into its ions with a corresponding reduction of the metal cations into its solid metal.
Fe + CuSO4 → FeSO4 + Cu
Salts sometimes react with acids to form another salt and acid. This is demonstrated by the following example:
H2SO4 + 2NaCl → NaSO4 + 2HCl
Salts can also react with bases to form a different salt and base. An example is given:
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O
Different types of salts
The different types of salts include acidic salts, basic salts, neutral salts, double salts, mixed salts, and complex salts.
Acidic salts refer to salts that form acidic solutions when dissolved in water. These salts are derived from a strong acid having a weak conjugate base (anion) and a weak base having a strong conjugated acid (cation). The cation, acting as a strong conjugate acid, can displace H+ from water, forming an acidic solution in a process known as hydrolysis.
HCl + NH3 → NH4+Cl–
NH4+ + H2O → NH4OH + H+
Basic salts are salts that form alkaline solutions. Basic salts are derived from a strong base having a weak conjugate acid (cation) and a weak acid having a strong conjugate base (anion). The anion, in this case, acting as a strong conjugate base, can displace OH– ion from water, forming an alkaline solution.
NaOH + CH3COOH → CH3COO–Na+ + H2O
CH3COO– + H2O → CH3COOH + OH–
Neutral salts can result from the reaction of a strong acid with a strong base, or a weak acid with a weak base. They are neither acidic nor basic. Neutral salts derived from strong acids and strong bases do not even hydrolyze, so the solution stays neutral. Table salt, NaCl, is an example of a strong salt.
HCl + NaOH → NaCl + H2O
Double salts are salts that contain more than one type of cation or anion. Many coordination complexes form double salts. Mohr’s salt ((NH4)2[Fe(H2O)6](SO4)2) is a common example. In Mohr’s salt, there are two different types of cations, NH4+ and [Fe(H2O)6]2+.
Mixed salts contain two cations that share an anion, or vice versa. They are produced by the reaction of more than one type of acid or base. An example is a sodium-potassium sulfate (NaKSO4).
Complex salts, as the name suggests, may contain a complex anion or cation, or both. Complex ions are formed when neutral or negative ligands surround a central metal ion by forming coordinate covalent bonds. The ions formed could be negative (anions) or positive (cations). Potassium ferrocyanide (K4[Fe(CN)6]) is an example of a complex salt with an anionic complex.
Concepts Berg
What are the properties of salts in chemistry?
Salts have some characteristic physical properties: they are brittle, are mostly soluble in water and other polar solvents, can be acidic or basic or neutral, can conduct electricity, and have high melting points because of strong electrostatic interactions between oppositely charged ions.
How are salts formed in chemistry?
The reactions to prepare salts are:
- An acid and a base; HCl + NaOH → NaCl + H2O
- An acid and a metal; H2SO4 + Zn → ZnSO4 + H2
- A metal and a non-metal; Mg + Cl2 → MgCl2
- A base and an acid-anhydride; KOH + SO3 → KHSO4
- An acid and a base-anhydride; 2HNO3 + CaO → Ca(NO3)2 + H2O
What is salt in chemistry?
Salt is an ionic compound made up of oppositely charged ions. The positive ions (cations) and negative ions (anions) are held together by strong electrostatic forces of attraction.
When is a salt solution basic or acidic?
Basic solutions are formed from salts that contain an anion which is a strong conjugate base and can displace OH– ions from water in a process known as hydrolysis. The cation, on the other hand, needs to be a weak conjugate acid.
Alternatively, acidic solutions are formed by salts that possess a strong conjugate acid as a cation. The cation is then able to displace H+ ions from water, forming an acidic solution. The anion needs to be a weak conjugate base in this case.
Write down some uses of salts.
There are plenty of uses for salts. Even the common table salt is used for several purposes including tanning, dyeing, bleaching, flavoring, and preserving food. Salts are required to manufacture several chemicals in the chemical industry including sodium hydroxide (caustic soda), sodium carbonate (baking soda), hydrochloric acid, chlorine, and soap.
Chemical properties of table salt:
Table salt is in fact sodium chloride made up of sodium ions, Na+, and chloride ions, Cl– held together in a lattice by strong electrostatic forces of attraction.
Some chemical properties of NaCl are:
- It is non-flammable.
- It is toxic in excessive amounts.
- Its standard enthalpy of formation is -411.12 kJmol-1.
- The coordination number of Na+ and Cl– is 6:6, which means that each Na+ ion is surrounded by 6 Cl– ions and vice versa.
- It is a neutral salt, not acidic or basic.
What are some examples of insoluble salts?
Insoluble salts usually refer to salts that do not dissolve in water, or to a very limited extent. Some examples of insoluble salts are lead (II) chloride (PbCl2), silver chloride (AgCl), lead sulfate (PbSO4), calcium sulfate (CaSO4), barium sulfate (BaSO4), and most of the metal carbonates.
Is salt a pure substance?
A pure substance is made up of one kind of element or compound. Salt is definitely pure unless there are impurities added.
What happens to salt when it’s heated?
It depends on the thermal stability of the salt. Thermally unstable salts are decomposed at higher temperatures. Salts such as metal carbonates give off carbon dioxide gas when heated. Thermally stable salts are melted if the provided heat is high enough.
Reference Books
- The Properties of Salts By Marylou Morano-Kjelle (Rutgers University)
Reference links
- About salts (seasalt.com)
- Properties of salt (saltassociation.co.uk)
