In chemistry, a salt is an electrically neutral chemical compound consisting of cations and anions connected by an ionic bond. The classic example is table salt or sodium chloride (NaCl), which consists of positively charged sodium ions (Na+) and negatively charged chlorine ions (Cl–). The non-chemistry definition of salt generally refers only to this compound.
Salt Definition in General Chemistry
In introductory chemistry courses, a salt is a compound forming from a neutralization reaction between an acid and a base, where the base exchanges its cation (typically a metal) with the hydrogen ion (H+) of the acid, forming a salt and water. However, the chemistry definition encompasses other salt-forming reactions.
Chemical Reactions That Form Salts
So, a neutralization reaction actually isn’t the only way a salt forms. The only hard-and-fast rules are that the reaction involves a cation and an anion and the resulting product is an ionic compound. The resulting compound is either organic or inorganic.
- An acid and a base react via a neutralization reaction.
Example: HCl + NH3 → NH4Cl
- An acid reacts with a base anhydride.
Example: 2 HNO3 + Na2O → 2 NaNO3 + H2O
- A base reacts with an acid anhydride.
Example: 2 NaOH + Cl2O → 2 NaClO + H2O
- A salt metathesis reaction occurs when two salts are mixed in water and their ions recombine and form a new insoluble salt that precipitates out of solution.
Example: Pb(NO3)2 (aq) + Na2SO4 (aq) → PbSO4↓ + 2 NaNO3 (aq)
- A metal and acid react.
Example: Mg + H2SO4 → MgSO4 + H2
- A metal reacts with a nonmetal.
Example: Ca + Cl2 → CaCl2
Naming Salts in Chemistry (Nomenclature)
The salt name begins with its cation (e.g., potassium or ammonium) followed by its anion (e.g., acetate or chloride). For example, NaCl is sodium chloride and KNO3 is potassium nitrate. Sometimes more general names are used, such as sodium salts or chloride salts.
Naming also describes salts that contain varying numbers of hydrogen atoms compared to their parent acid:
- A monobasic salt has one hydrogen atom replaced.
Example: sodium phosphate monobasic (NaH2PO4)
- A dibasic salt has two hydrogen atoms replaced.
Example: sodium phosphate dibasic (Na2HPO4)
- A tribasic salt forms when three hydrogen atoms are replaced.
Example: sodium phosphate tribasic (Na3PO4)
- A polybasic salt is one where more than one hydrogen atom is replaced.
Types of Salts in Chemistry
There are multiple ways of classifying salts. Grouping mainly depends on the way they form or the type of ions produced when they dissolve in water.
- Strong salts or strong electrolyte salts consist of strong electrolytes that completely dissociate in water. They salts often have Na, K, or NH4 as cations and NO3, ClO4, or CH3COO as anions, although most group 1 and 2 metals form them. An example of strong salt is potassium nitrate, (KNO3).
- Weak salts or weak electrolyte salts, in contrast, consist of weak electrolytes. Sodium acetate (CH3COONa) is an example of a weak salt.
- Simple salts form via neutralization reactions between acids and bases. These salts are further classified according to their pH levels when they dissolve in water as acid, alkali, or neutral.
- An acid salt produces hydrogen ions (H+) when it dissolves in a solvent. The resulting pH from dissolving an acid salt in water is acidic (pH<7). An example of an acid salt is sodium bisulfate (sodium hydrogen sulfate, NaHSO4).
- Alkali salts or basic salts form hydroxide ions (OH–) in water. The resulting aqueous solution is basic (pH>7). A more general definition is that an alkali salt salt forms from the incomplete neutralization of a strong base and weak acid. For example, sodium hydroxide (NaOH) is an alkali salt.
- A neutral salt is neither acidic nor basic. Dissolving a neutral salt does not impact the solution pH. Sodium chloride (NaCl) is an example of a neutral salt.
- A double salt forms via a reaction between simple salts and contains more than one cation or anion in the molecule. Potassium alum (potassium aluminum sulfate) is an example of a double salt [KAl(SO4)2].
- A complex salt or coordination compound is a combination of molecular compounds and ions, where a central metal atom is surrounded by coordination bonds to ligands. It is a salt because it contains cations bonded to anions. However, the compound does not completely dissociate into its ions when dissolved. An example of a complex salt is mercury iodide (HgI2).
- A mixed salt consists of a cation wither two anions or an anion with two cations. Mixed salts form via reactions between more than one acid or base. Potassium sodium carbonate (CKNaO3) is an example of a mixed salt.
- A zwitterion is not a salt at all, although it contains cationic and anionic centers within a single molecule. Examples of zwitterions include amino acids and peptides.
Anhydrous vs Hydrated Salts
An anhydrous salt does not contain water in its formula. Table salt (NaCl) and copper sulfate (CuSO4) are examples. In contrast, a hydrated salt contains water within its crystalline structure. An example is copper sulfate pentahydrate (CuSO4·5H2O). Some salts are only anhydrous, while take both anhydrous and hydrated forms.
Properties of Salts in Chemistry
The properties of a salt largely depend on the type of salt that it is. But, salts display characteristics relating to color, flavor, odor, solubility, conductivity, and melting point.
- Color: Most salts are transparent or translucent, at least as large crystals. They often appear opaque as powders because the tiny crystals reflect so much light.
- Flavor: Not all salts are “salty.” For example, sodium chloride tastes like salt, but lead diacetate tastes sweet, potassium bitartrate is sour, and monosodium glutamate is savory or umami.
- Odor: Strong salts usually are odorless, while weak salts smell like their conjugate acid or conjugate base. For example, acetates smell like acetic acid or vinegar and cyanides have the almond scent of hydrogen cyanide.
- Solubility: Not all salts dissolve in water, but they do tend to dissolve into polar solvent and not into nonpolar organic solvents. Most sodium, potassium, and ammonium salts dissolve in water. Most metal carbonates are insoluble in water.
- Conductivity: Solid salts are mostly insulators. Molten or dissolved salts are electrical conductors.
- Melting point: Salts typically have high melting points. Sodium chloride, for example, melts at 801 °C. However, salts with low lattice energies are liquids near room temperature.
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