Salts in Chemistry: Types, Properties, and Uses of Salts

Types of Salts

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Recall that a salt is an ionic compound formed when the hydrogen ion(s) of an acid are completely or partially replaced by a metal ion, or an ammonium ion (NH₄⁺). Generally, acids reacts with bases to form salt and water only. This reaction is termed neutralization reaction; and is shown below:

• HCl_(aq) + NaOH_(aq) →NaCl_(aq) + H₂O_(l)

 

Salts can be put into five types. These are:

1. Normal salt

Normal.salts are formed when all replaceable hydrogen ion (H⁺) of the acid had been replaced (completely). Examples of normal salts include Na₂SO₄, NaCl, KCl and Ca(NO₃)₂, KNO3, etc. 

The equation below shows the formation of a normal salt:

• HCl_(aq) + NaOH_(aq) → NaCl_(aq) + H₂O_(l)

• HNO_3(aq) + KOH_(aq) → KNO_3(aq) + H₂O_(l)

 

2. Acid salt

Acid salts can only be formed when the acid is not monobasic. This imply that the acid must be polybasic, in order to allow for the replacement of some replaceable H⁺;  while also retaining an ionizable hydrogen (H⁺). In this regard, acidic salts will always contain hydrogen within the salt molecule. Examples include NaHSO₄, NaHCO₃, CaHPO₄.

• H₂SO_4(aq) + NaOH_(aq) → NaHSO_4(aq) + H₂O_(l)

• H₃PO_4(aq) + NaOH(aq) → NaH₂PO_4(aq) + H₂O_(l)

• H₃PO_4(aq) + Ca(OH)_2(aq) → CaHPO_4(aq) + 2H₂O_(l)

Polybasic acids are acids capable of donating more than one hydrogen ion (proton) per molecule. They are also called polyprotic acids, and can form acidic salts when their replaceable hydrogen ions are partially replaced by a metallic or ammonium ion.. Examples include H₂SO₄, H₂CO₃, H₃PO₄, H₂S, C₆H₈O₇, and H₄P₂O₆.

 

3. Basic salt

Basic salts contains hydroxide ions (OH⁻) along with the acid's anion. They are formed from the incomplete neutralization of a base. Examples of basic salts include. Pb(OH)Cl, Mg(OH)Cl, Cu(OH)Cl, PH(OH)NO₃.

The equations below show the formation of basic salt:

• Pb(OH)_2(s) + HCl^(aq) → Pb(OH)Cl_(s) + H₂O_(l)

• 2Cu(OH)_2(s) + HCl → Cu(OH)Cl_(s) + CuCO_3(s)

Please read the introduction to salts in chemistry, their classifications, solubility, water of crystallization, preparations and colors here.

 

4. Double salt

A double salt is a crystalline compound made by combining two different simple salts in a fixed ratio. In this regard, a double salt ionizes to produce three different ions in solution. Two of these ions are positively charged (two metallic ions or one metallic and one ammonium ion), while the remaining ion is negatively charged.

Examples of double salts include the following:

• Potash alum: KAl(SO₄)₂.12H₂O. The ions comprising this salt include K⁺, Al³⁺, and SO₄^2-.

• Carnallite: KCl.MgCl₂.6H₂O: The ions comprising this salt include K⁺, Mg²⁺ and Cl^-.

• Mohr's salt: FeSO₄.(NH₄)₂SO₄.6H₂O. The ions comprising this salt include Fe²⁺, NH₄⁺, and SO₄^2-.

Double salts undergo complete dissociation into their individual (respective) ions when mixed in water. The equation below show a double salt (that is, two simple salts that had crystallized together), dissociating fully into all component ions in solution.

• KAl(SO₄)₂·12H₂O → K⁺ + Al³⁺ + 2SO₄²⁻

• FeSO₄·(NH₄)₂SO₄.12H₂O → Fe²⁺ + NH⁺ + 2SO₄²⁻

 

5. Complex salt

A complex salt is an ionic compound that contains a central metal atom or ion (which is often a transition metal), surrounded by molecules or ions called ligands, which are held together by coordinate covalent bonds.

Simply put, a double salt contains a complex ion (often a central transition metal + ligands) that stays intact in solution, (that is, they remain bound together, and do not dissociate in solution).

Examples of complex salt include:

• Na[Ag(CN)₂]: It dissociates into Na⁺ and a stable silver-cyanide complex ion - [Ag(CN)₂]^-

• K₄[Fe(CN)₆]: It dissociates into K⁺ and a stable ferrocyanide complex ion - [Fe](CN)₆]^4-

• [Cu(NH₃)₄]SO₄: It dissociates into SO₄^2- and a stable ferrocyanide complex ion - [Cu](NH₃)₄]²⁺.

• K[Ag(CN)₂]: It dissociates into K⁺ and a stable silver-cyanide complex ion - [Ag(CN)₂]^-.

Unlike simple or double salts that break down entirely into their respective individual ions when dissolved in water, the inner central metal  ions and its ligands (of a complex salt) do not break free. In this regard, it will not give a positive test for the respective central ion and it ligand ions. This is so because the core metal and ligands stay bound together in water.

Please read on bases and alkalis, their definitions, preparation, physical and chemical properties here.

 

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Properties of Salts

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• They are generally crystalline solids with high melting and boiling points; (as a result of strong ionic bonding).

• They conduct electricity when molten or in aqueous solution, but not as solids.

• Their solubility varies widely between different salts. For example, all trioxonitrate salts are soluble (e.g KNO₃, Pb(NO₃)₂, Mg(NO₃)₂, NH₄NO₃), while trioxocarbonate salts ere insoluble with the exceptions of Na, K and Ca trioxocarbonates.

• They can react with acids, alkalis, and other salts according to their ion reactivity. Such reaction could be precipitation reaction and displacement reaction.

• Many carbonates and nitrates decompose on strong heating.

• Aqueous solutions of salts may be acidic, neutral, or alkaline depending on the parent acid/base.

Please read on acids, their definitions, basicity and types here.

 

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Deliquescence, Efflorescence, Hygroscopy, and Drying Agents

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Deliquesce

This is a phenomenon whereby salts absorbs so much moisture that it dissolves in the water absorbed, forming a solution. Salts that undergo this phenomenon are termed 'deliquescent'. Examples of deliquescent salts include CaCl₂, NaOH, MgCl₂.

 

Efflorescence

This is a phenomenon whereby hydrated salt loses some or all of its water of crystallization to the air, becoming powdery, without needing heat. Salts that undergo this phenomenon are termed 'efflorescent'.

Examples include Na₂CO₃·10H₂O (washing soda), Na₂SO₄·10H₂O, CuSO_4.5H₂O (Glauber's salt), CuSO₄.5H₂O (Blue vitriol) and FeSO₄·7H₂O.

 

Hygroscopy

This is a phenomenon whereby a substance absorbs moisture from the air without changing state (remains solid). Substances that undergo this phenomenon are termed hygroscopic. Simply put, hygroscopic substances.absorbsoisture from the atmosphere without turning into solution.

Examples of hygroscopic substances include anhydrous CaCl₂, silica gel, concentrated H₂SO₄ (liquid). Other materials that are hygroscopic include honey, simple sugars, and cellulose fibers.

You can read on the chemical properties of matter here.

 

The table below summarizes the properties of hygroscopic, deliquescent and efflorescent substances.

Property of salt	Absorbs moisture?	Changes state?
Hygroscopic	Yes	No (stays solid)
Deliquescent	Yes (a lot)	Yes (dissolves into solution)
Efflorescent	Loses moisture to air	Yes (crumbles to powder)

 

 

Drying agents (or desiccants)

Drying agents are hygroscopic substances that functions to remove small amounts of water from organic solvents, solutions, or gases without chemically reacting with them.

Drying agents are hygroscopic/deliquescent substances used to remove moisture from gases or in desiccators.

Below are examples of drying agents and the substances they can dissicate (dry).

• Concentrated H₂SO₄: It can dry neutral gases like H₂, N₂, O₂ and acidic gases like SO₂, CO₂, HCl and Cl₂. However, it cannot dry ammonia gas (NH₃) and hydrogen sulphide gas (H₂S).

• Fused calcium chloride (fused CaCl₂): It can dry all neutral and acidic gases e.g H₂, O₂, N₂, Cl₂, HCl, CO, and CO₂. However, it cannot dry ammonia gas (NH₃),

• Calcium oxide or quicklime (CaO): It dries ammonia gas (NH₃) in particular. It can also dry hydrogen gas (H₂) and oxygen gas (O₂). It cannot dry acidic gases.

• Phosphorus pentaoxide (P₂O₅ / P₄O₁₀): It can dry most neutral and acidic gases. It cannot dry ammonia gas (NH₃ - which is a basic gas)

• Silica gel: It can dry all gases. This is so because it is inert (does not react with these gases).

Please read on relative acidity and alkalinity (pH scale), and its calculations here.

 

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Hydrolysis of Salts

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Hydrolysis of salts refers to the reaction of a salt's ions (cation, anion, or both) with water, which can make the resulting solution acidic, alkaline, or neutral, depending on the relative strengths of the parent acid and base. Below are examples of salt hydrolysis:

1. A salt of a weak base (e.g. NH₄Cl) releases NH₄⁺, which reacts slightly with water to produce ammonia (NH₃) and an extra H⁺ (H₃O⁺), making the resulting solution acidic. This is shown via the equation below:

• NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺ 

2. A salt of a weak acid (e.g. Na₂CO₃) releases CO₃²⁻, which reacts with water to produce HCO₃⁻ and an extra OH^-, making the resulting solution alkaline. This is shown via the equation below:

• CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻

3. A salt of a strong acid and strong base (e.g. NaCl) has ions that do not react significantly with water, resulting to an unchanging neutral solution.

The hydrolysis of salts is shown via the table below:

Salt Type (Parent Acid/Base)	Solution pH	Example
Strong acid + Strong base	Neutral (pH 7)	NaCl, KNO₃
Strong acid + Weak base	Acidic (pH < 7)	NH₄Cl, (NH₄)₂SO₄, FeCl₃
Weak acid + Strong base	Basic (pH > 7)	Na₂CO₃, CH₃COONa
Weak acid + Weak base	Depends on relative strengths (~neutral)	CH₃COONH₄

 

Please read on pH indicators, types and importance of pH and pOH here.

 

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Uses of Salts in Daily Life

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Salts aren't just the table salt (found in our kitchen). There are many kinds of salt found everywhere in daily life. Below are salts arn their uses.

1. Sodium chloride (NaCl): This is the common table salt (often present in the kitchen). It is used for seasoning and preserving food (e.g. salting/curing meat and fish), and as a raw material for the chlor-alkali industry.

2. Sodium hydrogentrioxocarbonate(IV) (NaHCO₃): NaHCO₃ is also called baking soda. It is  used in baking (as raising agent), and as an antacid for indigestion.

3. Sodium trioxocarbonate(IV) (Na₂CO₃): Na₂CO₃ is also called washing soda. It is used in the production of glass, softening of hard water, and as detergents.

4. Calcium trioxocarbonate(IV) (CaCO₃): CaCO₃ is also called limestone. It found in eggshells, seashells, and limestone/marble buildings. It is used as an antacid/calcium supplement. It also serves as the primary raw material used in the production of cement.

You can read on the preparation of bases and alkalis, their test and uses here.

5. Magnesium sulfate (MgSO₄): MgSO₄ is also called epsom salt. It is used in bath salts, as a laxative, and in the production of fertilizer (which supplies Mg to soil/plants).

6. Potassium nitrate (KNO₃): It is used in the production of fertilizers, in food preservation (curing meat), and fireworks/gunpowder.

7. Copper(II) sulfate (CuSO₄): It is used as a fungicide/algaecide. It is also used in electroplating, and in school laboratories to demonstrate crystallization.

8. Calcium chloride (CaCl₂): It is used to de-ice roads in cold climates and as a drying agent.

9. Silver bromide/silver chloride (AgBr/AgCl): Historically, it was used in photographic film (light-sensitive salts).

10. Ammonium salts, e.g. (NH₄)₂SO₄, NH₄NO₃): It is widely used as nitrogenous fertilizers.

Note: Salts also play essential biological roles. For example, sodium and potassium salts are vital for nerve impulse transmission and muscle function, and calcium/phosphate salts are crucial for bones and teeth.

Please read on the next topic (Carbon, allptropes of carbon, destructive distillation of coal and wood, gasification and uses of carbon) here.