What Is A Salt In Chemistry?

In chemistry, a salt is an ionic compound that’s typically in crystalline solid form when dry. As salts are often the product of a neutralisation reaction between an acid and a base, they generally have a neutral pH.

The most common type of salt on Earth exists as a dissolved substance in our oceans. Salts, particularly table salt or sodium chloride, also exist as dry solid minerals that can be extracted from underground salt mines. 

Many of us consume salt on an almost daily basis, whether intentionally or unintentionally. Table salt, for example, is often added to food either as a preservative or to improve the flavour. This is especially true for processed foods.

What is meant by a salt in chemistry?

Salt is defined in chemistry as a compound that has ionic bonds, forming crystalline structures.

As the product of the neutralisation reaction between acids and bases, salts usually have a neutral pH. Some salts, however, can produce either acidic or basic solutions when dissolved in water. When salt is dissolved in an aqueous solution, the ions separate into positively-charged anions and negatively-charged cations. This makes the solution an effective conductor of electrical current. 

Many types of salts are simple inorganic compounds with monatomic ions like NaCl. However, there are also organic salts that have polyatomic ions, such as acetate or C₂H₃O₂, which is a product of the reaction between acetic acid and a base.

Examples of salts used in chemistry

Salts occur naturally, either in solutions such as seawater or as dry solid minerals. The salinity of the oceans is about 3.5% or 35 parts per thousand, although this does vary between regions. Meanwhile, mineral salt called halite (another name for rock salt) is mined from natural underground salt deposits.

Here are some examples of salt other than sodium chloride:

• Calcium chloride (CaCl₂) – commonly used for deicing roads during the winter, calcium chloride is an edible salt that has a similar appearance to sodium chloride.
• Sodium bisulphate (NaHSO₄) – this type of salt is created from sulphuric acid and is commonly used to reduce the pH level in swimming pools.
• Copper sulphate (CuSO₄) – this salt is ubiquitous in school chemistry laboratories because of its ability to change colour when reacting with other substances. Copper sulphate is also used as a fungicide, algaecide, and herbicide.
• Potassium dichromate (K₂Cr₂O₇) – potassium dichromate is an orange salt that’s commonly used as an oxidising agent.
• Ammonium dichlorate ((NH₄)₂Cr₂O₇) – a strong oxidising agent, ammonium dichlorate is often used in lithography and photographic development.
• Sodium bicarbonate (NaHCO₃) – also known as baking powder, sodium bicarbonate is used as a leavening agent for the expansion of dough. It’s also a popular cleaning agent and is a common component in fire extinguishers.
• Ammonium hydrogen difluoride (NH₄HF₂) – this is an acid salt used in glass and silicate etching. Its other applications include protecting wood and printing and dyeing textiles. Ammonium hydrogen difluoride is also used as an antiseptic in breweries.

You can read more about the different types of salts in chemistry and their uses here.

The classification of salts

Salts can be classified into four broad categories, depending on the pH and type of ions they contain:

1. Normal salts

   Normal salts are formed from the complete replacement of an acid reactant with the metal or group of atoms acting as metals from a base. The most common example of a normal salt is sodium chloride, or table salt. Normal salts have a neutral pH.

2. Acid salts

   Acid salts are formed from the partial replacement of hydrogen atoms and produce an acidic solution when dissolved in water. An example of an acidic salt is magnesium hydroxychloride.

3. Basic salts

   Otherwise known as alkaline salts, basic salts are formed from the reaction between weak acids and strong bases. Sodium carbonate is an example of a basic salt.

4. Double salts

   These form two types of salts when dissolved in water. An example of a double salt is potash alum [K₂S0₄.Al₂(S0₄)₃.24 H₂0].

Different types of salts used in chemistry

In addition to the categories outlined above, some substances can also be classified as either anhydrous salts or hydrated salts.

Anhydrous salt

While some salts can be dissolved in water, they can’t always combine with the water molecules themselves. The most common example of this is table salt. Although it can be dissolved in water, table salt cannot exist in a ‘dry’ form that incorporates water molecules. However, some salts that are capable of incorporating water molecules may also be in an anhydrous state if the water molecules are removed through heat.

Hydrated salt

Hydrated salts, such as copper sulphate (CuSO₄.5H₂), can exist in ‘dry’ solid crystalline forms even though they’re able to integrate water molecules in their structures. The water molecules in these types of salts crystallised along with the salt crystals.

What is a soluble salt?

Soluble salts are those that dissociate into ions and separate the solvent’s molecules to form solutions. Sometimes heat is needed to dissolve salts or make a more saturated solution. Certain salts such as mercury iodide and lead sulphates aren’t soluble in water. 

Summary

Salts are ionic compounds that can be formed through the neutralisation reaction between acids and bases. Many common types of salts have a neutral pH, although some are either acidic or basic when dissolved in water. Salts can be classified as double salts, anhydrous salts, and hydrated salts.