Have you ever thought how the salt on your table gets to you? Do we just mine it? Or is there a chemical way of producing salts? In this article we will go through the chemistry of salts and reactions you probably didn't think could make salts.
- In this article, we will talk about how we make salts.
- Salt composition and formula.
- We will study what an ionic bond is, along with the concepts of a cation and anion.
- The different elements that can make up those ions.
- How acids and bases can produce salts through a neutralisation reaction.
- The formula and equation for neutralisation reactions.
- How you can purify soluble and insoluble salts.
- To finish, we will see some examples of salts and how they are made.
Salt Formula
What is salt really? We can define salts by a simple chemical formula that unites all salts.
A salt is an ionic chemical compound made of a (metal) cation and non-metal anion (usually).
So what are all those terms though? Below is a short summary of the concepts you will come across in this topic.
An ionic bond is one where one atom donates its electron and the other atom accepts it. This is different from covalent bonds, where electrons are shared equally between the two atoms. The formation of an ionic bond creates one atom with a lost electron, which will be positive when the bond dissociates, and one atom with an additional electron, which will be negative if the ionic bond dissociates. We term these two ions that are formed upon the breaking of an ionic bond the anion and cation, where the cation is the positive ion and the anion is the negative ion.
Here you can see how an ionic bond is formed (Fig. 1):
And here we have a covalent bond (Fig. 2):
Fig.2: A dot and cross diagram of a chlorine molecule with a covalent bond.
A cation is an ion that is positively charged.
An anion is an ion that is negatively charged.
But how are these two ions (the cation and anion) positioned in salts to make up their specific structure and give their chemical properties? The general formula of salts can help us shed a light into this.
Salts are generally comprised of an ionic bond between the cation and anion:
$$Cation^{a+}_{b}\ Anion^{c-}_{d}$$
$$Where\ a\cdot b=c\cdot d$$
It is worth mentioning that salts have no net electric charge, as the charges cancel out whilst the elements are ionically bonded to each other, such as seen in a crystalline structure
So the general formula for salts tells us that they are comprised of this electrostatic interaction between the cation and anion. But how do many molecules come together to make up a larger salt structure? This trend actually continues and forms large crystalline structures. Here the crystal of salt is comprised of large structures of alternating large negative anions and small positive cations.
This structure actually creates a very strong regular structure, which gives it its physical properties of high melting points. They can be dissolved in polar solvents.
The true determinant of the structure are actually the elements that are involved in the ionic bonding to form the salt. In the next section we will discuss what elements usually come together to make up salts, as well as go into detail which elements can become each ion (cation and anion) and which can't.
Elements That Make Up Salt
So can salt be made from any element? Why or why not?
Firstly, the cation.
The cation has to be an element that can easily donate an electron. This is why most cations are metals, as they can have few electrons in their outermost shell. It is easier for them to get rid of those few electrons than gain multiple electrons to complete the octet.
Additionally, some metals can have a variable oxidation state.
So if metals can easily lose electrons to become positively ionised, they act as cations in salts. Some common examples which are most commonly seen in salts are alkali metals and alkali earth metals, which comprise the first two groups of the periodic table:
Fig. 3: The periodic table.
Secondly, the anion.
The anion is different a bit from the cation, as it can not only be a single element but can also be a small covalently bonded molecule, which acts as an ion. Here we make a distinction between monoatomic anions and polyatomic anions.
Cations can also do this and they do it a lot ( Hg2+), + a complex anion /cation is also an option (FeCN6)3- anion or UO23+ cation.
Monoatomic anions are anions that are comprised of a single element. These are elements that can easily attract electrons and hold onto their electrons strongly. They are characterised by having a lot of electrons in their outermost shell, thus it is easier for them to accept a few electrons to complete the octet, than lose a bunch. These are typically electrons that are also very electronegative. Some common examples of these are halogens, which are in group 7 (or 17) of the periodic table. These elements can easily accept one electron to complete the octet, thus becoming a negative ion.
Polyatomic anions are anions that are small chemical molecules that act as negatively charged particles. This means that they will be a group of covalently bonded molecules that don't dissociate further when the ionic bond is broken, yet remain together acting as one ion. They can be organic or inorganic. Later on in the article, we will look at different examples of salts and with it discover different types of anions that can comprise salts.
Acid and Base Salt Reaction (Neutralisation)
In this section, we will cover how and why certain chemical reactions can produce salts.
You have probably heard of acids and bases, but maybe not always in the context of salts and ionic compound chemistry. Lets have a brief recap of acids and bases below:
Acids are chemical species that can produce H+ ions in water. On the other hand, bases are chemical species that produce hydroxide ions, OH-, in water.
So what importance do these two have in the production of salts? Let's take a deeper look at their chemical composition to see how those two can react.
Acids are usually characterised by a hydrogen being bonded to an anion. Here the hydrogen acts as the cation. You can write acids as H-A, where the H is the hydrogen and the A the anion moiety. On the other hand, the base is often a metal bonded to a hydroxide. This allows us to write bases as M-OH, where the OH is the hydroxide and the M is the metal cation in the compound. Note that the hydroxide in bases and the hydrogen in acids both dissociate when in contact with water, which results in the production of ionic species.
As acids and bases together have all the necessary parts to make a salt, that is a metal cation and an anion, we can react them together to produce a salt. This is called a neutralisation reaction.
In the next section, we will go into the specific chemistry of how this reaction goes on with equations, and you will get an understanding of the underlying chemistry that goes on in creating salts. There are actually two ways in which this neutralisation can turn out: it can produce a soluble salt or an insoluble salt.
Soluble Salt Production
Firstly, producing a soluble salt means that your salt product will be still dissolved. Remember that the neutralisation reaction you will be performing will be in an aqueous environment, meaning the reagents will be in water. So if you made your salt, but it is dissolved in water, what do you do?
There are purification steps you can take to retrieve the salt you just created. You can heat your reaction mixture to evaporate any extra solvent to be left with a very saturated solution. If you leave this saturated mixture to cool, salt crystals will start to form. You can harvest these crystals through filtration. Lastly, you can leave the crystals to dry fully (as they will be wet after the filtration) and you can collect your produced salt.
Insoluble Salt Production
Insoluble salts are determined by the chemical nature of the compound. The produced salt will not be soluble at standard conditions. Insoluble salts will appear immediately when you react the base and the acid, and will sediment at the bottom of your beaker. You can filter off the solvent to be left with the salt crystals only. You can then dry the crystals to get a pure salt.
Reacting compounds to produce insoluble salts is a common laboratory practice to qualitatively identify the presence or absence of certain elements or compounds in samples.
Making Salts Equation
Here we will go over the general chemistry of the neutralisation reaction.
Take. a look at the following two general equations:
$$acid + base \rightarrow salt + water$$
$$\text{H-Anion} + \text{Cation-OH} \rightarrow \text{Cation-Anion} + H_2O$$
Can you see how first the two compounds dissociate to produce ions (the hydrogen ion, hydroxide ion, a cation, and an anion) and then come together to make an ionic compound (the salt) and the water molecule? This is why this reaction is often referred to as a neutralisation reaction, as it produces water in the process it neutralises the pH of the acid and base, which are on different spectrums of the pH scale.
Water naturally has a pH of 7 at room temperature.
For every neutralisation reaction you can also write the ionic equation which only takes into account the cation and anion and disregards the spectator ions:
\[ Cation^+ + Anion^- \rightarrow \text{Cation-Anion} \]
Examples of Salts
Here we will go over a few examples of salts and how they are made.
The most common example of a salt you will come across is able salt, NaCl (sodium chloride). It can be made with sodium hydroxide (NaOH) and hydrochloric acid (HCl) Below is the reaction equation:
\[ NaOH_{(aq)} + HCl_{(aq)} \rightarrow NaCl_{(aq)} + H_2O_{(l)} \]
Another example is the production of a salt with an organic polyatomic anion. Let's take a look at magnesium acetate, Mg(CH3COO)2. Here the cation is an alkali earth metal. You can make this salt from acetic acid and magnesium hydroxide with the following reaction:
\[ 2CH_3COOH_{(aq)} + Mg(OH)_{2(aq)} \rightarrow Mg(CH_3COO)_{2(aq)} + 2H_2O_{(l)} \]
For the last example let's take a look at an insoluble salt, barium sulfate (BaSO4). The anion here is a polyatomic inorganic group. We can make this salt through a simple chemical reaction that is not a neutralisation reaction with the following reaction:
$$BaCl_{2(aq)} + MgSO_{4(aq)} \rightarrow BaSO_{4(s)} + MgCl_{2(aq)}$$
Making Salts - Key takeaways
- Salts are chemical compounds with a cation and an anion bonded through an ionic bond.
- An ionic bond is a chemical bond where one atom donates an electron and the other accepts it.
- Metals (such as group 1 an 2) make up cations
- Anions can be monoatomic or polyatomic, and if polyatomic can be organic or inorganic
- A neutralisation reaction is a reaction between an acid and a base producing salt and water.
- Soluble salts have to be heated and crystallised before extraction, whilst insoluble salts have to just be filtered off from the solvent.
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