Under the “O” level syllabus, all salts are considered to be neutral unless otherwise stated. However, in reality, there are actually acidic, basic salts as well as neutral salts.
Let me explain basic salts first, as it builds on topics I’ve covered in previous posts.
What are basic salts?
A basic salt is a salt that dissolves in water to produce a solution with a pH of more than 7. In other words, there must be more OH– ions than H+ ions present in the solution
How are they formed?
When a strong base reacts with a weak acid.
For instance, when sodium hydroxide (strong alkali) reacts with ethanoic acid, the salt formed, sodium ethanoate, is a basic salt.
Why is it basic?
When sodium ethanoate is dissolved in water, it dissociates to form 2 ions, Na+ and CH3COO–.
Recall what you learned in the previous post about the behaviour of the Na+ ion in water. Due to its low charge density of the Na+, it is unable to hydrolyse (break-up) the water molecule, hence will produce few if any H+ ions.
Recall what you learned in the previous post about the behaviour of the ethanoate ion (CH3COO-) in water. It forms an alkaline solution with plenty of OH– ions.
Thus, there will be more OH– ions than H+ ions in a solution containing dissolved sodium ethanoate and the solution is said to be basic/alkaline.
How about acidic salts?
An acidic salt is a salt that dissolves in water to produce a solution with pH less than 7. In other words, there must be more H+ ions than OH– ions in the solution
How are they formed?
- When a strong acid reacts with a weak base. (E.g. HCl + NH4OH, the salt formed, NH4Cl will be an acidic salt)
- When the salt contains a cation with high charge density. (E.g. AlCl3)
Why is it acidic?
Let’s talk about scenario 1 first!
HCl is a strong acid that dissociates completely in water to produce H+ and Cl– ions. This implies that the Cl– ion has relatively weak attraction to the H+ as it is willing to give it up easily. Hence, when dissolved in water, the Cl– ion, is unable to hydrolyse the water molecule to produce any OH– ions.
Put another way, the Cl– ion which is negative is attracted to the partial positive H atom of the water molecule. It is unlikely to react with water as it has relatively low charge density. Even if it reacts with water to produce HCl and OH–, since the HCl formed is a strong acid, it dissociates immediately to form back H+ and Cl–. The H+ then reacts with the OH– ion to form H2O. In other words, no net OH– ions are produced.
On the other hand, when the ammonium ion is dissolved in water, it reacts with the water to form H3O+ (which is commonly written as H+ in most textbooks*).
NH4+ + H2O ⇌ NH3 + H3O+
Hence, there will be more H+ than OH– ions in a solution containing dissolved ammonium chloride and the solution is said to be acidic.
Onwards to scenario 2!
When the metal ion has high charge density (E.g. Al3+), it is able to hydrolyse the water molecule to produce H+ ions. The Al3+ which has high charge density is attracted to the partial negative end (O) of the water molecule. It hydrolyses the water molecule to form the aluminate ion (Al(OH)4–) and 4 H+ ions**. The key point to note is that the OH– ions are covalently bonded to the Al3+ to form one large complex ion but there are 4 H+ ions. See diagram below.
However, the Cl– ion which has relatively lower charge density is unable to react with water to produce OH– ions as discussed above. Hence, there are more H+ ions than OH– ions in a solution containing dissolved AlCl3 and the solution is said to be acidic.
Likewise, if the anion has high charge density while the cation has low charge density, the resulting solution is most likely to be basic.
Ah ha! I hear voices again.. (not that its good thing mind you.. ) But.. What if the charge density of the cation and anion are both high? In that case, the ionic bonds between the ions will be very strong and it will probably not dissolve in water as it requires too much energy to break. (See previous post about why ionic compounds are soluble in water)
As the name suggests, neutral salts dissolve in water to produce a solution with pH 7. This means that there are equal amounts of H+ and OH– ions in the solution.
How are they formed?
They are formed when a strong acid reacts with a strong alkali. (E.g. HCl + NaH, the salt formed, NaCl will be neutral).
Tell me why! (Ain’t nothing but..)
As discussed in the earlier sections, due to its low charge density of the Na+, it is unable to hydrolyse (break-up) the water molecule, hence will produce few if any H+ ions. Likewise, the Cl– ion which also has relatively low charge density is unable to react with water to produce OH– ions. Hence the solution is neutral!
How about the salts formed by a weak acid and a weak base? Are they acidic/basic/neutral?
The solution can be acidic, basic or neutral depending on the relative strengths of the cation and the anion since both the cation and anion are able to hydrolyse the water molecule to produce H+ and OH– ions respectively.
*Note: The true form of H+ is H3O+. That is because when an acid dissociates in water, it is actually forming bonds with water, hence H3O+. But to simplify things, most textbooks will just put H+.
*For A level students, I have deliberately removed some jargon like conjugate acid/base to simplify the discussion. But you should know that the conjugate base of a strong acid is weak and vice versa, etc.
**For A level students, you should know that this is not strictly speaking true. It should form hydrated aluminium ions (hexaqua complex) first then form H+ ions when the complex is deprotonated. It need not necessarily form the aluminate ion. This is just a simplification to make it easier for O level students to understand.