1. The many names of sourness

When someone complains that a coffee is “too acidic”, we know that they find it too sour. Indeed, the word acid came from the Latin word for sour acidus. This is not just a linguistic coincidence. Acids do have a defining sour taste.

We have also seen the effect of acids in Lower Secondary Science. We know that acids turn moist blue litmus paper red.

But what explains these two observations about acids? Do acids share similar chemical composition or reactions at the atomic level? We shall dig deeper by thinking small.

2. It’s not about who they are

Common Acids Chemical Formulae
Hydrochloric acid HCl
Nitric acid HNO3
Sulfuric acid H2SO4

Early chemists used to think that all acids contained the same element that we now call oxygen. In fact, oxygen was so named because oxy- means acid in Greek and -gen means giving. It is the element that gives acid, or so they thought. This idea was overturned when later chemists confirmed the chemical composition of hydrochloric acid. It does not contain oxygen.

Perhaps hydrogen then? From the table above, all the common acids have at least one hydrogen atom. This seems plausible, until we realise water too contains hydrogen atoms. Yet, it is clearly not an acid. Therefore, acids are not defined by their chemical composition, even though many of them do contain hydrogen atoms.

3. Defining acids: it’s about what they do, in water

Common Acids Dissociation in Water
Hydrochloric acid HCl (aq) ⟶ H+ (aq) + Cl (aq)
Nitric acid HNO3 (aq) ⟶ H+ (aq) + NO3 (aq)
Sulfuric acid H2SO4 (aq) ⟶ 2H+ (aq) + SO42- (aq)

In water, all acids react to produce hydrogen ions, H+. It is these hydrogen ions that interact with cells in the tongue to inform the brain that you are drinking something sour (and hopefully not something soured). It is also them that react with compounds in litmus papers, turning them from blue to red. In short, hydrogen ions explain the properties of acids we observe.

We can further classify acids based on the number of hydrogen ions produced per molecule. We call this basicity of acids.

  1. Monobasic acids like hydrochloric acid and nitric acid, which produces just 1 hydrogen ion per molecule
  2. Dibasic acids like sulfuric acid. It produces 2 hydrogen ions per molecule.

4. Goodbye mate, let’s dissociate

Diagram showing how the dissociation of hydrogen chloride is also ionisation, whereby ions are produced as chlorine gains the hitherto shared pair of electrons.
Dot-and-cross diagram showing the dissociation of hydrochloric acids

This reaction is called dissociation, which is a cheem word for separation. We name it so because one molecule is separated into multiple ions in water.

It is also called ionisation, because ions are formed in the process. Initially, hydrogen and chlorine atoms share electrons in a simple molecule. In the presence of water, they stopped sharing and chlorine gains the valence electron of hydrogen. Simply put, water eggs chlorine on to steal the shared pair of electrons. Therefore, ions are formed.

This process of dissociation happens to all acids in water. And so we shall define acids as such:

Acids are compounds that dissociate in water to produce hydrogen ions.

Editor’s Note: The next part, points 5 and 6, are not tested in the Combined Chemistry syllabus. However, you may still want to read on to get a full understanding of acids. No harm at all!

5. Completely strong, partially weak

Diagram showing the partial dissociation of weak acid and complete dissociation of strong acid. Complete dissociation leads to a higher concentration of hydrogen ions.
Weak acid dissociates partially to produce some hydrogen ions, while strong acid dissociates completely to produce more hydrogen ions

While all acids do dissociate in water, they dissociate to different extents.

CH3COOH (aq) ⇄ CH3COO (aq) + H+ (aq)

Weak acids dissociate partially. An example is ethanoic acid, CH3COOH found in vinegar. When you add pure ethanoic acid into water, only a small percentage of the simple molecules of ethanoic acid dissociate to produce hydrogen ions. In chemical equations, we represent this partial dissociation with a double arrow (⇄). Therefore, in the resulting mixture, most ethanoic acid will exist as simple molecules, while only some will have dissociated into ions.

On the other hand, strong acids like hydrochloric acid, nitric acid, and sulfuric acid dissociate completely. When they are added to water, all of the simple molecules will dissociate to produce hydrogen ions.

6. Big brain time: which has the highest concentration of hydrogen ions?

Let’s compare three very common acids we always see in the lab to find out which produces the highest concentration of hydrogen ions.

  • 1 mol/dm3 hydrochloric acid, HCl (aq)
  • 1 mol/dm3 sulfuric acid, H2SO4 (aq)
  • 1 mol/dm3 ethanoic acid, CH3COOH (aq)

First, let’s understand what the concentration before the acid name means. It refers to the number of moles of pure acid added into each dm3 of water.

Next, we come to the crux of the question. For every mole of pure acid added into water, how many moles of hydrogen ions will be produced? To answer this question, we need to consider two factors in turn: strength and basicity.

  • STEP 1: Strong or weak acid?

    Ethanoic acid is a weak acid, while hydrochloric acid and sulfuric acid are strong acids. This means that ethanoic acid will only dissociate partially, to a much lower extent than the strong acids, to produce the lowest concentration of hydrogen ions.

  • STEP 2: Monobasic, dibasic, or tribasic?

    HCl (aq) ⟶ H+ (aq) + Cl (aq)
    H2SO4 (aq) ⟶ 2H+ (aq) + SO42- (aq)

    Hydrochloric acid is monobasic. 1 mol/dm3 hydrochloric acid will dissociate fully to give 1 mol/dm3 hydrogen ions. However, sulfuric acid is dibasic. 1 mol/dm3 sulfuric acid will dissociate fully to give 2 mol/dm3 hydrogen ions, which is twice that of hydrochloric acid.

  • STEP 3: Conclude

    Therefore, in ascending order of concentration of hydrogen ions in water:

    CH3COOH (weak acid) < HCl (strong monobasic acid) < H2SO4 (strong dibasic acid)

7. Concentration of hydrogen ions and pH

As the concentration of hydrogen ions increases, the corresponding pH decreases.
Acids with high concentration of hydrogen ions is translated to low pH.

We can represent the concentration of hydrogen ions as pH. While the exact mathematical formula relating concentration and pH is not required at the O Level, we have to know two general observations.

  1. As the concentration of hydrogen ions increases, the pH decreases.
  2. Neutral solutions have a pH of 7 at 25° C.

These two observations imply that acids have pH lower than 7. This is because acids have a higher concentration of hydrogen ions than a neutral solution, and hence a lower pH.

Amongst acids of the same concentration, strong acids tend to have a lower pH of around 1 than weak acids.

8. Colourful pH

Indicators have different colours in solutions of different pH.

We cannot see pH directly, but we can observe its effect on indicators. Universal Indicator is the most sensitive to pH change. We can use it to confirm the presence of acids, if it turns yellow, orange, or red. For the dilute acids we use in the laboratory, it can even differentiate strong acids that turn it red from weak acids that turn it orange.

Other indicators like methyl orange and phenolphthalein change their colour only once. They cannot confirm the exact pH, but gives us a broad range of possible pH. For example, if methyl orange remains orange, we can only tell that the solution has a pH more than 3. However, we cannot confirm that if it is a weak acid, neutral solution, or alkali.