What are acids?


In this chapter you will learn what are acids, what chemical properties have and why reactions do acids undergo. You will also learn about bases and alkali. Chemicals can be catagorised into two simple groups: acids or bases.

Alkalis are a part of bases that are soluble. Most bases are insoluble in water and they exist in the solid state, but there are some bases that are soluble in water.

What is an acid?

An acid is a substance that dissociates in water to produce H+ ions as the only positive ion.

Dissociates mean that the molecule break up to form ions. For example, hydrochloric acid (also known as hydrogen chloride) is an acid. It is a gas in room temperature and pressure and when the hydrogen chloride gas is bubbled into water, the hydrogen chloride molecule dissociates (break up) to form positive hydrogen ions and negative chloride ions. Only when hydrochloric acid is dissolve in water which is in its aqueous state, it can exhibit acidic properties.

The process of hydrogen chloride gas should look something like this:


Important thing to note when defining acids

It is important to note that hydrogen chloride gas does not display properties of acids, as it has not dissolve in water. An acid only gives its acidic properties only if it is dissolved in water as it is the hydrogen ions that gives acids their acidic properties. Acids in other state (solid, liquid or gas) DO NOT exhibit acidic properties because there are no positively charged hydrogen ions!

An important thing to note is that not all molecules that contains hydrogen atoms will dissociates in water to produce hydrogen ions, and therefore is an acid. Water, for example, has a chemical formula H2O, but it the water molecule does not dissociates (significantly) in water to produce H+ ions. Hence, water is not an acid. (This is true in O Level syllabus.)

However, there is something called the self-ionisation of water.

As acid dissolves in water to produce H+ ions. Hence, as the concentration of hydroxide ions remain the same, the concentration of hydrogen ions should be relatively higher than the hydroxide ions.

Hydrogen ions

There are two types of hydrogen ions, the negative hydrogen ions H- (also known as hydride ions) and the positive hydrogen ions (H+) ions. Hydride ions are formed when the neutral hydrogen atom gains and electron to from the negative hydride ion. Similarly, the positive hydrogen ion is formed when the hydrogen atom loses one electron.

When we are talking about acids, we are always talking about positively charged hydrogen ions. It is the hydrogen ions that the acid produce when dissolve in water that gives the acid its acidic properties. 

Examples of acids in O Level syllabus

  • Here are the acids you should know for our O Level syllabus. You should spend some time remembering the chemical name and chemical formula.
Name of acids Formula of acids Strength of acids What happens when acid dissolves in water pH of the solution
Nitric acid HNO3 Strong HNO3(aq) → H+(aq) + NO3-(aq)
Hydrochloric acid HCl Strong HCl(aq)   H+(aq) + Cl-(aq)  1
Sulfuric acid H2SO4  Strong  (i) H2SO4(aq)  H+(aq) + HSO4-(aq)

(ii) HSO4-(aq)  H+(aq) +SO42-(aq)

Carbonic acid H2CO3  Weak (i) H2CO3(aq)  H+(aq) + HCO3-(aq)

(ii) HCO3-(aq) → H+(aq) +CO32-(aq)


Phosphoric acid H3PO4  Weak (i) H2SO4(aq)  H+(aq) + HSO4-(aq)

(ii) HSO4-(aq) → H+(aq) +SO42-


Ethanoic acid CH3COOH  Weak  CH3COOH(aq) → H+(aq) + CH3COO-(aq)  4-5

There are some textbooks that refer sulfuric acid as 'sulphuric'. This is the old spelling and is not accepted in Cambridge O Level. You must learn to write 'sulfuric'. If an acid has more than one hydrogen ions in a molecule (sulfuric acid, carbonic acid and phosphoric acid), the positive hydrogen ions will dissociate in water one by one. 

For ethanoic acid, even though there are more than one hydrogen atoms in each molecule (CH3COOH), only the hydrogen in the COOH group can dissociate.

Important names of ions to know

Identify of polyatomic ions  Chemical formula of ions
 NO3-  Nitrate ions
CO32-  Carbonate ions
SO42- Sulfate ions
 NH4+ Ammonium ions




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