CNC Illustration by Gwendolyn Say (@saydrawings)

Acids, bases and salts

Many reactions occurring in water can be understood as a dance between acids and alkalis, whereby hydrogen ions and hydroxide ions reunite.

In fact, alkalis are a type of bases, on which different salts are built when reacted with different acids.




Acids ionise and dissociate in water to form hydrogen ions.

HCl(aq) ⟶ H+(aq) + Cl(aq)
CH3COOH(aq)  H+(aq) + CH3COO (aq)

Acidity and pH

A more acidic solution has a greater concentration of hydrogen ions, giving a lower pH.

Acidic pH < 7 (red Universal Indicator)
Neutral pH = 7 (green Universal Indicator)
Alkaline pH > 7 (blue Universal Indicator)

Acid Strength

Strong acids dissociate completely, but weak acids dissociate partially.

Strong: HCl, HNO3, H2SO4
Weak: H2CO3, CH3COOH

Acid-Metal Redox

Acids oxidise moderately reactive metal to form a salt and hydrogen gas. For example:

2HCl + Mg ⟶ MgCl2 + H2
2HNO3 + Zn ⟶ Zn(NO3)2 + H2

Acid-Carbonate Reaction

Acids react with carbonate to form a salt, water and carbon dioxide gas. For example:

2HCl + MgCO3 ⟶ MgCl2 + H2O + CO2
2HNO3 + ZnCO3 ⟶ Zn(NO3)2 + H2O + CO2

Acid-Alkali Neutralisation

Hydrogen ions of acid react with hydroxide ions of alkali to form water. A salt is also formed. The general ionic equation is:

H+(aq) + OH(aq) ⟶ H2O(l)


Alkalis are soluble bases that dissociate in water to form hydroxide ions, giving a more than 7 pH.

NaOH(s) ⟶ Na+(aq) + OH(aq)
NH3(g) H2O(l)  NH4+(aq) + OH(aq)

Alkali-Ammonium Reaction

Alkalis react with ammonium salts to form another salt, water and ammonia gas. For example:

NaOH + NH4Cl ⟶ NaCl + H2O + NH3

Alkali-Salt Precipitation

Hydroxide ions of alkalis can combine with some metal cations to form insoluble precipitate. The general ionic equation is:

Mn+(aq) + nOH(aq) ⟶ M(OH)n(s)


Bases are any compounds that react with acids to form salt and water only. They include:

Soluble alkalis: NaOH, Ca(OH)2, NH3
Metal oxides: CaO, CuO
Insoluble metal hydroxides: Cu(OH)2

Acid-Base Neutralisation

Bases can neutralise acids to form salt and water only. For example:

CuO + 2HCl ⟶ CuCl2 + H2O
Cu(OH)2 + 2HCl ⟶ CuCl2 + H2O

Uses of Acids and Bases

Acids like sulfuric acid and bases like ammonia and calcium hydroxide have important industrial uses.

Sulfuric acid: make fertiliser, detergent and car battery
Ammonia: make fertiliser
Calcium hydroxide: reduce acidity in soil


Diverse range of compounds containing oxygen. Their properties depend on whether the oxygen is bonded to a metal or non-metal.

Metal oxides are ionic.
Non-metal oxides are covalent.

Metal Oxides

They are either basic oxides that react with acids only, or amphoteric oxides that react with acids and bases.

Basic: most metal oxides
Amphoteric: ZnO, PbO, Al2O3

Non-Metal Oxides

They are either acidic oxides that react with bases only, or neutral oxides that do not react with acids or bases.

Acidic: most non-metal oxides
Neutral: monoxides like H2O, CO and NO


Salts are ionic compounds that can be formed by replacing the hydrogen ions of an acid with metal or ammonium cations.

Most salts are soluble except AgCl, PbCl2, BaSO4, PbSO4, ZnSO4 and most carbonate salts.

Soluble Salts

They are made by reacting acids with excess insoluble metals, oxides or carbonates. If the reactant is soluble, use titration instead.

Most soluble salts: add insoluble reactant to acid
Na+, K+ and NH4+ salts: titration

Insoluble Salts

They are made by mixing two soluble reactants to precipitate the insoluble salt product.

The metal cation of the insoluble salt is usually introduced as aqueous metal nitrate, and the anion in a sodium solution.


Acids & bases TYS questions – Combined Chemistry

Move from basic questions to acidic, I mean challenging, TYS questions taken from the Combined Science papers

Preparation of salts

The three methods to prepare pure samples of solid salt

Raising the question of baking powder vs baking soda

Your kitchen is actually a chemistry lab. Baking powder and soda are bases that react with acids to raise your bread.

Carbon dioxide & other acidic oxides

Some oxides can behave like an acid and neutralise bases. How do you identify them?

O Level titration TYS questions

Don't let the difficult question from the 2014 paper be your endpoint. Overcome it here!

Reactions of acids

Acids are dangerous and corrosive, as their hydrogen ions are unlocked to wreak havoc. Learn how they react with metals, carbonates, and bases.

Definition and properties of acids

Learn about the defining property of acids: acids dissociate in water to produce hydrogen ions, either partially or fully, depending on their strength

Prelim questions on acids

Try out challenging questions from prelim papers to apply what you have learned about acids: dissociation of hydrogen ions, acid formation, and reactions.