IGCSE Chemistry 0620: Classifying Oxides as Acidic or Basic

A fertiliser manufacturer tests three oxide powders recovered from a raw mineral blend: potassium oxide, sulfur trioxide and lithium oxide. Each oxide is tested separately with dilute nitric acid and with sodium hydroxide solution.

(a) Potassium oxide reacts with dilute nitric acid, forming a salt and water, but does not react with sodium hydroxide solution. State the type of oxide potassium oxide is, and write a word equation for its reaction with nitric acid. [2]

(b) Sulfur trioxide does not react with nitric acid, but reacts with sodium hydroxide solution to form a salt and water. State the type of oxide sulfur trioxide is, and write a word equation for its reaction with sodium hydroxide. [2]

(c) Lithium oxide gives the same pattern of results as potassium oxide: it reacts with the nitric acid but not with the sodium hydroxide solution. State the type of oxide lithium oxide is, and explain, in terms of metals and non-metals, why potassium oxide and lithium oxide both behave this way. [2]

Part (a): Potassium oxide

Potassium oxide reacts with an acid but not with an alkali, so it is a basic oxide, the typical behaviour of a metal oxide.

With dilute nitric acid, it undergoes a normal acid + base reaction, forming a salt and water:

$\text{potassium oxide} + \text{nitric acid} \rightarrow \text{potassium nitrate} + \text{water}$

The salt is named potassium nitrate because the nitrate ion comes from the nitric acid used.

Part (b): Sulfur trioxide

Sulfur trioxide reacts with an alkali but not with an acid, so it is an acidic oxide, typical of many non-metal oxides.

With sodium hydroxide solution, it forms a salt and water:

$\text{sulfur trioxide} + \text{sodium hydroxide} \rightarrow \text{sodium sulfate} + \text{water}$

Part (c): Lithium oxide, and the general pattern

Lithium oxide reacts with the acid but not with the alkali, so, like potassium oxide, it is a basic oxide.

Potassium and lithium are both metals. Metal oxides generally behave as basic oxides: they react with acids to form a salt and water, and do not react with alkalis. Non-metal oxides, such as sulfur trioxide in part (b), generally behave the opposite way, as acidic oxides: they react with alkalis but not with acids. This left-to-right pattern (metal oxides basic, non-metal oxides acidic) is a useful general rule, although a small number of oxides, such as aluminium oxide and zinc oxide, are exceptions, since they are amphoteric and react with both acids and alkalis.

Final answers

(a) Basic oxide; potassium oxide + nitric acid → potassium nitrate + water.
(b) Acidic oxide; sulfur trioxide + sodium hydroxide → sodium sulfate + water.
(c) Basic oxide; both potassium oxide and lithium oxide are metal oxides, and metal oxides typically behave as basic oxides, reacting with acids but not alkalis.