Classify oxides as acidic, basic, amphoteric or neutral, and describe neutralisation as the reaction of hydrogen ions with hydroxide ions to form water

What this dot point is asking

SEAB wants you to classify oxides into four types (acidic, basic, amphoteric and neutral) according to the element they come from and how they react, and to describe neutralisation at the ionic level as hydrogen ions reacting with hydroxide ions to form water. These ideas tie acids and bases together and explain why neutralisation always gives water whatever the acid and alkali used.

The answer

The four types of oxide

An oxide is a compound of an element with oxygen. Oxides fall into four classes:

• Basic oxides are metal oxides that react with acids to form a salt and water. Examples are sodium oxide, magnesium oxide and copper(II) oxide. Soluble basic oxides (such as sodium oxide) dissolve to give alkalis.
• Acidic oxides are non-metal oxides that react with bases (alkalis) to form a salt and water, and dissolve in water to give acids. Examples are carbon dioxide, sulfur dioxide and nitrogen dioxide.
• Amphoteric oxides react with both acids and bases, showing both characters. The key examples are aluminium oxide, zinc oxide and lead(II) oxide.
• Neutral oxides are non-metal oxides that are neither acidic nor basic. Examples are water, carbon monoxide and nitrogen monoxide.

A useful pattern: across the Periodic Table, metal oxides (on the left) tend to be basic and non-metal oxides (on the right) tend to be acidic, with amphoteric oxides in between.

Neutralisation as an ionic reaction

When an acid reacts with an alkali, the products are a salt and water. The acid provides hydrogen ions ($\text{H}^+$) and the alkali provides hydroxide ions ($\text{OH}^-$). The essential reaction is between these two ions:

This is neutralisation. The hydrogen ions and hydroxide ions combine to form water, so the solution moves toward pH $7$. The salt forms from the remaining ions (the metal ion from the alkali and the non-metal ion from the acid), which stay in solution as spectator ions.

Why neutralisation always gives water

Because the underlying reaction is always hydrogen ions plus hydroxide ions making water, every acid-alkali neutralisation forms water, regardless of which acid and which alkali are used. Only the salt differs, named from the particular acid and metal involved. This is why the general pattern acid + alkali gives salt + water holds so reliably.

Examples in context

Example 1. Acidic oxides and acid rain. Sulfur dioxide and nitrogen oxides are acidic non-metal oxides released by burning fossil fuels. They dissolve in rainwater to form acids, lowering its pH and producing acid rain, a direct consequence of how non-metal oxides behave.

Example 2. Basic oxides treating acidity. Magnesium oxide, a basic metal oxide, is used to neutralise excess acid in soil and in the stomach. Its reaction with hydrogen ions raises the pH toward neutral, an everyday application of a basic oxide reacting with an acid.

Try this

Q1. State whether carbon dioxide is an acidic, basic or neutral oxide, and the type of element it comes from. [1 mark]

• Cue. Acidic oxide, from a non-metal (carbon).

Q2. Write the ionic equation for neutralisation and state what it shows. [2 marks]

• Cue. $\text{H}^+(aq) + \text{OH}^-(aq) \rightarrow \text{H}_2\text{O}(l)$; it shows hydrogen ions and hydroxide ions combine to form water.

Q3. Explain what is meant by an amphoteric oxide and give one example. [2 marks]

• Cue. An oxide that reacts with both acids and bases; for example aluminium oxide (or zinc oxide).