---
title: "Period 3 Oxides and the Acid-Base Divide"
description: "Metal oxides on the left of period 3 give alkalis with water, non-metal oxides on the right give acids, and aluminium oxide does both. The bonding changes across the row and the reactions follow it."
canonical: https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ
source: https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ.md
retrieved: 2026-09-12
---

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# Period 3 Oxides and the Acid-Base Divide

Metal oxides on the left of period 3 give alkalis with water, non-metal oxides on the right give acids, and aluminium oxide does both. The bonding changes across the row and the reactions follow it.

Subject: Science · Area: Chemistry · Ages 16 to 18
Page: https://lightmysky.com/learn/science/period-3-oxides-and-the-acid-base-divide-mt_3kC5hTavIQ

## Ready when they can

- Writes equations for sodium oxide and for sulfur trioxide reacting with water
- Links the change from ionic to covalent bonding across the row to the change from basic to acidic
- Explains what amphoteric means using aluminium oxide with acid and with alkali
- Predicts the approximate pH of the solution a named period 3 oxide makes

## Lesson: Oxides from alkali to acid

The left of the row gives alkalis. Sodium oxide meets water to make sodium hydroxide solution: Na2O plus H2O gives 2NaOH. Magnesium oxide does the same basic job. These oxides are ionic, and their solutions turn alkaline.

The right of the row gives acids. Sulfur trioxide meets water to make sulfuric acid: SO3 plus H2O gives H2SO4. Phosphorus oxides do the same acidic job. These oxides are covalent, and their solutions turn acidic.

**Example.** Aluminium oxide sits on the fence and reacts both ways. It takes on acids to give a salt plus water, and it takes on alkalis to give an aluminate. Oxides that behave like this are called amphoteric.

**Tip.** To predict a pH, read the bonding. Metal oxides from the left give pH above 7, non-metal oxides from the right give pH below 7, and aluminium oxide offers no simple pH.

**Recap.** Ionic oxides turn water alkaline, covalent oxides turn it acidic, and aluminium oxide does both.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Trends Across Period 3: Radius, Ionisation Energy and Melting Point](https://lightmysky.com/learn/science/trends-across-period-3-radius-ionisation-energy-and-melting-point-mt_stFFQ6GPIh)
- [Strong and Weak Acids and the pH Scale](https://lightmysky.com/learn/science/strong-and-weak-acids-and-the-ph-scale-mt_Y4nSn5XH56)

## Opens up

- [Group 2: Reactivity and Solubility Trends](https://lightmysky.com/learn/science/group-2-reactivity-and-solubility-trends-mt_Ch8FEkh-dV)
- [Acid-Base Models Beyond Bronsted: Lewis, Solvents and Hard and Soft](https://lightmysky.com/learn/science/acid-base-models-beyond-bronsted-lewis-solvents-and-hard-and-soft-mt_jarui-9yfx)
- [Main-Group Chemistry: What Changes Down a p-Block Group](https://lightmysky.com/learn/science/main-group-chemistry-what-changes-down-a-p-block-group-mt_wq15-pBrI5)