Oxides

Oxide classification is a 2-3 mark question that doubles as an MCQ filter, and it is won or lost on a single sorting rule plus four named exceptions. The Extended version has a sharper edge: “describe how you could show zinc oxide is amphoteric” demands two pieces of experimental evidence, and answers offering one reaction cap at half marks.

The sorting rule (Core)

Element type predicts oxide character:

Oxide type	Formed by	Behaviour	Examples
Acidic	Non-metals	React with bases; dissolve in water to give acidic solutions	CO2, SO2
Basic	Metals	React with acids to give salt + water	CuO, Na2O, MgO
Neutral (S)	A few non-metals	React with neither acids nor alkalis	CO, H2O
Amphoteric (S)	Al, Zn	React with both acids and alkalis	Al2O3, ZnO

Carbon dioxide dissolving in water gives carbonic acid (the reason rainwater is naturally slightly acidic), and sulfur dioxide gives an acidic solution implicated in acid rain, which crosses over into Chemistry of the Environment. Basic oxides that dissolve, such as sodium oxide, give alkaline solutions; most metal oxides (CuO, Fe2O3) are insoluble bases.

The classification connects to the periodic table: moving left to right across a period, oxides shift from basic through amphoteric to acidic, mirroring the metal-to-non-metal transition tested in The Periodic Table.

Basic oxides in action (Core)

A basic oxide neutralises an acid: base + acid → salt + water. Copper(II) oxide warmed with dilute sulfuric acid gives blue copper(II) sulfate solution, the exact reaction used in the insoluble-base method of salt preparation. The black solid disappearing into a blue solution is a stock observation mark.

Acidic oxides do the mirror-image job: carbon dioxide reacts with sodium hydroxide solution, which is why CO2 is absorbed by alkalis and why limewater (a dilute alkaline solution of calcium hydroxide) turns milky in the CO2 test.

Amphoteric oxides (Supplement)

Aluminium oxide and zinc oxide react with both acids and alkalis, forming a salt and water each time:

• With acid: ZnO + 2HCl → ZnCl2 + H2O (behaving as a base)
• With alkali: ZnO + 2NaOH → Na2ZnO2 + H2O (behaving as an acid; the salt is sodium zincate)

0620 does not require the alkali-side equations from memory. The marks are for the concept and the evidence. To show an oxide is amphoteric: add it to dilute hydrochloric acid and observe it react/dissolve; add a fresh sample to sodium hydroxide solution and observe it react/dissolve again. Both observations together prove the classification; either alone proves only basic or only acidic behaviour.

The same amphoteric logic reappears with the hydroxides: aluminium hydroxide and zinc hydroxide precipitates dissolve in excess sodium hydroxide, the distinguishing step in cation analysis covered in qualitative analysis ion and gas tests.

Worked exam question

Four oxides are listed: MgO, SO2, ZnO, CO. (a) Classify each oxide as acidic, basic, neutral or amphoteric. [4] (b) Describe experiments to show that zinc oxide is amphoteric. Include the expected observations. [3]

Model answer: (a) MgO basic (1); SO2 acidic (1); ZnO amphoteric (1); CO neutral (1). (b) Add zinc oxide to dilute hydrochloric acid: the solid reacts and dissolves, showing it behaves as a base (1). Add zinc oxide to aqueous sodium hydroxide: the solid again reacts and dissolves, showing it behaves as an acid (1). Reacting with both acid and alkali shows the oxide is amphoteric (1).

Mark-by-mark: (a) is the sorting rule plus the two memorised exceptions (ZnO amphoteric, CO neutral). (b) distributes one mark per reagent-plus-observation and one for the conclusion; naming “acid” without a specific acid, or omitting what is seen, erodes the experiment marks. A named alkali (sodium hydroxide) is safer than “an alkali”.

The mistakes that cost marks

1. Calling carbon monoxide acidic because carbon dioxide is. CO and H2O are the two neutral oxides. Learn them as exceptions.
2. Proving amphoteric with one reaction. The definition requires both: reacts with acids and with alkalis.
3. Writing “dissolves” without “reacts”. Amphoteric evidence is a chemical reaction; pure dissolving describes sugar in tea.
4. Classifying by appearance or state. The test is chemical behaviour towards acids and alkalis, not whether the oxide is a white solid or a gas.

How examiners want it phrased

Student wording	Mark-scheme wording
”Metal oxides are the opposite of acids"	"Basic oxides react with acids to form a salt and water"
"Zinc oxide is both"	"Zinc oxide is amphoteric: it reacts with both acids and alkalis"
"CO2 makes water sour"	"Carbon dioxide dissolves in water to form an acidic solution"
"It fizzed away in the acid"	"The solid reacted and dissolved in the dilute acid, forming a salt solution”

One table, four examples, two exceptions: this is among the cheapest reliable marks in Section 7, mapped in full on the Acids, Bases and Salts pillar. If classification questions still feel like coin-flips, a free 1-hour trial lesson with one of our Chemistry specialists will turn the rule-plus-exceptions into ten seconds of certainty.

Test yourself

Sort each oxide before you peek. The answers stay hidden until clicked.

Q1 (2 marks). Classify sodium oxide and carbon dioxide as acidic or basic, and state which one dissolves in water to give an alkaline solution.

Show answer

• sodium oxide is basic; carbon dioxide is acidic [1] • sodium oxide dissolves in water to give an alkaline solution (of sodium hydroxide) [1]

Q2 (2 marks). Copper(II) oxide is warmed with dilute hydrochloric acid. Write the balanced symbol equation and state one observation.

Show answer

• CuO + 2HCl → CuCl2 + H2O [1] • the black solid reacts and dissolves, forming a blue-green solution [1]

Q3 (2 marks). (Extended) Name the two neutral oxides in the 0620 syllabus and state what neutral means here.

Show answer

• carbon monoxide (CO) and water (H2O) [1] • a neutral oxide reacts with neither acids nor alkalis [1]