Metal oxides are basic, non-metal oxides acidic
Basic oxide = a metal oxide, reacting with acid to give a salt and water: , , , . Acidic oxide = a non-metal oxide, reacting with a base to give a salt and water: , , ; many dissolve to give an acid, e.g. . Metallic character (left/bottom of the Periodic Table) means basic, non-metallic (right/top) acidic.
(Extended) Amphoteric oxides react with acid AND base
An amphoteric oxide reacts with an acid AND with a base, each time giving a salt and water. The two named here are aluminium oxide, , and zinc oxide, ; both elements sit near the metal/non-metal boundary. Toward acid it acts basic: . Toward alkali it acts acidic: .
Salt and water is the pattern every time
Oxide reactions all share one shape: metal oxide + acid gives salt + water, non-metal oxide + base gives salt + water, and for an amphoteric oxide either direction works. The salt name comes from the metal (or ammonium) in the oxide plus the anion named by the acid: hydrochloric acid gives a chloride, sulfuric acid a sulfate, nitric acid a nitrate. Water is always the second product, so an equation is complete only once both salt and water are shown.
Drawn from real examiner reports.
Metal = basic, non-metal = acidic
Classification is fixed by the element the oxide contains, not by how it looks. A metal oxide is basic — it neutralises an acid and, if soluble, gives a high-pH solution. A non-metal oxide is acidic — it neutralises a base and, if soluble, gives a low-pH solution. Candidates reverse this, calling gaseous basic or a coloured solid like acidic.
Jun 2023 P11 Q22; Nov 2023 P11 Q20
(Extended) Amphoteric needs BOTH reactions
Amphoteric is a two-part definition. Writing only "aluminium oxide reacts with acid to form a salt and water" describes a basic oxide; the base reaction must be given too. Calling or basic "because it contains a metal" omits the base-reacting half.
Basic oxide is not the same as alkali
A basic oxide reacts with an acid to give a salt and water, but it need not dissolve in water — is insoluble and still basic. An alkali is specifically a soluble base, one that dissolves to release ions. Every alkali is a base, but many basic oxides are not alkalis, so "it does not dissolve, so it cannot be basic" is wrong.
Keep the sulfate group as one unit
When balancing an oxide + acid equation, treat and as single units that pass through unchanged. Splitting sulfate into separate S and O atoms, or writing the product as a sulfite, a sulfide or an oxide, is a recurring error: the sulfate ion stays intact, giving .
Jun 2022 P41 Q5aiii
Dropping the coefficient on NaOH
In the 2 in front of is easily lost: sodium carbonate needs two sodium atoms, and the two hydrogens those units supply are what form the water. Count sodium and hydrogen on both sides before moving on — an unbalanced equation scores nothing even when the products are right.
(Extended) Aluminium formula slips: AlCl, AlO, Al3O2
Aluminium forms , so its chloride is and its oxide is . Writing AlCl wrecks the chlorine and hydrogen balance in ; writing AlO or Al3O2 comes from pairing the charge numbers instead of balancing against at the lowest ratio.
Inverting the ratio in a mass calculation
For a 1 : 1 reaction, product mass = (reactant mass divided by the reactant ) multiplied by the product . So 8.0 g of gives 8.0/80 x 160 = 16.0 g of . Using 80/160 instead returns 4.0 g — less than you started with, even though has the larger . Sanity-check the direction against the two values.
Balance in a fixed order
Name the salt first (the metal from the oxide plus the acid's anion), then balance the metal atoms and the anion group as whole units, then hydrogen, and check oxygen last — water absorbs any oxygen imbalance. Adjusting oxygen first forces a repeated re-balance.
Give the reason, not just the label
Classification questions want a reason as well as a label. "Basic" alone earns half; "basic, because magnesium is a metal" earns both marks. The reason must name the metal or non-metal identity of the element — "it is a gas" or "it is a coloured solid" scores nothing.
Count your points against the tariff
Match the number of distinct points to the mark tariff. A 4-mark "classify these oxides and give a reason" question wants four classifications each with its own reason, not one general paragraph. Two-part definitions need both parts written out separately.
Basic oxide: an oxide of a metal that reacts with an acid to produce a salt and water. Examples: , , .
Acidic oxide: an oxide of a non-metal that reacts with a base to produce a salt and water. Examples: , , .
Amphoteric oxide (Ext): an oxide that reacts with both an acid and a base, each time producing a salt and water. The only two amphoteric oxides required on this specification are (aluminium oxide) and (zinc oxide).
Link to the periodic table: metallic character increases toward the left/bottom of the periodic table (basic oxides); non-metallic character increases toward the right/top (acidic oxides). Aluminium and zinc sit close to the metal/non-metal boundary, which is why their oxides can react both ways.
Define an acidic oxide.
Copper(II) oxide is a basic oxide. Write a balanced symbol equation for its reaction with dilute sulfuric acid.
