IMAT Chemistry · Inorganic Compounds and Nomenclature

Oxides, Hydroxides, Acids, Hydrides and Salts

Inorganic compounds fall into a few families; metal oxides make bases, non-metal oxides make acids, and acids with bases make salts.

All Inorganic Compounds and Nomenclature notes

Why this matters

The most asked page of the chapter. The papers asked to sort oxides into acidic, basic and amphoteric (2014, 2015), to pick the oxide that gives a hydroxide with water (2024), to recognise a gaseous acidic oxide (2023), and to name the class of an acid (2026).

Concept 1 of 4: Basic, acidic, amphoteric and neutral oxides

Metals hold their electrons loosely, so in an oxide the oxygen is present as the oxide ion, which grabs

\mathrm{H^+}

: a base. Non-metals share electrons with oxygen covalently, and the oxide pulls on water's oxygen to release

\mathrm{H^+}

: an acid. Elements on the border between metals and non-metals do a bit of both.

Definition

Basic oxides: oxides of metals, especially Groups 1 and 2. They react with acids to form a salt and water; the soluble ones dissolve in water to give hydroxides (alkalis).
Acidic oxides: oxides of non-metals (and of metals in high oxidation states, such as $\mathrm{Mn_2O_7}$ ). They react with bases; the soluble ones dissolve in water to give oxoacids. In traditional naming they are anhydrides: $\mathrm{CO_2}$ is carbonic anhydride, $\mathrm{SO_3}$ sulfuric anhydride.
Amphoteric oxides react with both acids and bases: $\mathrm{Al_2O_3}$ , ZnO, PbO, BeO.
Neutral oxides react with neither: CO, NO, $\mathrm{N_2O}$ .
Across period 3 the oxides go from basic ( $\mathrm{Na_2O}$ , MgO) to amphoteric ( $\mathrm{Al_2O_3}$ ) to acidic ( $\mathrm{SiO_2}$ , $\mathrm{P_4O_{10}}$ , $\mathrm{SO_3}$ , $\mathrm{Cl_2O_7}$ ).

Type	Formed by	Behaviour	Examples
Basic	Metals, especially Groups 1 and 2	Neutralises acids; with water gives a hydroxide	$\mathrm{Na_2O}$ , $\mathrm{K_2O}$ , CaO, MgO, CuO
Acidic	Non-metals	Neutralises bases; with water gives an oxoacid	$\mathrm{CO_2}$ , $\mathrm{SO_2}$ , $\mathrm{SO_3}$ , $\mathrm{NO_2}$ , $\mathrm{P_4O_{10}}$ , $\mathrm{SiO_2}$
Amphoteric	Metals near the metal and non-metal border	Reacts with both acids and bases	$\mathrm{Al_2O_3}$ , ZnO, PbO
Neutral	A few non-metals	Reacts with neither acids nor bases	CO, NO, $\mathrm{N_2O}$

Silicon dioxide is acidic although it does not dissolve in water: it reacts with hot concentrated alkali.

Practice this conceptself-check · 3 quick reps

Not every non-metal oxide is acidic

CO, NO and

\mathrm{N_2O}

are neutral: they do not form acids in water. Most other non-metal oxides are acidic, including

\mathrm{NO_2}

, which reacts with water to give nitric acid.

Concept 2 of 4: Oxides with water: hydroxides and oxoacids

Adding water to an oxide just adds the atoms of water to it. A metal oxide becomes a metal hydroxide. A non-metal oxide becomes an acid in which the non-metal keeps exactly the same oxidation number, which is the quickest way to find which acid you get.

Definition

Basic oxide + water gives a hydroxide: $\mathrm{Na_2O + H_2O \rightarrow 2NaOH}$ , $\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}$ .
Acidic oxide + water gives an oxoacid: $\mathrm{SO_3 + H_2O \rightarrow H_2SO_4}$ , $\mathrm{CO_2 + H_2O \rightarrow H_2CO_3}$ , $\mathrm{N_2O_5 + H_2O \rightarrow 2HNO_3}$ .
The non-metal's oxidation number does not change: S is $+4$ in $\mathrm{SO_2}$ and in $\mathrm{H_2SO_3}$ ; $+6$ in $\mathrm{SO_3}$ and in $\mathrm{H_2SO_4}$ .
Method: write oxide + $\mathrm{H_2O}$ , add up the atoms, and if the non-metal appears twice, split the result into two acid molecules.

Oxides and water

\text{metal oxide} + \mathrm{H_2O} \rightarrow \text{hydroxide} \qquad \text{non-metal oxide} + \mathrm{H_2O} \rightarrow \text{oxoacid}

Worked example

Write the products when (a) potassium oxide and (b) dichlorine heptoxide,

\mathrm{Cl_2O_7}

, react with water.

Practice this conceptself-check · 4 quick reps

The acid keeps the oxidation number of its oxide

\mathrm{SO_2}

gives sulfurous acid,

\mathrm{H_2SO_3}

+4

), not sulfuric acid. Only

\mathrm{SO_3}

gives

\mathrm{H_2SO_4}

+6

). Match the oxidation numbers before choosing the acid.

Concept 3 of 4: Binary acids, oxoacids, hydroxides and hydrides

Look at which elements a formula contains and the family is usually obvious. Hydrogen plus one non-metal is a binary acid; add oxygen and it is an oxoacid; a metal with hydroxide groups is a base; and hydrogen joined to a single other element is a hydride.

Definition

Binary acids (hydracids): hydrogen and one non-metal, acidic in water: HF, HCl, HBr, HI (the hydrohalic acids) and $\mathrm{H_2S}$ . Named hydro...ic acid: hydrochloric acid.
Oxoacids (oxyacids): hydrogen, a non-metal and oxygen; the acidic H atoms are joined to O. HClO, hypochlorous acid, is an oxoacid even though it has only one O.
Hydroxides: a metal ion with $\mathrm{OH^-}$ ions: NaOH, $\mathrm{Ca(OH)_2}$ . Most are bases; $\mathrm{Al(OH)_3}$ and $\mathrm{Zn(OH)_2}$ are amphoteric.
Hydrides: hydrogen with one other element. Metal hydrides (NaH, $\mathrm{CaH_2}$ ) are ionic, contain $\mathrm{H^-}$ , and react with water to give hydrogen gas and a hydroxide. Non-metal hydrides ( $\mathrm{CH_4}$ , $\mathrm{NH_3}$ , $\mathrm{H_2O}$ ) are covalent molecules.
An acid with two or three acidic H atoms is diprotic ( $\mathrm{H_2SO_4}$ ) or triprotic ( $\mathrm{H_3PO_4}$ ).

Class	Made of	Examples
Binary acid (hydracid)	H + one non-metal	HCl, HBr, HF, $\mathrm{H_2S}$
Oxoacid	H + non-metal + O	$\mathrm{H_2SO_4}$ , $\mathrm{HNO_3}$ , HClO, $\mathrm{H_3PO_4}$ , $\mathrm{H_2CO_3}$
Hydroxide	Metal ion + $\mathrm{OH^-}$	NaOH, $\mathrm{Ca(OH)_2}$ , $\mathrm{Al(OH)_3}$
Metal hydride	Metal + H (H is $-1$ )	NaH, $\mathrm{CaH_2}$ , LiH
Non-metal hydride	Non-metal + H (H is $+1$ )	$\mathrm{CH_4}$ , $\mathrm{NH_3}$ , $\mathrm{H_2O}$

Practice this conceptself-check · 3 quick reps

An acid is not an oxide, and a hydroxide is not an acid

HClO contains H, Cl and O, so it is an oxoacid, not an acidic oxide (which has no H at all, like

\mathrm{Cl_2O}

). A metal hydroxide such as NaOH is a base. Classify by the elements present before thinking about behaviour.

Concept 4 of 4: Salts: normal, acid, basic and hydrated

A salt is what is left when the acidic hydrogen of an acid has been replaced by a metal ion or ammonium. If every acidic hydrogen is replaced you get a normal salt; if some are left over you get an acid salt, which can still give up

\mathrm{H^+}

Definition

A salt is an ionic compound made of a cation (metal or ammonium) and the anion of an acid. Its name is cation then anion: potassium nitrate, ammonium sulfate.

Ways to make a salt: acid + base $\rightarrow$ salt + water; acid + metal $\rightarrow$ salt + hydrogen; acid + carbonate $\rightarrow$ salt + water + $\mathrm{CO_2}$ ; basic oxide + acidic oxide $\rightarrow$ salt ( $\mathrm{CaO + CO_2 \rightarrow CaCO_3}$ ).
Acid salts come from acids with two or three acidic H atoms when only some are replaced: $\mathrm{NaHCO_3}$ , $\mathrm{NaHSO_4}$ , $\mathrm{KH_2PO_4}$ , $\mathrm{Na_2HPO_4}$ . Their names say how many H remain: sodium dihydrogen phosphate.
"Acid salt" describes the formula, not the solution: $\mathrm{NaHSO_4}$ is acidic in water but $\mathrm{NaHCO_3}$ is slightly basic.

Type	Contains	Examples
Normal salt	No replaceable H left	NaCl, $\mathrm{K_2SO_4}$ , $\mathrm{CaCO_3}$ , $\mathrm{CH_3COONa}$
Acid salt	Anion still carries acidic H	$\mathrm{NaHCO_3}$ , $\mathrm{NaHSO_4}$ , $\mathrm{KH_2PO_4}$
Basic salt	$\mathrm{OH^-}$ as well as another anion	Mg(OH)Cl, $\mathrm{Cu_2(OH)_2CO_3}$
Hydrated salt	Water of crystallisation	$\mathrm{CuSO_4 \cdot 5H_2O}$

Practice this conceptself-check · 3 quick reps

Hydrogen in a formula does not make an acid salt

Only acidic H counts. Sodium ethanoate,

\mathrm{CH_3COONa}

, and ammonium chloride contain H atoms, but none of them is a leftover acidic H of the parent acid, so they are normal salts.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Oxides with water: hydroxides and oxoacids
Oxides and water
$\text{metal oxide} + \mathrm{H_2O} \rightarrow \text{hydroxide} \qquad \text{non-metal oxide} + \mathrm{H_2O} \rightarrow \text{oxoacid}$

Reference tables (3)

Basic, acidic, amphoteric and neutral oxides4 rows

Type	Formed by	Behaviour	Examples
Basic	Metals, especially Groups 1 and 2	Neutralises acids; with water gives a hydroxide	$\mathrm{Na_2O}$ , $\mathrm{K_2O}$ , CaO, MgO, CuO
Acidic	Non-metals	Neutralises bases; with water gives an oxoacid	$\mathrm{CO_2}$ , $\mathrm{SO_2}$ , $\mathrm{SO_3}$ , $\mathrm{NO_2}$ , $\mathrm{P_4O_{10}}$ , $\mathrm{SiO_2}$
Amphoteric	Metals near the metal and non-metal border	Reacts with both acids and bases	$\mathrm{Al_2O_3}$ , ZnO, PbO
Neutral	A few non-metals	Reacts with neither acids nor bases	CO, NO, $\mathrm{N_2O}$

Silicon dioxide is acidic although it does not dissolve in water: it reacts with hot concentrated alkali.

Binary acids, oxoacids, hydroxides and hydrides5 rows

Class	Made of	Examples
Binary acid (hydracid)	H + one non-metal	HCl, HBr, HF, $\mathrm{H_2S}$
Oxoacid	H + non-metal + O	$\mathrm{H_2SO_4}$ , $\mathrm{HNO_3}$ , HClO, $\mathrm{H_3PO_4}$ , $\mathrm{H_2CO_3}$
Hydroxide	Metal ion + $\mathrm{OH^-}$	NaOH, $\mathrm{Ca(OH)_2}$ , $\mathrm{Al(OH)_3}$
Metal hydride	Metal + H (H is $-1$ )	NaH, $\mathrm{CaH_2}$ , LiH
Non-metal hydride	Non-metal + H (H is $+1$ )	$\mathrm{CH_4}$ , $\mathrm{NH_3}$ , $\mathrm{H_2O}$

Salts: normal, acid, basic and hydrated4 rows

Type	Contains	Examples
Normal salt	No replaceable H left	NaCl, $\mathrm{K_2SO_4}$ , $\mathrm{CaCO_3}$ , $\mathrm{CH_3COONa}$
Acid salt	Anion still carries acidic H	$\mathrm{NaHCO_3}$ , $\mathrm{NaHSO_4}$ , $\mathrm{KH_2PO_4}$
Basic salt	$\mathrm{OH^-}$ as well as another anion	Mg(OH)Cl, $\mathrm{Cu_2(OH)_2CO_3}$
Hydrated salt	Water of crystallisation	$\mathrm{CuSO_4 \cdot 5H_2O}$

Watch out for (4)

Not every non-metal oxide is acidic
→ Basic, acidic, amphoteric and neutral oxides
The acid keeps the oxidation number of its oxide
→ Oxides with water: hydroxides and oxoacids
An acid is not an oxide, and a hydroxide is not an acid
→ Binary acids, oxoacids, hydroxides and hydrides
Hydrogen in a formula does not make an acid salt
→ Salts: normal, acid, basic and hydrated

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