Classes of inorganic compounds
Most simple inorganic compounds belong to a few families. An oxide has oxygen and one other element (CaO, CO₂). A hydride has hydrogen and one other element (NaH, HCl, NH₃). A hydroxide has the OH⁻ group (NaOH, Ca(OH)₂). An acid gives H⁺ in water (HCl, H₂SO₄). A salt is made when the H of an acid is replaced by a metal (NaCl, CuSO₄).
Two special salts: an acid salt still has some replaceable H (NaHSO₄, NaHCO₃). A hydrate has water locked inside the crystal (CuSO₄·5H₂O, blue; when heated it turns white).
Naming compounds and writing formulas
Name the metal (or the less electronegative element) first, then the other element with the ending -ide: sodium oxide, magnesium chloride, hydrogen sulfide. If an element can have more than one charge, put the charge in Roman numerals: iron(II) oxide FeO, iron(III) oxide Fe₂O₃. For non-metal pairs use number words: carbon monoxide CO, carbon dioxide CO₂, phosphorus pentoxide P₄O₁₀.
To write a formula from a name: write the charges, then cross them so the total charge is zero. Al³⁺ and O²⁻ give Al₂O₃ (2 × 3 = 3 × 2). To get the name from a formula, do it the other way round.
How oxides are made
Most elements react with oxygen when heated: 2Mg + O₂ → 2MgO (bright white flame); 4Na + O₂ → 2Na₂O; S + O₂ → SO₂; C + O₂ → CO₂ (plenty of air) or 2C + O₂ → 2CO (little air). Some oxides form by heating a hydroxide, carbonate or nitrate, for example CaCO₃ → CaO + CO₂. Noble gases and gold do not make oxides this way, and nitrogen only reacts at very high temperature (lightning, engines).
Across a period from sodium to chlorine, the oxides change from basic (Na₂O, MgO) through amphoteric (Al₂O₃) to acidic (SiO₂, P₄O₁₀, SO₃, Cl₂O₇). More in the lesson on Period 3 oxides.
Four kinds of oxides
Basic oxides are oxides of metals (Na₂O, MgO, CaO, CuO). They react with acids to give salt and water, and the soluble ones react with water to give an alkali: CaO + H₂O → Ca(OH)₂. They do not react with bases.
Acidic oxides are oxides of non-metals (CO₂, SO₂, SO₃, P₄O₁₀, SiO₂). They react with bases to give salt and water, and most react with water to give an acid: CO₂ + H₂O ⇌ H₂CO₃. They do not react with acids. (SiO₂ is acidic but does not dissolve in water.)
Amphoteric oxides (Al₂O₃, ZnO, PbO) react with both acids and bases: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O and Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄].
Neutral oxides (CO, N₂O, NO, H₂O) react with neither acid nor base.
Hydrides: acidic, basic and neutral
Hydrides follow a similar story. Hydrides of very reactive metals are basic: NaH + H₂O → NaOH + H₂. Hydrogen halides and H₂S are acidic: HCl in water gives hydrochloric acid. Ammonia is a basic hydride: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. Methane (CH₄) and water are neutral. The pattern: the more electronegative the other element, the more acidic the hydride in water (HF, HCl, H₂S are acids; CH₄ is not).
Try it: the three-beaker test
In the 3D, step through the oxides. For each one note: (1) colour of the water, (2) does it dissolve in acid, (3) does it dissolve in base. Then predict first for ZnO, SO₂ and MgO, and check your guess in free play. At home: blow through a straw into water with a few drops of red-cabbage juice. The colour turns pinkish because CO₂ (an acidic oxide) made carbonic acid.
Key formulas and definitions
- Metal + O₂ → metal oxide (usually basic)
- Non-metal + O₂ → non-metal oxide (usually acidic)
- Basic oxide + acid → salt + water
- Acidic oxide + base → salt + water
- Amphoteric oxide + acid → salt + water; + base → salt + water
- CaO + H₂O → Ca(OH)₂; CO₂ + H₂O ⇌ H₂CO₃
Worked examples
1. Write the formula of aluminium oxide.
Al is 3+ and O is 2−. Cross the charges: Al₂O₃ (total 6+ and 6−).
2. Name Fe₂O₃ and FeO.
Fe₂O₃: iron(III) oxide, because 2 Fe must balance 3 O²⁻ = 6−, so Fe is 3+. FeO: iron(II) oxide.
3. Classify: Na₂O, SO₃, ZnO, CO.
Na₂O basic (metal). SO₃ acidic (non-metal). ZnO amphoteric. CO neutral.
4. Write the equation for CO₂ passing into lime water, Ca(OH)₂.
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O. The white solid (milky water) is calcium carbonate. An acidic oxide reacts with a base.
5. Write the equations for ZnO with HCl and with NaOH solution.
ZnO + 2HCl → ZnCl₂ + H₂O. ZnO + 2NaOH + H₂O → Na₂[Zn(OH)₄]. It reacts with both, so it is amphoteric.
6. An unknown oxide dissolves in water and turns red litmus blue. What kind of oxide is it, and what does it contain?
It forms an alkali, so it is a basic oxide of an active metal (like Na₂O or CaO).
Common mistakes
- Saying every oxide is acidic or basic. CO, N₂O and water are neutral, and Al₂O₃ and ZnO are amphoteric.
- Thinking an insoluble oxide cannot be acidic or basic. SiO₂ is acidic even though it does not dissolve in water; it reacts with hot concentrated base.
- Using Roman numerals for non-metal oxides. Write carbon dioxide, not carbon(IV) oxide, in everyday naming.
- Mixing up the pair: basic oxides react with acids, acidic oxides react with bases. A basic oxide does not react with a base.