6d. Making salts by direct combination of elements

e.g. combining a metal with a non-metallic element

[Author © Dr Phil Brown PhD: Doc Brown's chemistry exam revision notes on pH-reactions-acids-bases-neutralisation-salt preps, suitable for students of UK GCSE level and international IGCSE/O level chemistry courses, ~US grades 9-10 chemistry notes [page updated Mar 11th 2026 *]

Sub-index of salt preparations

Method (a) Making a salt by neutralising a soluble acid with a soluble base (alkali) - neutralisation reaction

Method (b) preparing a salt by reacting an acid with a metal or with an insoluble base - oxide, hydroxide, carbonate

Method (c) Preparing an insoluble salt by mixing solutions of two soluble compounds

Method (d) Making a salt by directly combining its constituent elements (this page)

Example salt preparations by direct combination of the constituent elements

Uses of compounds whose synthesis is mentioned

'redox' theory of salt synthesis by direct combination of elements

See also on this page uses of compounds mentioned

and 'redox' theory of salt synthesis by direct combination of elements

How to make a salt by direct combination of elements?

Sometimes it isn't appropriate to prepare a soluble salt by reacting an acid with an insoluble base or alkali, so it may be possible to prepare the salt by directly combining the metal and the non-metal elements.

Two such examples are the preparation of anhydrous aluminium chloride and anhydrous iron(III) chloride (anhydrous here means without any water of crystallisation).

Example of salt preparations by direct combination of the constituent elements

Preparation of aluminium chloride AlCl₃

• How can we make aluminium chloride? How do we prepare iron(III) chloride?

• METHOD (d) both preparations illustrated above and below.

• Don't forget to wear safety glasses or goggles when doing this preparation - but it is a teacher demonstration.

• These compounds can be made by direct combination of the elements to form anhydrous salts e.g. if dry chlorine gas Cl₂ is passed over heated iron or aluminium, the chloride is produced.

  • These experiment preparations (shown above) should be done very carefully by the teacher in a fume cupboard.

  • The aluminium can burn intensely with a violet flame, white fumes of aluminium chloride sublime from the hot reacted aluminium and the white solid forms on the cold surface of the flask.

  • aluminium  +  chlorine  ===>  aluminium chloride

  • 2Al_(s) + 3Cl_2(g) ===> 2AlCl_3(s)

  • The aluminium chloride is often discoloured yellow from the trace chlorides of copper or iron that may be formed from traces of these metals that might be present in the original aluminium).

See also on this page uses of compounds mentioned

and 'redox' theory of salt synthesis by direct combination of elements

Preparation of iron(III) chloride FeCl₃

• Again, you pass dry chlorine gas over the heated iron wool.

  The iron (e.g. as steel wool) glows red and brown fumes of iron(III) chloride stream off, the brown solid collects on the cold flask surface.

  • iron + chlorine ===> iron(III) chloride

  • 2Fe_(s) + 3Cl_2(g) ===> 2FeCl_3(s)

• Note (i): Both these chlorides react exothermically and hydrolyse with water to give the metal hydroxide and fumes of hydrogen chloride, and so dry conditions are needed.

• Note (ii): Both these chlorides cannot be made in an anhydrous form from aqueous solution neutralisation. This is because on evaporation the compounds contain 'water of crystallisation'. On heating the hydrated salt  hydrolyses and decomposes into water, the oxide or hydroxide and fumes of hydrogen chloride, so, basically it a mess in terms of trying to make pure AlCl₃ and FeCl₃ in this way.

See also on this page uses of compounds mentioned

and 'redox' theory of salt synthesis by direct combination of elements

Other element combinations

• Theoretically you can make many anhydrous simple salts like the metal chlorides or metal sulfides by direct combination of the constituent elements.

  • e.g. you can burn sodium with a bright yellow flame in chlorine gas to make sodium chloride ('salt').

  • sodium  +  chlorine  ===>  sodium chloride

  • 2Na(s)  +  Cl₂(g)  ===>  2NaCl(s)

  • BUT, it would be the most ridiculously expensive and impractical way of making it.

  • Its a lot cheaper just to evaporate seawater and recrystallising the salt from the hot concentrated solution, if you need a particularly pure sample of sodium chloride.

  • Another examples of direct combination of elements to give a salt is to strongly heat silvery-grey iron filings with yellow sulfur powder to make iron sulfide (a black solid).

  • 

  • The mixture glows red, an exothermic reaction to form iron sulfide.

  • iron  +  sulfur  ===>  iron sulfide

  • Fe(s)  +  S(s)  ===>  FeS(s)

See also on this page uses of compounds mentioned

and 'redox' theory of salt synthesis by direct combination of elements

A redox reaction is one in which one chemical species is oxidised by electron loss and simultaneously another chemical species is reduced by electron gain.

e.g sodium + chlorine  ==>  sodium chloride

sodium atoms are oxidised by electron loss to form sodium ions: Na  ===>  Na⁺  +  e^–

chlorine molecules are reduced by electron gain to form chloride ions: Cl₂  +  2e^–  ===>  2Cl^–

So overall the balanced reaction is: 2Na  +  Cl₂  ===>  2Na⁺  +  2Cl^–  (= 2NaCl)

No electrons show up in this 'ionic redox equation' because, overall, 2 electrons are lost and 2 electrons gained.

Its the same for the formation of FeCl₃ and AlCl₃, just a bit more tricky to balance, M = Fe or Al.

Metal oxidised by electron loss: M  ==>  M³⁺  +  3e^–

Non-metal chlorine molecules reduced by electron gain:  Cl₂  +  2e^–  ===>  2Cl^–

So overall the balanced reaction is: 2M  +  3Cl₂  ===>  2M³⁺  +  6Cl^–  (= 2MCl₃)

Example salt preparations by direct combination of the constituent elements

Uses of compounds whose synthesis is mentioned