Doc Brown's Chemistry GCSE/IGCSE Science–Chemistry Revision Notes

The pH scale of acidity and alkalinity, acids, alkalis, salts and neutralisation

6. Methods of making salts and chemical tests for salts (for GCSE Level)

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 These revision notes on methods of making salts and chemical tests for ions (positive cations and negative anions) should prove useful for the new AQA chemistry, Edexcel chemistry & OCR chemistry GCSE (9–1, 9-5 & 5-1) science courses.

How do we make salts? What preparations are available to us?

Four basic methods for preparing salts are described on this page, with annotated diagrams.

BEFORE preparing a salt there are two important facts to know ...

(i) Is the salt is soluble or insoluble?

(ii) If using a base, is it soluble (alkali)? or insoluble?

... because these facts decide which method you use!

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

Method (b) Making a salt by from an acid with a metal or insoluble base – oxide, hydroxide or 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

A summary of chemical tests to identify ions in a salt, hence the identity of a salt

For chemical analysis analysts have developed a wide range of qualitative tests to detect specific chemicals which may be molecules or ions. Chemical tests are based on reactions that produce a gas with distinctive properties, or a colour change produced by adding a reagent or the production of an insoluble solid that appears as a precipitate.

Apart from knowing how to make salts, you need to know how to identify salts and other compounds from their constituent ions. There is no single test for a salt, you must do at least two tests to confirm the identity of the two constituent ions.  Most of the methods described below are simple precipitation tests.

Tests for METAL IONS – cations (positive ions)


Simple method for a flame test to identify metal ions: The metal salt or other compound  is mixed with concentrated hydrochloric acid and a sample of the mixture is heated strongly in a bunsen flame on the end of a cleaned nichrome wire (or platinum if you can afford it!). Before doing the test the nichrome/platinum wire should be cleaned in conc. hydrochloric acid and heated in the hottest part of the flame to make sure there is no contaminating flame colours. It doesn't matter whether the salt compound is soluble or insoluble.

the lithium ion Li+ gives a red-crimson (carmine–red) colour in the flame

the sodium ion Na+ gives a  yellow-orange colour in the flame

the potassium ion K+ gives a  lilac-purple colour in the flame

the calcium ion Ca2+ gives a  brick red colour in the flame

the copper ion Cu2+ gives a  blue–green colour in the flame


A non–chemical test method for identifying elements – atomic emission line spectroscopy
An instrumental method for METALS from LINE SPECTRA

If the atoms of an element are heated to a very high temperature in a flame they emit light of a specific set of frequencies (or wavelengths). These are all due to electronic changes in the atoms, the electrons are excited and then lose energy by emitting energy as photons of light. These emitted frequencies can be recorded on a photographic plate, or these days a digital camera.

Each emission line spectra is unique for each element and so offers a different pattern of lines i.e. a 'spectral fingerprint' by which to identify any element in the periodic table .e.g. the diagram on the left shows some of the visible emission line spectra for the elements hydrogen, helium, neon, sodium and mercury.

Note the double yellow line for sodium, hence the dominance of yellow in its flame test colour. In fact the simple flame test colour observations for certain metal ions relies entirely on the observed amalgamation of these spectral lines.

This is an example of an instrumental chemical analysis called spectroscopy and is performed using an instrument called an optical spectrometer (simple ones are called spectroscopes). This method, called flame emission spectroscopy, is a fast and reliable method of chemical analysis. This type of optical spectroscopy has enabled scientists to discover new elements in the past and today identify elements in distant stars and galaxies. The alkali metals caesium (cesium) and rubidium were discovered by observation of their line spectrum and helium identified from spectral observation of our Sun.

You can use the flame emission effect to measure the concentration of metal ions in solution.

Using a flame photometer instrument you can do quantitative analysis based on the light emitted from a solution of a metal ion.

The sample is evaporated at high temperature in a flame and the light emitted is measured with a special detector.

You can determine the precise concentration of a metal ion in dilute solution by using a calibration curve (right).

Solutions of known concentration are tested and a measure of the emitted light (flame photometer signal) can be plotted against the concentration to produce a linear calibration curve with an x,y origin of 0,0

Then, a solution of unknown concentration can be tested with the same set-up, and from the emitted light value you can obtain the unknown concentration from the calibration curve.

You can use special light filters to exclude the colour produced by other ions that may be present so improving the accuracy of a specific metal ion measurement.

Many instrumental methods of analysis are available and that these can improve sensitivity, accuracy and speed of tests.  More on instrumental methods of analysis


Quiz on identifying salts and other compounds


Some metal ions (cations) can be identified by the formation of coloured precipitates with sodium hydroxide solution

The non–metallic ion, the ammonium ion, can be detected with the same reagent, sodium hydroxide, because ammonia gas is released, especially if the mixture is gently warmed.

A few drops of sodium hydroxide solution are added to a solution of the salt under investigation to see if any precipitate (insoluble solid) is formed, and, from the observations e.g.

Metal ion detected colour of precipitate with NaOH ionic equation for the reactions
calcium, Ca2+


white precipitate Ca2+(aq) + 2OH(aq) ==> Ca(OH)2(s)
copper(II), Cu2+


blue precipitate (3 in diagram below) Cu2+(aq) + 2OH(aq) ==> Cu(OH)2(s)
iron(II), Fe2+

pale green

dark green precipitate (1 in diagram below) Fe2+(aq) + 2OH(aq) ==> Fe(OH)2(s)
iron(III), Fe3+


orange–brown precipitate (2 in diagram below) Fe3+(aq) + 3OH(aq) ==> Fe(OH)3(s)
zinc, Zn2+


white precipitate (4a in diagram below), which dissolves in excess to give a clear colourless solution  (4b in diagram below) Zn2+(aq) + 2OH(aq) ==> Zn(OH)2(s)

Zn(OH)2(s) + 2OH(aq) ==> Zn(OH)4]2–(aq)

aluminium, Al3+


white precipitate, which dissolves in excess, to give a clear colourless solution  (same as zinc ion, 4a + 4b in diagram below)

Al3+(aq) + 3OH(aq) ==> Al(OH)3(s)

Al(OH)3(s) + OH(aq) ==> [Al(OH)4](aq)

ammonium, NH4+


no precipitate formed, but ammonia gas released which you can smell, the gas turns damp red litmus paper blue NH4+(aq) + OH(aq) ==> H2O(l) + NH3(g)
************************** ************************************************************** ********************************************************

The above reactions are illustrated in the diagram below


For more on ionic equations see How to write equations and Making salts by precipitation


Quiz on identifying salts and other compounds


Tests for NON–METAL IONS – anions (negative ions)


Tests to detect and identify halide ions X,  the negative ions (anions) formed from the halogens, chloride, bromide and iodide.

To the suspected halide ion solution add a little dil. nitric acid and a few drops of silver nitrate solution.

Depending on the halide ion you get a different coloured silver halide precipitate, summarised below.

halide ion Colour of precipitate with silver nitrate Ionic equation to show precipitate formation
chloride Cl white precipitate of AgCl silver chloride (slowly darkens when exposed to light) Ag+(aq) + Cl(aq) ==> AgCl(s)
bromide Br cream precipitate of AgBr silver bromide Ag+(aq) + Br(aq) ==> AgBr(s)
Iodide I yellow precipitate of  AgI silver iodide  Ag+(aq) + I(aq) ==> AgI(s)

The silver nitrate tests for halide ions is illustrated in the diagram below.

You can only use this silver nitrate test on soluble chlorides.


Quiz on identifying salts and other compounds



test for CO2

Test for the carbonate ion CO32–

Addition of dil. hydrochloric acid to any carbonate or hydrogen carbonate results in fizzing! The effervescence is due to the evolution of carbon dioxide gas. If a sample of the evolved gas is carefully collected and bubbled into limewater a white precipitate is formed. The formation of the carbon dioxide confirms the original compound was a carbonate.  It doesn't matter whether the compound is soluble or insoluble.

carbonate + acid ==> salt + water + carbon dioxide

The ionic equation is

CO32–(s) + 2H+(aq) ==> H2O(l) + CO2(g)



Test for the sulfate ion SO42–

The suspected sulfate is dissolved in water. A little dilute hydrochloric acid added followed by a few drops of barium chloride solution. If a sulfate is present a white precipitate of barium sulfate is formed.

barium ion + sulfate ion ==> barium sulfate

Ba2+(aq) + SO42–(aq) ==> BaSO4(s)

This can only be done on soluble sulfate compounds.


Quiz on identifying salts and other compounds


Qualitative TESTS for common gases

CHEMICAL TEST FOR ... TEST METHOD OBSERVATIONS Chemical equation and comments
hydrogen gas H2 a colourless and odourless gas Apply a lit splint or spill. A squeaky pop! (might see condensation on test tube) 2H2(g) + O2(g) ==> 2H2O(l) + energy!
Chemical test for carbon dioxide gas CO2 a colourless and odourless gas Bubble the gas into limewater (aqueous calcium hydroxide solution). It turns cloudy – fine milky white precipitate of calcium carbonate. BEWARE Ca(OH)2(aq) + CO2(g) ==> CaCO3(s) + H2O(l

test for CO2

Chemical test for oxygen gas O2  a colourless and odourless gas Apply a glowing splint or spill. It re–ignites to a flame. C(in wood) + O2(g)  ==> CO2(g)

The relighted splint is mainly combustible carbon.

Chemical test for ammonia gas NH3 colourless gas with a strong pungent odour Damp red litmus.


(i) Litmus turns blue.

(ii) Gives white clouds with HCl fumes.

(i) Ammonia is the only common alkaline gas.

(ii) It forms fine ammonium chloride crystals with HCl

Chemical test for chlorine gas Cl2 pungent green gas - irritating and potentially toxic if breathed in - take care! Apply damp blue litmus. (Can use red litmus and just see bleaching effect.) (i) litmus turns red and then is bleached white. Non–metallic chlorine is acid in aqueous solution and a powerful oxidising agent which is why you get the double colour change and its the bleaching effect that distinguishes chlorine for some other acidic gases.


Quiz on identifying salts and other compounds

Advanced Chemistry Page Index and Links

Quiz on identifying salts and other compounds

ALL chemical tests for GCSE/IGCSE/A Level etc.

GCSE/IGCSE Acids & Alkalis revision notes sub–index: Index of all pH, Acids, Alkalis, Salts Notes 1. Examples of everyday acids, alkalis, salts, pH of solution, hazard warning signs : 2. pH scale, indicators, ionic theory of acids–alkali neutralisation : 4. Reactions of acids with metals/oxides/hydroxides/carbonates, neutralisation reactions : 5. Reactions of bases–alkalis like ammonia & sodium hydroxide : 6. Four methods of making salts : 7. Changes in pH in a neutralisation, choice and use of indicators : 8. Important formulae of compounds, salt solubility and water of crystallisation : 10. More on Acid–Base Theory and Weak and Strong Acids

See also Advanced Level Chemistry Students Acid–Base Revision Notes – use index

6. METHODS of MAKING SALTS – salt preparation procedures, now on four separate pages

6a. METHOD (a) Neutralising a soluble acid with a soluble base (alkali) to give a soluble salt

6b. METHOD (b) Reacting an acid with a metal or with an insoluble base to give a soluble salt

6c. Method (c) Preparing an Insoluble Salt

6d. Method (d) Making a salt by direct combination of elements


Revision notes on methods for making salts procedures for preparing salts tests for ions in salts KS4 Science GCSE/IGCSE/O level Chemistry Information on methods for making salts procedures for preparing salts tests for ions in salts for revising for AQA GCSE Science, Edexcel Science chemistry IGCSE Chemistry notes on methods for making salts procedures for preparing salts tests for ions in salts OCR 21st Century Science, OCR Gateway Science notes on methods for making salts procedures for preparing salts tests for ions in salts WJEC gcse science chemistry notes on methods for making salts procedures for preparing salts tests for ions in salts CIE O Level chemistry CIE IGCSE chemistry notes on methods for making salts procedures for preparing salts tests for ions in salts CCEA/CEA gcse science chemistry (help for courses equal to US grade 8, grade 9 grade 10) science chemistry courses revision guides explanation chemical equations for methods for making salts procedures for preparing salts tests for ions in salts educational videos on methods for making salts procedures for preparing salts tests for ions in salts guidebooks for revising methods for making salts procedures for preparing salts tests for ions in salts textbooks on methods for making salts procedures for preparing salts tests for ions in salts

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