METHODS OF MAKING SALTS & tests for ions
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
Original page now split into four
sections, NEW links below 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!
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!
A summary of chemical tests to identify ions in a salt, hence the identity of a salt
Apart from knowing how to make salts, you may need to know how to identify salts and other compounds from their constituent ions
Tests for METAL IONS - cations
Simple method for a flame test: 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.
Tests for NON-METAL IONS - anions
Tests for halide ions X-, the negative ions (anions) formed from the halogens
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.
You can only use this silver nitrate test on soluble chlorides.
Test for the carbonate ion CO32-
Addition of dil. hydrochloric acid to any carbonate results in fizzing! The effervesence 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
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.
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
6. METHODS of MAKING SALTS - salt preparation procedures, now on four separate pages
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Describing how to make a salt by four different methods, how to do it, what laboratory equipment is needed and how to crystallise or collect the salt in the end