Testing for Ions
Introduction


Sometimes we need to find out which compounds are dissolved in water. For example, an environmental scientist might analyse water for pollutants. Nowadays a lot of this work is done by machines, but laboratory analysis is still carried out by scientists. In this unit we will learn how to test for a variety of ions.

Testing for negatively charged ions
Halide ions (Cl, Br, I)

Most of the tests for negative ions, called
anions
Anions are negatively charged ions (so called because they are attracted to the anode – the positive electrode – during electrolysis).
anions
, rely on precipitation reactions. Examples include testing for halide ions (e.g. chloride, bromide, iodide), which are formed by the halogen elements (
Group
Groups are the vertical columns in the Periodic Table, consisting of elements with similar properties (chemical 'families').
Group
7 in the
Periodic Table
The Periodic Table is a table listing the elements in order of atomic number, arranged so that similar elements appear in columns.
Periodic Table
). Look at the experiments below:

Figure 1.   Testing for halide ions.
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Match the halide
ion
An ion is a charged particle made when an atom (or group of atoms) loses or gains one or more electrons, e.g. Na+ and Cl.
ion
to the colour of its
precipitate
A precipitate is an insoluble solid formed when two solutions react together.
precipitate
formed with silver nitrate solution.
  • Chloride
    Bromide
    Iodide
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Here is the equation for the reaction of sodium chloride with silver nitrate:


NaCl(aq)  +  AgNO3(aq)    NaNO3(aq)  +  AgCl(s)

We can show this by an
ionic equation
An ionic equation is one which shows the atoms and ions that actually change in a reaction, e.g.
Zn(s)  +  Cu2+(aq)    Zn2+(aq)  +  Cu(s).
ionic equation
:

Ag+(aq)  +  Cl(aq)    AgCl(s)

If we show the halide ion as X (to stand for any halide ion), the general ionic equation is:

Ag+(aq)  +  X(aq)    AgX(s)

What
compound
A compound is a substance made up of more than one type of atom.
compound
is precipitated in the test for an iodide ion?
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Complete the following equation.

  • Ag+(aq)  +  Br(aq)    (s)
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The differences between the colours of the precipitates of silver chloride and silver bromide are difficult to see unless you have both together to compare. To help distinguish them, we can add dilute ammonia solution. Silver chloride dissolves in the ammonia solution, but silver bromide is insoluble. (Silver bromide will dissolve in concentrated ammonia solution, but silver iodide is insoluble even in concentrated ammonia solution.)

Sulfate ion (SO42−)

Look at the test for the sulfate ion in Fig.2 below:

Figure 2.   Testing for the sulfate ion.
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If the unknown compound tested is sodium sulfate, the equation for the reaction is:


Na2SO4(aq)  +  BaCl2(aq)    2 NaCl(aq)  +  BaSO4(s)

The ionic equation is:

Ba2+(aq)  +  SO42−(aq)    BaSO4(s)

What colour is the barium sulfate precipitate?
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Nitrate ion (NO3)

Look at one way of testing for the nitrate ion shown in Fig.3 below:

Figure 3.   Testing for the nitrate ion.
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In this test, the nitrate ion is reduced by the aluminium, and ammonia gas is given off.

What is the test for ammonia gas?
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Carbonate ion (CO32−)

We can test for the carbonate ion by adding dilute
acid
An acid is a substance that forms a solution with a pH value of less than 7. Acidic solutions contain an excess of hydrogen ions, H+(aq).
acid
. If the gas given off turns limewater cloudy, then the test is positive. For example, if we add dilute hydrochloric acid to sodium carbonate, sodium chloride is formed and carbon dioxide gas – which turns limewater cloudy – is given off. The equation for this reaction is:

2 HCl(aq)  +  Na2CO3(s)    2 NaCl(aq)  +  H2O(l)  +  CO2(g)

The ionic equation is:

2 H+(aq)  +  CO32−(aq)    H2O(l)  +  CO2(g)

Testing for positively charged ions
We can also use precipitation reactions to identify positively charged ions, called
cations
Cations are positively charged ions (so called because they are attracted to the cathode – the negative electrode – during electrolysis).
cations
. Look at the experiments to test for cations shown in Fig.4 below:

Figure 4.   Testing for copper(II), iron(II), and iron(III) ions.
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The ionic equation for copper(II) ions reacting with an
alkali
An alkali is a substance that dissolves in water to form a solution with a pH value greater than 7. Alkaline solutions contain an excess of hydroxide ions, OH-(aq).
alkali
is:

Cu2+(aq)  +  2 OH(aq)    Cu(OH)2(s)

For iron(III) ions we get:

Fe3+(aq)  +  3 OH(aq)    Fe(OH)3(s)

Select the option below that completes the ionic equation for the test for iron(II) ions:

Fe2+(aq)  +  2 OH(aq)    .....
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Match the metal ion to the colour of its precipitate formed with sodium hydroxide solution.
  • Copper(II) ions
    Iron(II) ions
    Iron(III) ions
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Aluminium, magnesium, and calcium ions also form precipitates with sodium hydroxide solution. The only problem is that all their hydroxide precipitates are white. However, we can still use this test to establish that a compound contains some or all of these ions, then distinguish between the three ions by carrying out further tests.

Aluminium hydroxide dissolves in excess sodium hydroxide solution, while magnesium hydroxide and calcium hydroxide do not. That leaves magnesium ions and calcium ions. Here we can use a flame test to tell the difference. Look at the test below:

Figure 5.   Distinguishing between calcium and magnesium ions.
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How can we distinguish between magnesium and calcium in their compounds?
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Here are some other flame tests you can use to identify other metal ions:

Figure 6.   Lithium flame test.
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Figure 7.   Sodium flame test.
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Figure 8.   Potassium flame test.
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Figure 9.   Barium flame test.
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Match the metal ion to its flame colour.
  • Lithium
    Sodium
    Potassium
    Barium
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All the positively charged ions we have looked at so far have been metal ions. But we can also use sodium hydroxide to test for ammonium ions (NH4+). The solution being tested is heated with sodium hydroxide solution. If ammonium ions are present, ammonia gas is given off. For example:

NH4Cl(aq)  +  NaOH(aq)    NaCl(aq)  +  H2O(l)  +  NH3(g)

You test the ammonia with damp red litmus paper, which turns blue in the gas.

Summary


There is a variety of simple chemical tests we can use to find out what ions are present in a solution.

We can test for common anions (negative ions) as shown in the table below.


Anion Test
chloride Dissolve in dilute nitric acid, then a white precipitate forms with silver nitrate solution. (The precipitate dissolves in dilute ammonia solution.)
bromide Dissolve in dilute nitric acid, then a cream precipitate forms with silver nitrate solution. (The precipitate is insoluble in dilute ammonia solution, but will dissolve in concentrated ammonia solution)
iodide Dissolve in dilute nitric acid, then a pale yellow precipitate forms with silver nitrate solution. (The precipitate is insoluble in dilute and concentrated ammonia solution)
sulfate Dissolve in dilute nitric acid, then a white precipitate forms with barium chloride solution.
nitrate Add sodium hydroxide solution and warm with powdered aluminium, then test the gas given off (ammonia) with damp red litmus paper, which turns blue.
carbonate Add dilute acid, then pass the carbon dioxide gas through limewater, which turns milky (cloudy).



We can test for positive ions by adding sodium hydroxide solution and noting the colour of the precipitate, as shown in the table below.


Cation Result of adding sodium hydroxide solution
copper(II) pale blue precipitate
iron(II) dirty green precipitate
iron(III) rusty brown precipitate
aluminium white precipitate which dissolves in excess sodium hydroxide
magnesium white precipitate
calcium white precipitate



Ammonium ions give off ammonia gas when we heat them with sodium hydroxide solution.


Some metal ions also give out coloured light when we heat them in a Bunsen flame. Their characteristic colours are shown in the table below.


Cation Colour of flame test
lithium red (scarlet)
sodium bright yellow
potassium lilac
barium apple green
calcium brick red

Exercises
1. Match the following test results to the correct ions (as represented by the letters shown below).

a) Br     
    f) NH4+
b) SO42−    
    g) K+
c) Cu2+    
    h) Fe3+
d) Fe2+    
    i) CO32−
e) Al3+    
    j) Na+

  • A dirty green precipitate is formed with sodium hydroxide solution.
    Dissolve in nitric acid, then add silver nitrate solution which gives a cream precipitate (that is insoluble in dilute ammonia solution).
    Add dilute acid; the gas given off turns limewater milky.
    Bright yellow flame test.
    Lilac flame test.
    Dissolve in nitric acid, then add barium chloride solution which gives a white precipitate.
    Add sodium hydroxide solution and heat, then test the gas given off with red litmus paper, which turns blue.
    Add sodium hydroxide solution which gives a rusty brown precipitate.
    Add sodium hydroxide solution which gives a white precipitate that dissolves in excess sodium hydroxide.
    Add sodium hydroxide solution which gives a pale blue precipitate.
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