Chemical tests for ions
Lajoy Tucker
Teacher
Introduction and Definition
Chemical tests allow chemists to identify ions in unknown samples.
Tests can be used to identify the positive metal ions (cations) and negative ions (anions) that make up ionic compounds.
Test for Metal Ions (Cations) Using Sodium Hydroxide Solution
Sodium hydroxide solution (NaOH) is added to metal-ion solutions to form insoluble metal hydroxide precipitates.
Metal Ions Forming White Precipitates
Ion | Observation with limited NaOH | Identity of precipitate | Observation with Excess NaOH |
Al³⁺ | White precipitate | Aluminium hydroxide | Precipitate dissolves |
Ca²⁺ | White precipitate | Calcium hydroxide | Precipitate does not dissolve |
Mg²⁺ | White precipitate | Magnesium hydroxide | Precipitate does not dissolve |
All three ions above form white metal hydroxide precipitates with sodium hydroxide.
The precipitate will dissolve when excess sodium hydroxide is formed.
Metal Ions Forming Coloured Precipitates
Transition metal ions form coloured precipitates.
Ion | Observation with limited NaOH | Identity of precipitate | Observation with Excess NaOH |
Cu²⁺ | Blue precipitate | Copper (II) hydroxide | Precipitate does not dissolve |
Fe²⁺ | Green precipitate | Iron (II) hydroxide | Precipitate does not dissolve |
Fe³⁺ | Brown precipitate | Iron (III) hydroxide | Precipitate does not dissolve |
Ionic equations
Metal hydroxide formation can be shown using ionic equations. The number of hydroxide ions required for balancing is equal to the charge of the metal ion. The state symbols show that a solid precipitate (s) is formed from two aqueous solutions (aq)
Worked example
A white precipitate dissolves in excess NaOH. Which ion is present?
Test for negative ions (anions)
Test for Carbonates (CO₃²⁻)
Carbonates react with dilute acids to produce carbon dioxide gas.
Test Method
Add dilute acid.
Collect gas and bubble through limewater.
Positive Result
Limewater turns milky/cloudy.
Example Equation
Test for Halide Ions (Cl⁻, Br⁻, I⁻)
Halides react with silver nitrate solution in the presence of dilute nitric acid. The colour of the precipitate formed identifies which ion was present in the sample.
Example equation
Ion | Precipitate Colour |
Cl⁻ | White |
Br⁻ | Cream |
I⁻ | Yellow |
Test for Sulfate Ions (SO₄²⁻)
Sulfate ions form a white precipitate with barium chloride solution in the presence of dilute hydrochloric acid.
Equation
Practice Questions
Question 1:
Barium chloride solution with hydrochloric acid is added to a solution containing sulfate ions. State the colour of the precipitate formed.
Answer:
White
Question 2
A solution of an unknown ionic compound is analysed. Use the information below to identify ions present.
Test carried out | Observation |
Dilute hydrochloric acid added | No visible change |
Dilute nitric acid and silver nitrate solution added | Cream precipitate formed |
Sodium hydroxide solution added | Greeb precipitate formed |
Answer:
Positive ion: (green precipitate with sodium hydroxide.
Negative ion: (cream precipitate with silver nitrate).