Tests for ions · Grade 11
In Grade 11 you learn: tests for metal ions with sodium hydroxide; tests for carbonates, halides and sulfates.
Metal ions can be found with flame tests and by adding sodium hydroxide solution to see the color of the precipitate. Negative ions have their own tests: carbonates fizz with acid, halides give colored precipitates with silver nitrate, and sulfates give a white precipitate with barium chloride. Using one test for each ion identifies a salt.
Key facts
- Flame tests: lithium crimson, sodium yellow, potassium lilac, calcium orange-red, copper green.
- With sodium hydroxide solution: copper(II) blue, iron(II) green, iron(III) brown precipitates; aluminum, calcium and magnesium white. Only aluminum hydroxide dissolves in excess.
- Carbonates: add dilute acid; the gas (carbon dioxide) turns limewater milky.
- Halides: add dilute nitric acid then silver nitrate: chloride white, bromide cream, iodide yellow precipitate.
- Sulfates: add dilute hydrochloric acid then barium chloride: a white precipitate.
- Flame emission spectroscopy is an instrumental method: it identifies metal ions in mixtures and measures their concentrations.
Worked examples
Example 1
What color flame do lithium ions give in a flame test?
Answer: crimson
Example 2
Why is the wire for a flame test cleaned in acid first?
Answer: to remove any metal ions left from earlier tests
Example 3
A compound gives a yellow flame in a flame test. Which metal ion does it contain?
Answer: sodium ions
Example 4
Why can a flame test be hard to read for a mixture of metal ions?
Answer: one color can hide another
Example 5
What color is the precipitate when sodium hydroxide solution is added to a solution of copper(II) ions?
Answer: blue
Example 6
Sodium hydroxide solution is added to solutions P, Q and R. P gives a blue precipitate, Q a brown one and R a white one that dissolves in excess. Which metal ions are present?
Answer: P copper(II), Q iron(III), R aluminum
Example 7
A solution gives a cream precipitate with dilute nitric acid and silver nitrate solution. Which negative ion is present?
Answer: bromide ions
Example 8
A white solid gives a lilac flame. Its solution gives a cream precipitate with dilute nitric acid and silver nitrate. Name the solid.
Answer: potassium bromide
Example 9
Why is hydrochloric acid added before barium chloride when testing for sulfate ions?
Answer: to remove carbonate ions, which would also give a white precipitate
Example 10
Why is flame emission spectroscopy better than a flame test?
Answer: it can identify ions in mixtures and measure their concentration