Lesson 7 of 12Solutions & Solubility

Unit 4 · Lesson 7 of 12

Qualitative Analysis: Identifying Unknown Ions

Using selective precipitation and flame tests to identify which ions are present in an unknown solution.

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Key things to remember

1Selective precipitation: choose a reagent that precipitates only some possible ions, narrowing down the unknown.
2Flame test colours: Li+ = crimson red, Na+ = bright yellow, K+ = lilac, Ca2+ = orange-red, Sr2+ = red, Ba2+ = pale green, Cu2+ = blue-green.
3Flame colour arises from electrons absorbing heat energy then releasing it as light of specific wavelengths on falling back down.
4A single test is rarely conclusive — combine flame tests, selective precipitation, and solubility rules for confident identification.
5In a separation scheme, add the MOST selective reagent first (e.g. Cl- before SO4 2- before OH-) and filter after each step.
6Anion tests: Cl- + Ag+ → white AgCl; SO4 2- + Ba2+ → white BaSO4; CO3 2- + acid → CO2 bubbles that turn limewater milky. Cu2+ solutions are blue.

Suppose a lab gives you a colourless solution and asks: which metal ion is dissolved in it? You can't just look at it — most dissolved ionic compounds are colourless or pale in solution. Qualitative analysis is the set of chemical techniques used to identify which ions are present (not how much — that's quantitative analysis), mainly by watching for a characteristic reaction: a precipitate forming, or a flame changing colour. Think of it as detective work. Each test gives a clue that rules some suspects in and others out, and a good analyst plans the tests so that every result — positive or negative — narrows the list. The same logic is used every day in water-testing labs checking drinking water for lead, in mining labs identifying metals in ore, and in hospital labs testing blood and urine.

First clue: the colour of the solution

Before adding anything, look at the solution. Ions of Group 1 and Group 2 metals, and common anions such as chloride, nitrate and sulfate, are colourless in water. Ions of many transition metals, and a few polyatomic ions containing them, absorb some colours of visible light and so appear coloured. A blue solution strongly suggests copper(II); a colourless one rules out every coloured ion in the table below at once. Colour alone rarely identifies an ion for certain (several ions are green), but it is free, fast and never destroys the sample.

Ion in water	Colour
Cu2+ (copper(II))	Blue
Fe2+ (iron(II))	Pale green
Fe3+ (iron(III))	Yellow-brown to orange
Ni2+ (nickel(II))	Green
Co2+ (cobalt(II))	Pink
MnO4- (permanganate)	Purple
CrO4 2- (chromate)	Yellow
Cr2O7 2- (dichromate)	Orange
Na+, K+, Ca2+, Mg2+, Cl-, NO3-, SO4 2-	Colourless

Common coloured ions in aqueous solution; most main-group ions are colourless.

Selective precipitation

The idea behind selective precipitation is to add a reagent that will only precipitate certain ions, while leaving others dissolved — narrowing down which ions could possibly be present. For example, adding a chloride solution ( $Cl^{-}$ ) to an unknown will only produce a precipitate if $Ag^{+}$ , $Pb^{2 +}$ , or $Hg_{2}^{2 +}$ is present (per the solubility rules), since those are the only cations that form insoluble chlorides. If no precipitate forms, you've ruled those three ions out. By testing with a sequence of different reagents (adding one, filtering off any precipitate, then testing what remains with the next reagent), an unknown mixture of ions can be separated and identified step by step. The ORDER of reagents is the key design decision: start with the reagent that precipitates the FEWEST ions, so that each precipitate contains only one kind of cation. Adding a very unselective reagent such as hydroxide first would make several cations precipitate together, and you could not tell which were there.

Step	Add to the solution	Precipitate if present	Why this order works
1	NaCl(aq), then filter	AgCl (white)	Of Ag+, Ba2+ and Cu2+, only Ag+ forms an insoluble chloride
2	Na2SO4(aq) to the filtrate, then filter	BaSO4 (white)	Ag+ is already gone; CuSO4 is soluble, so only Ba2+ precipitates
3	NaOH(aq) to the filtrate	Cu(OH)2 (pale blue)	Only Cu2+ is left; Ba(OH)2 would have been soluble anyway

A separation scheme for a mixture that may contain Ag+, Ba2+ and Cu2+. The sodium ions added with each reagent are spectators.

A negative result is information too

If no precipitate forms at a step, that ion is absent (at detectable levels) — record it and carry on. But a negative test only rules out the ions that reagent is known to precipitate. No precipitation test can detect

Na^{+}

K^{+}

NH_{4}^{+}

, because all of their common compounds are soluble; those ions need a flame test or another method.

Tests for common anions

Chloride ( $Cl^{-}$ ): add acidified silver nitrate; a white precipitate of AgCl forms. Used to check road-salt contamination in wells and streams.
Sulfate ( $SO_{4}^{2 -}$ ): add barium chloride (after acidifying); a white precipitate of $BaSO_{4}$ forms.
Carbonate ( $CO_{3}^{2 -}$ ): add dilute acid; bubbles of $CO_{2}$ gas fizz out, and the gas turns limewater milky.
Hydroxide ( $OH^{-}$ ): the solution turns red litmus blue and gives a high pH reading.
The acid added in the chloride and sulfate tests destroys any carbonate first, so it cannot give a misleading white precipitate of silver or barium carbonate.

Flame tests

Many metal ions give off a distinct colour of light when heated strongly in a flame — this happens because the heat excites electrons to higher energy levels, and as they fall back down they emit light at specific wavelengths characteristic of that element. Dipping a clean wire loop in a sample and holding it in a flame is one of the fastest ways to screen for certain metal ions.

Metal ion	Flame colour
Li+ (lithium)	Crimson red
Na+ (sodium)	Bright yellow-orange
K+ (potassium)	Lilac / pale violet
Ca2+ (calcium)	Orange-red (brick red)
Sr2+ (strontium)	Red
Ba2+ (barium)	Pale green / yellow-green
Cu2+ (copper)	Blue-green

Fig 1Characteristic flame colours produced by common metal ions.

Combining techniques

A flame test alone can be ambiguous — sodium's bright yellow can mask other, dimmer colours, and several ions have similar shades (Ca2+ vs Sr2+, both reddish). In practice, chemists combine flame tests with selective precipitation and other tests to confirm an identification with more confidence. Viewing the flame through blue cobalt glass filters out sodium's yellow, which makes potassium's faint lilac visible. Choose the tool that fits the question: a flame test for Group 1 and Group 2 metal ions, precipitation for cations and anions that form insoluble compounds, a gas test for carbonate, and solution colour as a first screen.

Worked examples

Reveal one step at a time and try to beat it to the next line.

Example 1

An unknown colourless solution produces a bright yellow flame in a flame test. Which cation is most likely present, and what confirming test could you do?

Example 2

A chemist adds a few drops of chloride solution to an unknown and sees a white precipitate form. Using the solubility rules, list the possible cations present.

Example 3

An unknown solution gives a brick-red flame test. A student wants to confirm this is Ca2+ rather than Sr2+ (similar reddish colours). Describe a follow-up test using solubility rules.

Example 4

A colourless solution may contain any combination of Ag+, Ba2+ and Cu2+. An analyst adds NaCl(aq): no precipitate. She adds Na2SO4(aq): a white precipitate forms. She filters, then adds NaOH(aq) to the filtrate: no precipitate. The original sample gave a pale green flame. Which ions are present? Justify each conclusion.

Where marks get lost

Relying on flame test colour alone without considering that similar-looking colours (e.g. Ca2+ orange-red vs Sr2+ red) can be confused, especially by an inexperienced observer.
Forgetting that a strong sodium contamination can mask other flame colours, since sodium's bright yellow is very intense even in trace amounts.
Assuming a selective precipitation test that produces NO precipitate proves an ion's absence with total certainty — it only rules out ions whose compounds with that reagent are known to be insoluble.
Using a dirty wire loop for a flame test, which can carry contamination from a previous sample and give a false colour.
Adding reagents in the wrong order (e.g. NaOH first), so several cations precipitate together and none can be identified — or forgetting to filter between steps, so a later reagent is tested on a mixture that still contains the earlier precipitate.

Flashcards 8

Selective precipitation

Flame test

Why does a flame test produce colour?

Na+ flame colour

+ 4 more

Practice

Which metal ion produces a lilac (pale violet) flame test colour?

Why is a bright blue-green flame test colour a strong indicator of copper?

What flame colour would you expect from a solution containing Li+ ions?

A solution gives no precipitate when chloride ions are added, ruling out Ag+, Pb2+, and Hg2(2+). How many cations were ruled out by this single test?

A solution may contain Ag+, Ba2+ and Cu2+. Put the reagents in the order that separates them one at a time (filtering after each).

1Na2SO4(aq)
2NaCl(aq)
3NaOH(aq)

Match each ion to its flame test colour.

Na+

K+

Sr2+

Ba2+

Name one technique (other than flame tests) used to help identify unknown ions in solution.

Dilute acid is added to a white solid and the gas produced turns limewater milky. Which anion does the solid most likely contain?

A clear, blue solution gives no precipitate with NaCl(aq) but a pale blue precipitate with NaOH(aq). Which cation is present?

Which reagent is added to a water sample to test for sulfate ions, giving a white precipitate if they are present? (Give the name or formula.)

Match each observation to the ion it indicates.

White precipitate with acidified AgNO3

Fizzing with dilute acid

Blue solution

Crimson flame

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