Qualitative Analysis – Cations, Anions & Gas Tests

Vocabulary flashcards covering key ions, reagents, test results and colour changes required for Chapter 12 Qualitative Analysis.

Qualitative Analysis

Process of identifying ions or gases in a sample by adding reagents and observing changes, without measuring amounts.

what is a precipitate

Insoluble solid that appears when two solutions react, often coloured and diagnostic for ions.

Aluminium Ion + NaOH

White precipitate Al(OH)3 forms, dissolves in excess NaOH to give a colourless solution.

Aluminium Ion + NH3(aq)

White precipitate Al(OH)3 forms, remains insoluble in excess ammonia.

Zinc Ion + NaOH

White precipitate Zn(OH)2 forms, soluble in excess to give a colourless solution.

Zinc Ion + NH3(aq)

White precipitate Zn(OH)2 forms, soluble in excess ammonia to give a colourless solution.

Calcium Ion + NaOH

White precipitate Ca(OH)2 forms, insoluble in excess.

Calcium Ion + NH3(aq)

No precipitate or only very slight white precipitate observed.

Copper(II) Ion + NaOH

Light-blue precipitate Cu(OH)2 forms, insoluble in excess NaOH.

Copper(II) Ion + NH3(aq)

Light-blue precipitate dissolves in excess ammonia to give a dark-blue solution

Iron(II) Ion + sodium hydroxide

Green precipitate Fe(OH)2 forms, insoluble in excess NaOH

Iron(II) Ion + aqueous ammonia

Green precipitate Fe(OH)2 forms, insoluble in excess ammonia and oxidises to a reddish-brown

Iron(III) Ion + sodium hydroxide

Red-brown precipitate Fe(OH)3 forms, insoluble in excess NaOH

Iron(III) Ion + aqueous ammonia

Red-brown precipitate Fe(OH)3 forms, insoluble in excess NH3.

Ammonium Ion + NaOH

On warming, ammonia gas is evolved which turns damp red litmus paper blue.

Carbonate Ion Test

Add dilute acid; effervescence of CO2 in limewater that turns it milky confirms carbonate.

Chloride Ion Test

Acidify with dilute HNO3 then add (dilute silver nitrate) AgNO3(aq); white precipitate (AgCl) forms.

Iodide Ion Test

Acidify with dilute HNO3 then add AgNO3; yellow precipitate (AgI ) appears.

Acidify with dilute HNO3 then add Pb(NO3)2; bright-yellow precipitate (PbI2) forms.

Nitrate Ion Test

Add NaOH(aq) and aluminium foil, warm; ammonia gas produced turns moist red litmus blue.

effervescence is observed on warming.

Sulfate Ion Test

Acidify with dilute HNO3 then add Ba(NO3)2(aq); white precipitate (BaSO4) forms.

Ammonia Gas Test

Damp red litmus paper turns blue near the colourless, pungent NH3 gas.

Carbon Dioxide Gas Test

Bubble gas into limewater; white CaCO3 precipitate turns limewater milky.

Chlorine Gas Test

Greenish-yellow gas turns damp blue litmus red then bleaches it white.

Hydrogen Gas Test

Colourless gas gives a ‘pop’ when a lighted splint is introduced.

colourless, odourless gas

Oxygen Gas Test

Colourless gas relights a glowing splint.

colourless, odourless gas

Sulfur Dioxide Gas Test

Place a piece of filter paper soaked with acidified potassium

manganate(VII) solution /acidified potassium dichromate (VI) solution

at the mouth of the test tube.

Or

Bubble sulfur dioxide into acidified potassium manganate(VII)

solution or acidified potassium dichromate (VI) solution.

The acidified potassium manganate(VII) turns from purple to

colourless.

The acidified potassium dichromate(VI) turns from orange to green.

Water Vapour Test (Cobalt Paper)

Place cobalt (II) chloride paper near the mouth of the test tube

containing the vapour, if the vapour turns cobalt (II) chloride paper

from blue to pink.

To test for water, the vapour must be cooled and condensed into

liquid first. Add a few drops of liquid to anhydrous copper (II) sulfate,

if the liquid caused the white anhydrous copper (II) sulfate to turn

blue, liquid is water.

Ammonium Chloride Decomposition

NH4Cl(s) sublimates on heating, decomposing to NH3(g) and HCl(g) which recombine on cooling.

Hydrated Salt Heating

Gentle heat drives off water of crystallisation; salt becomes white anhydrous powder and droplets appear on tube walls.

Metal Carbonate Decomposition

Some carbonates produce metal oxide + CO2 on heating

metal carbonate → metal oxide + carbon dioxide

(e.g., CuCO3 → CuO + CO2; from green solid to black solid

ZnCO3 → ZnO +CO2; from white solid to yellow solid when hot and white solid when cold).

Group I Carbonate Stability

Sodium and potassium carbonates do not decompose on heating.

Limewater

Dilute Ca(OH)2 solution used to detect CO2 by formation of CaCO3 precipitate.

Acidified Potassium Manganate(VII)

Purple oxidising agent used to detect SO2 by decolourisation to colourless.

Potassium element colour, ion, compound colour, hydroxide formed and colour of salt solution

K

Silvery (polished) /Grey (powder)

K+

K2O white

KOH, soluble in water, colourless

colourless

Sodium element colour, ion, compound colour, hydroxide formed and colour of salt solution

Na

Silvery (polished) /Grey (powder)

Na+

Na2O white

NaOH, soluble in water, colourless

Colourless

Calcium element colour, ion, compound colour, hydroxide formed and colour of salt solution

Ca

Silvery (polished) /Grey (powder)

Ca+

CaO white

Ca(OH)2, slightly soluble in water

Colourless

Zinc element colour, ion, compound colour, hydroxide formed and colour of salt solution

Silvery (polished) /Grey (powder)

Zn2+

ZnO yellow when hot, white when cold

Zn(OH)2,white , insoluble in water

Colourless

Iron element colour, ion, compound colour, hydroxide formed and colour of salt solution

Silvery (polished) /Grey (powder)

Fe2+ , Fe3+

FeO  Iron(II) oxide  is black.  Fe2O3  Iron(III) oxide  is reddish   brown

Fe(OH)2,green  or dirty green ,  insoluble in   water  Fe(OH)3,   reddish brown  precipitate,   insoluble in   water

Iron(II) salt   solutions are   pale green.  e.g  aq FeSO4, aq  FeCl2  Iron(III) salt   solution is   yellowish brown  (lower   concentration)  /reddish brown  (higher conc)  e.g aq FeCl3

Copper element colour, ion, compound colour, hydroxide formed and colour of salt solution

Reddish   brown or   brown

Cu+ , Cu2+

CuO  Copper(II)   oxide is black solid  Cu2O  Copper(I)   oxide is   reddish   brown solid  CuCO3   Copper(II)   carbonate is  green solid  CuSO4  Copper(II)   sulfate is blue solid

Cu(OH)2, blue  precipitate,   insoluble in   water

Copper(II) salt  solutions are   usually blue.  e.g  aq Cu(NO3)2,   aq CuSO4   Copper(II)   chloride, CuCl2 solution is   bluish green or  green.

Carbon substance colour & remarks

C

black solid

Fluorine substance colour & remarks

F2

Pale yellow gas at r.t.p

Chlorine substance colour & remarks

Cl2

greenish-yellow gas at r.t.p

Bromine substance colour & remarks

Reddish brown liquid at r.t.p. 

Aqueous bromine and bromine gas are also reddish brown.

Iodine substance colour & remarks

I2

Black solid at r.t.p. 

Iodine vapour is violet. 

Iodine solution is yellowish-brown at lower concentration but reddish-brown at higher concentration.

Nitrogen dioxide substance colour & remarks

NO2

Reddish brown gas at rtp

nitrogen monoxide substance colour & remarks

NO

colourless gas at rtp

neutral gas

carbon monoxide substance colour & remarks

CO

colourless gas at rtp

neutral gas

Manganese(IV) oxide substance colour & remarks

MnO2

Black solid

Potassium manganate(VII) substance colour & remarks

KMnO4

Purple solution. 

Turns colourless when reduced.

Potassium dichromate(VI) substance colour & remarks

K₂Cr₂O7

Orange solution. 

Turns green when reduced.

Universal indicator substance colour & remarks

Red at pH 1 

Green at pH 7 

Violet/Purple at pH 14

Methyl orange substance colour & remarks

Red/Orange at pH < 4 

Yellow at pH > 4