QA Notes - Test for cations and anions

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QUALITATIVE ANALYSIS NOTES

Tests for Cations (Positive Ions):

Cations are identified based on their reactions with 2 reagents: aqueous sodium hydroxide and

aqueous ammonia. The colour of the precipitates (ppt) formed and their solubility in excess of

the reagent serves as a useful guide for the identification of cations in aqueous solutions.

Cation

Observations:

Identity of precipitate Reaction with aqueous sodium

hydroxide Reaction with aqueous ammonia

Al3+ White precipitate formed, soluble in _{excess aqueous sodium hydroxide to} give a colourless solution.

White precipitate formed, insoluble

in excess aqueous ammonia. Aluminium hydroxide_Al(OH)

3

Ca2+ White precipitate formed, insoluble

in excess aqueous sodium hydroxide. No visible reaction. Calcium hydroxideCa(OH)2

Cu2+ Blue precipitate formed, insoluble in _{excess aqueous sodium hydroxide.} Blue precipitate formed, soluble in _{excess aqueous ammonia to give a} dark blue solution.

Copper(II) hydroxide Cu(OH)2

Fe2+

Dirty-green precipitate formed, insoluble in excess aqueous sodium

hydroxide. Dirty-green precipitate turns reddish-brown on standing in air.

Dirty-green precipitate formed, insoluble in excess aqueous ammonia. Dirty-green precipitate turns reddish-brown on standing in air.

Iron(II) hydroxide Fe(OH)2

Fe3+ Reddish-brown precipitate formed, _{insoluble in excess aqueous sodium} hydroxide.

Reddish-brown precipitate formed, insoluble in excess aqueous ammonia.

Iron(III) hydroxide Fe(OH)3

Pb2+ White precipitate formed, soluble in _{excess aqueous sodium hydroxide to} give a colourless solution.

White precipitate formed, insoluble

in excess aqueous ammonia. Lead(II) hydroxide_Pb(OH)

2

Zn2+ White precipitate formed, soluble in _{excess aqueous sodium hydroxide to} give a colourless solution.

White precipitate formed, soluble in

excess aqueous ammonia to give a colourless solution.

Zinc hydroxide Zn(OH)2

NH4+

On warming, a colourless, pungent gas evolved which turned moist red litmus paper blue. The gas is ammonia.

No visible reaction.

-Na+ _{No visible reaction.} _{No visible reaction.}

-K+ _{No visible reaction.} _{No visible reaction.}

-Note: To distinguish between Al

3+

_{and Pb}

2+

_{, add aqueous potassium iodide (KI).}

If Pb

2+

_{is present, a bright yellow ppt is formed. This ppt is PbI}

2

.

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Tests for Anions (Negative Ions):

Anions are identified based on their reactions with specific reagents.

Anion Test Observations Identity of precipitate

NO3

-Add aqueous sodium hydroxide. Then add a little aluminium powder or Devarda’s alloy.

Warm.

A colourless, pungent gas evolved which turned moist red litmus paper blue. The gas is ammonia.

-CO32- Add any dilute acid.

Brisk effervescence. A colourless, odourless gas evolved which gives a white precipitate with limewater (calcium hydroxide). The gas is carbon dioxide.

A white precipitate is obtained because of the

calcium carbonate (CaCO3) formed.

Cl- Add dilute nitric acid.

Add aqueous silver nitrate. A white precipitate is formed. Silver chlorideAgCl

I- Add dilute nitric acid.

Add aqueous lead(II) nitrate. A yellow precipitate is formed.

Lead(II) iodide PbI2

SO4

2-Add dilute nitric acid.

Add aqueous barium nitrate. A white precipitate is formed.

Barium sulphate BaSO4 Add dilute hydrochloric acid.

Add aqueous barium chloride. A white precipitate is formed.

Note:

The purpose of adding dilute nitric acid before adding the specific reagents is to prevent the

precipitation of CO

32-

ions (i.e. to eliminate the possible presence of CO

32-

ions) and at the same

time, not interfere with the test (because nitrates ions do not form precipitates as nitrate

compounds are soluble in water).

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Tests for Gases:

Gas Colour & Odour Test Observations

Ammonia Colourless gas with _{a pungent smell} Place a piece of moist red litmus _{paper in the gas.} Red litmus paper turns blue.

Carbon

dioxide Colourless and odourless gas Bubble the gas through aqueous calcium hydroxide (limewater). Limewater turns milky.

Chlorine _{with a pungent smell}Greenish-yellow gas Place a piece of moist blue _{litmus paper in the gas.} Blue litmus paper turns red, and is _{then bleached.}

Hydrogen Colourless and _{odourless gas} Place a lighted wooden splint _{near the gas.} Lighted splint is extinguished with a _{`pop’ sound.}

Oxygen Colourless and _{odourless gas} Place a glowing wooden splint _{near the gas.} Glowing splint is relighted.

Sulphur

dioxide Colourless gas with a pungent smell

Place a drop of acidified

potassium dichromate(VI)

solution on a piece of filter paper, and then place it in the gas.

Acidified orange potassium dichromate(VI) solution turns green.

Tests for Water or Water Vapour:

There are 2 chemical tests to detect the presence of water or water vapour:

1. Use anhydrous copper(II) sulphate

Water will change the colour of anhydrous copper(II) sulphate from white to blue.

2. Use dry cobalt(II) chloride paper

Water will change the colour of dry cobalt(II) chloride paper from blue to pink.

Note that these 2 tests only show the presence of water. They cannot be used to test for the

purity of water.

3

carbon dioxide gas

acidified silver nitrate

acidified lead (II) nitrate

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SUMMARY (CATIONS)

Concept Map 1:

Reaction with aqueous sodium hydroxide

Concept Map 2:

Reaction with aqueous ammonia

4 Salt solution

White ppt:

Al3+_{, Ca}2+_{, Pb}2+_{, Zn}2+ Blue ppt:_Cu2+ Dirty-green ppt:_Fe2+ Reddish-brown ppt:_Fe3+

No ppt: NH₄+_{, Na}+_{, K}+ aqueous sodium hydroxide White ppt soluble: Al3+_{, Pb}2+_{, Zn}2+ White ppt insoluble:_Ca2+ Ammonia produced: NH₄+ Excess aqueous

sodium hydroxide Warm

carbon dioxide gas

acidified barium nitrate / barium chloride

Salt solution

White ppt:

Al3+_{, Pb}2+_{, Zn}2+ Blue ppt:_Cu2+ Dirty-green ppt:_Fe2+ Reddish-brown ppt:_Fe3+

No ppt: Ca2+_{, NH} 4 +_{, Na}+_{, K}+ aqueous ammonia White ppt soluble: Zn2+ White ppt insoluble:_Al3+_{, Pb}2+ Excess aqueous ammonia Blue ppt soluble: Cu2+

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SUMMARY (ANIONS)

1. Test for carbonate (CO3 2- ) dilute acid

2. Test for nitrate (NO3 - )

sodium hydroxide and aluminium powder (Devarda’s alloy) nitrate

ammonia gas

3. Test for chloride (Cl -₎

4. Test for iodide ( I-₎

5. Test for sulphate ( SO42-₎

5

carbonate

carbon dioxide gas

chloride

white precipitate

iodide

yellow precipitate

acidified barium nitrate / barium chloride

sulphate

white precipitate warm

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IONIC EQUATIONS (CATIONS)

Cation

Add aqueous sodium

hydroxide/aqueous ammonia:

Ionic Equation:

Colour of

precipitate

Identity of precipitate

Al

3+

_White

_{Aluminium hydroxide}

_Al

3+

_{(aq) + 3OH}

-

_(aq)

_Al(OH)

3

(s)

Ca

2+

_White

_{Calcium hydroxide*}

Cu

2+

_Blue

_{Copper(II) hydroxide}

Fe

2+

_Dirty-green

_{Iron(II) hydroxide}

Fe

3+

_{Reddish-brown}

_{Iron(III) hydroxide}

Pb

2+

_White

_{Lead(II) hydroxide}

Zn

2+

White

Zinc hydroxide

NH

4+

-

NH4

+_{(aq) + OH}-_(aq) _NH

3 (g) + H2O (l)

* For the calcium ion (Ca

2+

_{), very little or no precipitate is formed with aqueous ammonia.}

Note:

For the ammonium ion (NH

4+

), ammonia gas (NH

3

) is evolved when warmed with aqueous

sodium hydroxide.

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IONIC EQUATIONS (ANIONS)

Anion

Test:

_precipitate

Identity of

Ionic Equation:

NO

3

-Aqueous sodium

hydroxide +

Aluminium powder

(with warming)

-

-CO

32-

Dilute hydrochloric

acid

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

Cl

-

_{Dilute nitric acid +}

Silver nitrate

Silver chloride

I

-

_{Dilute nitric acid +}

Lead(II) nitrate

Lead(II)

iodide

SO

4

2-Dilute nitric acid +

Barium nitrate

Barium

sulphate

Dilute hydrochloric

acid + Barium

chloride

Note:

For the nitrate ion (NO

3-

), ammonia gas (NH

3

) is evolved when warmed with aqueous sodium

hydroxide and aluminium powder.

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PRECIPITATION ILLUSTRATION (CATIONS)

To illustrate how a precipitation reaction takes place for cations, consider the following model

for the test for Cu

2+

_ions.

1. Suppose the above test solution is copper(II) nitrate, can you work out the ionic equation for

the precipitation reaction?

2. Suppose the above test solution is copper(II) sulphate, can you work out the ionic equation

for the precipitation reaction?

8

+

OH -Na+ OH -Na+ Na+ OH -OH -OH -X m-Cu2+ X m-Cu2+ Cu2+ _X m-Cu2+ Cu2+ X

m-Test solution

Reagent

OH -Cu2+ X m-Cu2+ _Cu2+ OH -X m-X m-Cu2+ Cu2+ OH- _OH -OH -Na+ Na+ Na+ X

m-precipitate (ppt)

spectator ions

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PRECIPITATION ILLUSTRATION (ANIONS)

To illustrate how a precipitation reaction takes place for anions, consider the following model

for the test for Cl

-