Test for an unknown compound / substance

Preliminary examination of substance

Basically we note the following:-

Appearance: Note the nature of the solid, i.e. is it a powder, crystal, or lamp; its colour (or its aqueous solution) and its smell.

Observation: Inference

Blue, blue-green or Copper (II) salt

green in solution.

Green colour Iron (II) or copper(II) salt

Black colour Oxide or sulphide

Yellow colour Lead (II) oxide or

Iron (II) salt

Deliquescent Chloride or nitrate

Smell of ammonia Ammonia salt

Smell of sulphur dioxide Sulphite

Smel of hydrogen sulphide Sulphide

IDENTIFICATION OF GASES

Gases are often given off by the action of heat or reagent on substances.

Gases can commonly be identified by the following tests:-

(a) The colour and smell of the gas

(b) Put a burning splint in the gas

(c) Put moist red and moist blue litmus paper in the gas

Summary of identification of gases

Gas	Colour	Smell	Action with splint
Chlorine	Greenish-yellow	Irritating smell	Extinguished
Carbon dioxide	Colourless	Very faint	Extinguished
Ammonia	Colourless	Chocking	Extinguished
Water vapour	Colourless	None	Extinguished
Nitrogen dioxide	Reddish-brown	Irritating	Extinguished
Oxygen	Colourless	None	Relights glowing splint
Sulphur dioxide	Colourless	Irritating	Extinguished
Nitric acid vapour	Pale yellow fumes	Irritating	Splint burns
Hydrogen chloride	Colourless, misty in	Irritating	Extinguished

Confirmatory tests for some gases

Hydrogen chloride: Hold moist stopper of conc. ammonia bottle in the gas. Dense white fumes are formed.

Ammonia: Turns moist litmus paper blue and forms dense white fumes with concentrated hydrochloric acid.

Carbon dioxide: Turns limewater milky.

Sulphur dioxide: Decolourise acidified KMnO4 solution.

Flames test

Place a little of the substance on a watch glass, moisten it with pure, concentrated hydrochloric acid, and heat a little on a platinum or nichrome wire. Note the colour of the flame:

Colour of flame Inference

Yellow (visible through Blue glass) Sodium

Lilac (purple through Blue glass) Potassium

Brick red Calcium

Bright green or blue Copper

Grey - blue Lead

Note: The absence of a characteristic flame colouration would indicate that all of the above metals are probably absent. Cations which do not cause flame colouration include silver aluminum, zinc magnesium, ammonium.

Action of heat

Heat a little of the substance in an ignition tube or in a small dry test-tube until no further change.

Observation: Inference

Carbon dioxide Carbonate, or Hydrogen carbonate

Ammonia Ammonium salt

Nitrogen dioxide Nitrate

Oxygen Nitrate, peroxide or (IV oxide)

Sulphur dioxide Water of crystallization; hydrogen

carbonate, hydrogensulphate;

White sublimate Hydroxide, ammonium salt

Yellow Zinc oxide

Hot, white cold

Reddish -brown hot;

yellow cold Lead (II) oxide

Cracking sound Nitrate salt.

Action of heat on the substances can be summarized by the following equations:-

For carbonates

MCO₃(s) MO(s) + CO₂(g)

For hydrogencarbonates

MHCO₃(s) M₂CO₃(s) + H₂O(I) + CO₂(g)

For ammonium salts

NH₄X(s) NH₃(g) + HX(g)

For nitrates

2M(NO₃)₂(s) 2MO₂(s) + 4NO₂(g) + O₂(g)

For hydroxide

M(OH)₂(s) MO(s) + H₂O(g)

For oxide

2MO(s) 2M(s) + O₂(g)

2FeSO₄(s) Fe₂O₃(s) + SO₂(g) + SO₃(g)

Note: The only hydrgencarbonates that exist in solid state are those of sodium, potassium and ammonium.

The nitrates of potassium, sodium and ammonium on heating decompose in the following way:

For sodium and potassium:

2NaNO₃(1) 2NaNO₂(1) + O₂(g)

For ammonium nitrate:

NH₄NO₃ (s) N₂O(g) + 2H₂O(1)

A student is highly advised to carry out test for gases concurrently during heating of the substances.

The nature of residue after heating

In most cases, the oxide of the metal is left after heating. The residue may be different in colour from the original compound. For example hydrated copper (II) sulphate is blue in colour after heating, the reside is white. If it is heated strongly, the reside may be black.

Action of Acids

Dilute hydrochloride acid

Add cold, dilute hydrochloric acid to the solid substance in a test-tube.

If there is no reaction warm gently.

Note: The smell of hydrogen chloride is to be expected if the mixture is heated too strongly. This is because of the volatile nature of hydrochloric acid. The action of dilute HCI is summarized below:-

Observation	Inference
Vigorous effervescence and carbon dioxide	Carbonates or hydrogencarbonates
Evolved (turns lime water milky)
Colours, odourless gas pops with lighted splint	Metals above hydrogen in the metal activity series
Pale yellow-green	Chlorine gas from strong oxidizing agent.

Summary of equations

	2-

CO₃ (s) + 2H⁺(aq) CO₂(g) +H₂O(1)

-

M(s) + 2H⁺ (aq) M²⁺(aq) + H₂(g)

2HCI(aq) + O H₂O(1) + CI₂(g)

2-

-

2-

-

Dilute sulphuric acid shows similar reactions like those of dilute hydrocroloric acid as shown above.

Reactions of Test Solutions with selected Reagents

(a) Sodium hydroxide solution

To the solution of the unknown add NaOH solution drop by drop until in excess.

+

presence of Na⁺, K⁺ or NH₄

Ammonia gas evolved on warming indicates presence of NH₄.

(ii) If a precipitate forms

Observation	Inference
White precipitate insoluble in excess	Mg2+, Ca2+, Ba2+
White precipitate soluble in excess	Al2+, Pb2+, Zn2+
Blue precipitate insoluble in excess	Cu2+
Green precipitate insoluble in excess	Fe2+
Reddish-brown precipitate insoluble in excess	- Fe3+
Ammonia gas given off on warming	NH4

Summary of equations

Ca²⁺(aq) + 2OH(aq) Ca(OH)₂(s)

Pb²⁺(aq) + 2OH^-(aq) Pb(OH)₂(s)

Zn²⁺ + 2OH^-(aq) Zn(OH)₂(s)

Al³⁺ (aq) + 3OH^-(aq) Al(OH)₃(s)

Cu²⁺ (aq) + 2OH^-(aq) Cu(OH)₂(s)

Fe³⁺ (aq) + 3OH^-(aq) Fe(OH)₃(s)

-

NH₄ (aq) + OH^-(aq) NH₃(g) + H₂(g)

(b) Ammonium hydroxide solution (aqueous ammonia)

Add ammonia solution, drop by drop to the solution until in excess.

Observation	Inference
White precipitate solution in excess	Zn2+
White precipitate insoluble in excess	Pb2+, Al3+, Mg2+
Pale blue precipitate soluble in excess forming a deep blue solution	Cu2+
Green precipitate insoluble in excess	Fe2+
Reddish-brown precipitate insoluble in excess	- Fe3+

-

-

2+

2+

Confirmatory Tests for Cations

(1) Copper (II) ion

Cu²⁺ forms pale blue precipitate with ammonia solution, the precipitate

is soluble in excess ammonia solution to give a deep blue solution.

(2) Lead (II) ion

Pb²⁺ forms a yellow participate with Potassium iodide. The precipitate is

-

Pb²⁺(aq) + 2I (aq) Pb2I₂(s)

(3) Iron (II) ion

Fe2+(aq) forms a dark blue precipitate with potassium

hexacyanoferrate (III) solution.

(4) Iron (III) ion. Fe³⁺ gives a deep red colouration with ammonia or potassium thiocyanate solution.

(5) Zinc ion forms white precipitate with sodium hydroxide and ammonium hydroxide. In both cases, the precipitate is soluble in excess.

(6) Aluminum ion. With sodium hydroxide a white precipitate forms. The precipitate is soluble in excess. With aqueous ammonia a white precipitate forms insoluble in excess.

(7) Ammonium ion. Ammonia gas is evolved on heating with sodium hydroxide solution.

	+

NH₄(aq) + OH^-(aq) NH₃(g) + H₂O(1)

Confirmatory Tests for Anions

1. Chlorides ion CI^-

(a) With silver nitrate solution, a white precipitate forms. The

precipitate is soluble in ammonia solution and insoluble in

dilute HNO₃.

Ag⁺(aq) + CI^-(aq) AgCI(s)

(b) With a solution containing lead(II) ions, CI^- ion forms a

white precipitate, which dissolves on heating and reappears

on cooling.

			-
	2-

2. Carbonates, CO₃, and hydrogencarbonate is HCO₃.

(a) With dilute HCI or H₂SO₄, with effervescence occurs with evolution of carbon dioxide.

	2-

CO₃(s) + 2H⁺(aq) H₂O(1) + CO₂(g)

	-

HCO₃(s) + H⁺(aq) H₂O(1) + CO₂(g)

Note: The carbonates of those cations which form

insoluble sulphates (Pb²⁺, Ca²⁺, Ba²⁺) will not react

with satisfactorily with dilute sulphuric acid.

(b) Aqueous barium salt (Ba(NO₃)₂, BaCI₂) or calcium salt

(CaCO₃) is formed. A white precipitate of BaCO₃ or CaCO₃ is

formed.

The precipitate is soluble in dilute acids.

(c) Magnesium sulphate or other magnesium salt in solution. A

white precipitate of magnesium carbonate is formed in the cold

with carbonates. No precipitate is formed in the cold with

hydrogencarbonates but precipitate forms on heating.

	2-

3. Sulphate ion, SO₄

To some if the solution add dilute nitric acid and then barium nitrate

2-

Ba²⁺(aq) + SO₄ (aq) BaSO₄(s)

OR: To some of the solution add dilute hydrochloric acid and then basium chloride solution. A white precipitate forms.

Ba²⁺(aq) + 2CI^- (aq) BaCI₂(s)

	-

4. Nitrate ion, No₃

The some of the solution add cold ion (II) sulphate solution. Carefully

pour cold, concentrated sulphuric acid down the side of the test-tube so

that the acid sinks to the bottom and forms separate layer there.

Formation of a brown ring where the acid meets the other liquid confirms

nitrate.

 

     Solubility of substances in water

    See section on the solubility of salts in water under "her concepts in   

    selected topic". 

    Substances in water can be

-          soluble

-          sparingly soluble

-          insoluble

If the substance does not dissolve in cold, warm abit.

If the salt dissolves, note the colour of the solution.

If the solution is:

(i)       coloured, probably, Fe²⁺,  Fe³⁺, Cu²⁺ present.

 

Ion	Colour in solution
Fe2+ Fe3+ Cu2+	Pale blue-green or pale gree Yellow-brown, or brown Blue, blue-green, or green in solution.

-

(ii)    Colourless, probably

                            Zn²⁺, Al³, Pb²⁺, Ca²⁺, Mg²⁺, NH4, present

If the solid is insoluble in cold, but soluble on heating and reprecipitates on cooling, lead(II) chloride present.

If the solid is insoluble in both cold and on heating, this indicates insoluble salts of metals e.g. lead, aluminum, zinc, copper, iron are present.