SUBJECT – CHEMISTRY

PROJECT – TO DETECT THE PRESENCE

OF RADICAL ACIDIC IN
GIVEN SALT.
THEORY-
Identification Of Acid Radicals (Anions)
The identification of the acid radicals is first done on the basis of
preliminary tests. Dry heating test is one of the preliminary tests
performed earlier which may give some important information about the
acid radical present. The other preliminary tests are based upon the fact
that:
1. CO3 2-, S2-, SO 32-and NO 2– react with dil. H2S04 to give out CO2, H2S,
SO2 and NO2 gas respectively which can be identified by certain tests.
2. Cl–, Br–, I–, NO3 –, C2 O4 2- and CH3COO– react with cone. H2S04 but not
with dil. H2S04 to produce characteristic gases.
3. S042- and P043- react neither with dil. H2S04 nor with cone. H2S04.
These are therefore, identified by individual tests.
Thus, the acid radicals may be identified by performing the following tests in
the order given below :
(i) Dil. H2S04 test. Treat a pinch of the salt with dil. H2S04 and identify
the gas evolved.
(ii) Cone. H2S04 test. If no action takes place with dil. H2S04, warm a
pinch of the salt with cone. H2S04 and identify the gas evolved.

(iii) Independent Group. (SO42- and PO43-). If the salt does not
react with dil. H2S04 as well as with cone. H2S04, test for SO42-
and PO43- by performing their individual tests.
Let us now discuss these tests in detail one by one.
Dilute Sulphuric Acid Test-
Take a small quantity of the salt in a test-tube and add 1-2 ml of dilute
sulphuric acid. Observe whether some gas is evolved or not. If some gas is
evolved, identify the gas and draw inferences from Table .

S.NO OBSERVATION INFERENCE

GAS POSSIBLE
EVOLVED RADICAL

1 Colourless, odourless gas with

brisk effervescence, turns lime
water milky.
2
CO2 CO3

2 Colourless gas, smell like that of

rotten eggs, turns ‘ lead acetate H2S S2-
paper black.

3 Colourless gas, smell like that of

2-
burning sulphur, turns acidified SO2 SO3
potassium dichromate paper
green.

4 Reddish brown gas, pungent

smell, turns ferrous sulphate
solution black. NO2 NO2-

2- 2- 2-
5 No gas is evolve CO3 , S , SO3
-----
–
---- , NO2 absent
Note:
1. Do not treat the salt with a large quantity of dilute acid.
2. Do not heat the salt with dilute acid.

Chemical Reactions Involved in Dil. H2SO4 Test-

Dilute H2S04 (or dilute HCl) decomposes carbonates, sulphides sulphites and
nitrites in cold to and liberates different gases. These gases on identification
indicate the nature of the acid radical present in the salt.
1. Carbonates. On treating the solid carbonate, CO2 is given off in the cold
with brisk effervescence.

2.
Sulphides. Sulphides when treated with dil. H2S04 give H2S gas.

Sulphites. Sulphites when treated with dil. H2S04 give SO2 gas.

4. Nitrites. On treating the solid nitrite with dil. H 2S04, nitric oxide (NO)
gas is evolved which readily gives dense brown fumes of NO 2 with oxygen of
the air.
Potassium Permanganate Test-
To a pinch of salt in test tube add about 2 ml of dilute sulphuric acid. Boil
off any gas evolved, add little more of dilute acid and then potassium
permanganate solution d’ropwise. Note the changes as given in Table .
This test helps in detection of Cl–, Br–, I–, C2O42-, and Fe2+ radicals.

S.NO OBSERVATION INFERANCE

1 Potassium permanganate decolourised Presence of Fe2+
without the evolution of any gas. salts.

2 Potassium permanganate decolourised

:
(a) In cold
Cl–
(i) With the evolution of chlorine.
Br–
(ii) With the evolution of bromine.
I–
(iii) With the evolution of iodine.

(b) On warming

(i) With the evolution of carbon dioxide 𝐂𝟐𝐎 𝟒𝟐−

3 KMnO4 not decolourised. Absence of Cl–, Br–, I–

2- 2+
, C2O4 and Fe
Note:
Do not perform this test if the salt reacts with dilute sulphuric acid
because sulphides, sulphites and nitrites also decolourise KMn04
solution.

Chemical Reactions Involved

Concentrated Sulphuric Acid Test-
This test is performed by treating small quantity of the salt with cone,
sulphuric acid (2-3 ml) in a test tube. Identify the gas evolved in cold and
then on heating. Draw inferences from Table . Cone. Sulphuric Acid Test.

S.NO OBSERVATION GAS POSSIBLE

EVOLVED RADICALS
1 Colourless gas with pungent smell.

The gas gives white fumes with

HCL Cl–
aqueous ammonia (NH4OH) and

white ppt. with AgNO3 solution.

2 Reddish brown vapours with pungent

smell, turns starch paper yellow. It

Br2 Br–
does not turn

FeSO4 solution black.

3 Deep violet vapours with pungent

smell, turns starch paper blue. A

I2 vapours I–
sublimate is formed on the sides of the

tube.
 4 Reddish brown gas with pungent

smell, turns FeSO4 solution black. NO2 𝑵𝑶−

𝟐

5 Colourless gas which turns lime

water milky and a gas which burns

with blue flame. CO2 and

𝐂𝟐𝐎 𝟒𝟐−
CO

6 No gas/vapours evolved. Cl–, Br–, I–,

CH3COO–,
2-
C2O4 absent

Note :
1. If some gas is evolved with dilute sulphuric acid, then there is no
need for performing cone, sulphuric acid test.
2. Do not boil the salt with cone, sulphuric acid. On boiling, the acid may
decompose to give SO2 gas.
3. Nitrates give vapours of nitric acid (colourless) when heated with
cone, sulphuric acid. When a paper pellet or copper chips is added,
dense brown fumes evolve. Paper pellet acts as a reducing agent and
reduces nitric acid to NO2 (Reddish brown gas).
Chemical Reactions Involved in cone. H2S04 Test

Tests For Independent Radicals (SO42- and PO43-)

As already discussed these radicals are not detected by dilute or
concentrated H2SO4. They are tested individually.
1. SULPHATE (SO42-)
Boil a small amount of salt with dilute HCl in a test tube. Filter the contents,
and to the filtrate add few drops of BaCl2 solution. A white precipitate,
insoluble in cone. HCl, indicates presence of sulphate.
2. PHOSPHATE (PO43- )
Add cone. HNO3 to the salt in a test tube. Boil the contents and add excess of
ammonium molybdate solution. A yellow
precipitate indicates presence of phosphate.
S.NO EXPERIMENT OBSERVATION INFERENCE
1 PHYSICAL White colour C𝑢+,F𝑒+,F𝑒+,N𝑖+,
2 2 3 2
EXAMINATION M𝑛+,C𝑜+, are
2 2
Colour of the absent
sample is noted
2 A little Pungent smell N𝐻+ may be present
4
amount of the evolved
sample is
rubbed
between
thumb and
finger.

3 Dry Test Tube Pungent gas N𝐻4+ ,C𝒍−may be

Heating : evolved along present
Heated small with formation of
amount of the white sublimate
sample in a dry test
tube.
4 Flame test was No characteristic C𝑎+ ,Ba2+ Cu2+ may
2
performed colour in the be absent.
flame

5 Charcoal activity No characteristic Pb2+, Cu2+,Zn2+,As3+

test was performed. bead but white may be absent and
incrustation NH4+ may be present
around the edge
of cavity.

6 Small amount of Effervescence S2-, NO2-,may be

the sample is with evolution of absent.
2-
treated with dilute colourless, CO3 may be present.
H2SO4 and and odourless gas
pass the gas
through lime wate

7 Small amount of Colourless and NO3-, Cl-, Br- be

2-
the solid sample is odourless gas absent. Co3 be
treated with evolved with present.
effervescence
 concentrated
H2SO4
2-
8 Small amount of Effervescence Co3 is present and
the solid sample is with evolution of confirmed.
treated with Dilute colourless,
H2SO4 and the odourless gas
evolved gas is which turns lime
passed through water milky.
lime water.

Preparation of aqueous extract :

Aqueous extract of the sample was prepared and following test
for acid radical was performed.
2-
9 Small amount of Lime water turns Co3 is present and
the sample is milky and confirmed
treated with dilute milkiness
H2SO4 and and disappears on
pass the gas passing excess lime
through lime water water

2-
10 To the aqueous White precipitate Co3 is present and
solution of sample was formed confirmed
MgSO4 solution was
added

11 Acidified the solution No precipitate

with dilute HNO3 formed Cl- is absent
and AgNO3 solution
was added
2-
12 Acidified the solution No precipitate SO4 is absent.
with dilute HCl and formed
BaCl2 solution was
added

13 Freshly prepared No brown ring NO3- was absent.

FeSO4 solution is was formed
added to the
sample solution
 and 2ml of
conc.H2SO4 was
added carefully
along the wall of the
container

3-
14 Acidified the No precipitate is PO4 is absent
solution with conc. produced
HNO3 and a pinch
of ammonium
molybdate was
added to it.

2-
15 To the aqueous No precipitate Co3 is absent
solution of sample was formed
MgSO4 solution was
added

16 Acidified the Curdy White Cl- is present and

solution with dilute precipitate confirmed
HNO3 and AgNO3 formed which
solution was added. dissoved in excess
NH4OH

17 Freshly prepared No brown ring NO3- was absent.

FeSO4 solution is was formed
added to the sample
solution and 2ml of
conc.H2SO4 was
added carefully
along the wall of
the container.