Test for Anions, Identification of Gases


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In today’s class, we will be talking about the test for anions, identification of gases. Enjoy the class!

Test for Anions, Identification of Gases

Test for Anions | classnotes.ng

Test for anions

Identification of gases e.g. CO2, SO2 and O2.

Characteristics test for anions e.g. SO42-, SO32-, CO32-,NO-3, Cl, etc.

The anions are negatively charged ions examples of anions are SO42-, SO32-, CO32-, S2- etc.

Identification of gases

Gases                       Identification

H2                                       It gives a ‘pop’ sound when the gas in contact with a lighted splint.

O2                             It rekindles a glowing splint.

CO2                           It is colourless and odourless. It turns lime water milky.

Cl2                            A green wish yellow gas, it turns moist iodide paper black.

SO2                          Colourless gas with an irritating smell. It turns damp blue litmus paper to red. That is acidic gas.


  1. State two (2) examples of anions with their gases.
  2. Identify Cl2 and NH3

Test for anions

  • Test for SO42-

Test                             Observation                                    Inferences

Sample +H2O          dissolve in water                             soluble

Soln +Bacl2+            white ppt                                          SO42-, SO32-,CO32-

Dil HCl                        ppt insoluble                                    SO42- confirmed.

Soln + dil HCl            No visible reaction                         SO32-,CO32-,S2- are absent

Soln + Bacl2              white precipitate                            SO42-, confirmed

  • Test for SO32-

Soln+Bacl2                white precipitate

+ dil HCl                     dissolves in dil HCl                            SO32- or CO32-

soln+acidified         decolourise the purple

soln  of KMnO4        colour to colourless                                   SO32- confirmed

soln +                         the orange colour change                    SO32- confirmed

acidified K2Cr2O7     to green (reducing property)

  • Test for CO32-

Soln+dil HCl              Effervescence occurred and a Colourless gas is librated           SO32-, CO32-, S2- may be present

Gas+litmus paper      it changes moist blue litmus paper to red                   acidic gas

Gas + lime water    The gas turned lime water milky.                                  gas is CO2 from CO32-

  • Test for S2-

Test                                                   Observation                                               Inferences

Soln + dil HCl            A colourless gas evolved with rotten egg smell              H2S gas from S2-

Gas +KMnO4 soln     purple colour is decolourised with a deposit of sulphur       S2- present

  • Test for Cl

Soln + dil HNO3                   No visible reaction             SO32- or CO3 are absent

Soln + AgNO3                      white precipitate                Cl- present

General evaluation
  1. Give the common reagents for a confirmatory test of anions
  2. State how you would confirm/test for NO3-
  3. Name three gases that are colourless and acidic to litmus.
  4. Sodium chloride and silver trioxonitrate(V) crystals are separately soluble in water to give colourless solutions. Explain what happens when their solutions are mixed together.

Reading assignment

School Chemstry by O.Y Ababio pages 165 – 183

Practical Chemistry by R. I Makanjuola pages 27-33.

Practical Chemistry for Schools and Colleges by Godwin O. Ojokuku pages 30 – 98.

Weekend assignment

  1. Brown ring test is used to confirm (a) S2- (b)NO3- (c) SO42- (d) SO32-
  2. The presence of SO32- change the colour of acidified K2Cr2)7 from (a) green to red (b) purple to colourless  (c) orange to green  (d) green to orange
  3. A greenish-yellow gas that changes moist iodide paper black is (a) Cl2 (b) SO2            (c) H2 (d) O2
  4. Sometimes in the presence of HsSO4, copper turning and heat NO3- gives (a) brown ring reaction  (b) pure brown fume   (c ) reddish-brown   (d) effervescence
  5. The gas that turns lime-water milky is (a) H2 (b) SO2 (c) NH3 (d) CO2.


  1. State the colour of these solutions a. KMnO4 K2Cr2O3 c. HOBr d. CuSO4.5H2O e. Ca(OH)2
  2. Carry out the following exercises on sample K. add about 10cm3 of distilled water to K in a test tube. Divide the solution into four.
  3. To the 1st portion add NaOH dropwise and in excess.
  4. To the 2nd portion add NH3 solution dropwise and then in excess.
  5. To the 3rd portion, add it drops of BaCl2 follow by the addition of dil HCl
  6. To the 4th portion, add dil HCl followed by the addition of BaCl2.

Record your observation and inferences then name the salt K.


In our next class, we will be talking about Volumetric Analysis.  We hope you enjoyed the class.

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