Test for Anions

Identification of gases e.g CO₂, SO₂ and O₂. Characteristics test for anions e.g SO₄^2-,SO₃^2-,CO₃^2-,NO^-3, Cl^–, etc. The anions are negatively change ions examples of anions are SO₄^2-,SO₃^2-,CO₃²-,S^2- etc.

Identification of Gases

Gases               identification

1. H₂ : It gives a ‘pop’ sound when the gas is contact with a lighted splint.
2. O₂ :It rekindles a glowing splint.
3. CO_{2 :}It is colourless and odourless. It turns lime water milky.
4. Cl_{2  :}A green wish yellow gas, it turns moist iodide paper black.
5. SO₂ Colourless gas with irritating smell. It turns damp blue litmus paper to red. That is acidic gas.

Test For Anions

Test for SO₄^2-

Test                             Observation                                          Inferences

Sample +H₂O                dissolve in water                                   soluble

Soln +Bacl₂+                  white ppt                                              SO₄^2-, SO₃^2-,CO₃^2-

Dil HCl                         ppt insoluble                                         SO₄^2-, confirmed.

Soln + dil HCl               No visible reaction                               SO₃^2-,CO₃^2-,S^2- are absent

Soln + Bacl₂                  white precipitate                                  SO₄^2-, confirmed

 Test for SO₃^2-

Soln+Bacl₂                    white precipitate

+ dil HCl                       dissolve in dil HCl                                 SO₃^2- or CO₃^2-

soln+acidified               decolourise the purple

soln  of KMnO₄ colour to colourless                              SO₃^2- confirmed

soln +                            the orange colour change                     SO₃^2- confirmed

acidified K₂Cr₂O₇                   to green (reducing property)

 

Test for CO₃^2-

Soln+dil HCl                 Effervescence occurred and a             SO₃^2-,CO₃^2-, S^2-

Colourless gas is librated                      may be present

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

To red.

Gas + lime water           The gas turned lime                              gas is CO₂ from CO₃^2-

Water milky.

Test for S^2-

Test                             Observation                                          Inferences

Soln + dil HCl               A colourless gas evolved                       H₂S gas from S^2-

With rotten egg smell

Gas +KMnO₄ soln           purple colour is decolourised

With a deposit of sulphur                    S^2- present

Test for Cl^–

Soln + dil HNO₃                        No visible reaction                    SO₃^2- or CO₃ are absent

Soln + AgNO₃                                       white precipitate                      Cl- present

GENERAL EVALUATION

1. Give the common reagents for confirmatory test of anions
2. State how you would confirm/test for NO^3-
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

1. School Chemstry by O.Y Ababio pages 165 – 183
2. Practical Chemistry by R. I Makanjuola pages 27-33.
3. Practical Chemistry for Schools and Colleges by Godwin O. Ojokuku pages 30 – 98.

WEEKEND ASSIGNMENT

1. Brown ring test is used to confirm (a) S^2- (b)NO^3- (c) SO₄^2- (d) SO₃^2-
2. The presence of SO₃^2- 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 change moist iodide paper black is (a) Cl₂ (b) SO₂ (c) H₂  (d) O₂
4. Sometimes in the presence of conc HsSO₄, copper turning and heat NO^3- 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.

THEORY

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 1^st portion add NaOH drop wise and in excess.
4. To the 2^nd portion add NH₃ solution drop wise and then in excess.
5. To the 3^rd portion, add it drops of BaCl2 follow by the addition of dil HCl
6. To the 4^th portion, add dil HCl follow by the addition of BaCl₂.
7. Record your observation and inferences then name the salt K.