Class-IX Subject-Chemistry

Half-Yearly Syllabus Handout-4

Chapter-18 Chemical Tests

Pg- 190

All the topics of this chapter are included in Paper-1.

Testing for gases (Pg-191):

1.Test for hydrogen,H2: When a lighted splint is held at the mouth of a test tube, hydrogen
burns with a squeaky pop sound as it combines explosively with oxygen in the air to make
water.

2H2(g) + O2(g) → 2H2O(l)

2. Test for oxygen,O2: A glowing splint is put into the tube containing the gas. Oxygen relights
a glowing splint.

3. Test for carbon dioxide,CO2: The carbon dioxide is bubbled through limewater. Carbon
dioxide turns limewater milky/chalky/cloudy.

Limewater is calcium hydroxide solution. Carbon dioxide reacts with it to form a white
precipitate of calcium carbonate.

Ca(OH)2(aq) + CO2(g) → CaCO3(s) + H2O(l)

4. Test for chlorine gas,Cl2: A piece of damp blue litmus paper or an universal indicator paper
is put into the test tube or held over its mouth. Chlorine is a green gas that turns the damp blue
litmus paper or universal indicator paper red at first and then bleaches it (turns white).

5. Test for ammonia gas,NH3: A piece of universal indicator paper or red litmus paper is held
at the mouth of the test tube. Ammonia is an alkaline gas which turns the universal indicator
paper/litmus paper blue.

Testing for water (Pg-192):

Using anhydrous copper(II)sulfate: Water turns white anhydrous copper(II)sulfate blue.

Anhydrous copper(II)sulfate lacks water of crystallization and is white. Dropping water onto
it replaces the water of crystallization and turns it blue.

1
anhydrous copper(II)sulfate + water → hydrated copper(II)sulfate

CuSO4(s) + 5H2O(l) → CuSO4.5H2O(s)

white blue

This test works for anything that contains water, e.g. any solution or acids. But it does not show
that the water is pure.

Physical Test to show that water is pure(Pg-192):

Pure water freezes at exactly 0oC and boils at exactly 100oC at 1 atmosphere pressure.

If the water is impure, it will usually freeze at a lower temperature and boil at a higher
temperature.

Testing for ions (Pg-192):

Flame Test (Test for few cations: Na+, Li+, K+, Ca2+, Cu2+ ):

Procedure: A platinum or nichrome wire is dipped into concentrated hydrochloric acid and
then into the salt sample, so that some salt sticks on the end. The wire and the salt are then held
just within a non-luminous (roaring) Bunsen burner flame and the colour of the flame is
observed.

The following table shows the flame colours of the cations present in the salt.

Cation Flame colour

Testing for Cations (Positive ions) using sodium hydroxide solution (Pg-193):

1.Test for copper(II) ions: To an aqueous solution of the sample, few drops of sodium
hydroxide solution is added. A blue precipitate of copper(II)hydroxide is produced which
indicates the presence of copper(II) ions, Cu2+.

Chemical equation: CuSO4(aq) + 2NaOH(aq) → Cu(OH)2(s) + Na2SO4(aq)

2
Ionic equation: Cu2+(aq) + 2OH-(aq) → Cu(OH)2(s)

2. Test for iron(II) ions: To an aqueous solution of the sample, few drops of sodium hydroxide
solution is added. A green precipitate of iron(II)hydroxide is produced which indicates the
presence of iron(II) ions, Fe2+.

Chemical equation: FeSO4(aq) + 2NaOH(aq) → Fe(OH)2(s) + Na2SO4(aq)

Ionic equation: Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)

Note: The green precipitate darkens on standing and turns orange-brown around the top of the
tube. This is due to the iron(II)hydroxide being oxidized to iron(III)hydroxide by the air.

3. Test for iron(III) ions: To an aqueous solution of the sample, few drops of sodium
hydroxide solution is added. An orange-brown precipitate of iron(III)hydroxide is produced
which indicates the presence of iron(III) ions, Fe3+.

Chemical equation: FeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq)

Ionic equation: Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

4. Test for ammonium ions: To an aqueous solution of the sample or to a solid sample, few
drops of sodium hydroxide solution is added. No precipitate is formed. Ammonia gas is
produced. On warming, the gas is tested with a piece of damp red litmus paper or universal
indicator paper. Ammonia is alkaline and turns the litmus paper/universal indicator blue. This
indicates the presence of ammonium ions, NH4+.

Chemical equation: NH4Cl(s) + NaOH(aq) → NaCl(aq) + NH3(g) + H2O(l)

Ionic equation: NH4+(aq) + OH-(aq) → NH3(g) + H2O(l)

Study Table 18.1 from Pg-194.

Testing for Anions (Negative ions) (Pg-195):

1.Test for carbonates CO32-: To a solid sample, few drops of dilute hydrochloric acid is added.
A colourless gas is produced which is bubbled through limewater. The limewater turns cloudy
due to the formation of white precipitate of calcium carbonate. This shows that the gas is carbon
dioxide and the sample contains carbonate ions, CO32-.

For example, using zinc carbonate and dilute nitric acid:

3
Chemical equation: ZnCO3(s) + 2HNO3(aq) → Zn(NO3)2(aq) + CO2(g) + H2O(l)

Ionic equation: CO32- (s) + 2H+(aq) → CO2(g) + H2O(l)

2. Test for sulfates SO42-: To an aqueous solution of the sample, few drops of dilute
hydrochloric acid followed by barium chloride solution is added. A white precipitate of barium
sulfate is produced which indicates the presence of sulfate ions, SO42-.

Ionic equation: Ba2+(aq) + SO42- (s) → BaSO4(s)

Note: Here hydrochloric acid should be added at first before adding barium chloride solution
so that the acid reacts with the solution and removes other anions which might also produce
white precipitate.

Read Testing for sulfates from Pg-195.

3. Test for Chlorides (Cl-), Bromides (Br-), Iodides (I-):

Test for Chlorides (Cl-):To an aqueous solution of the sample, dilute nitric acid followed by
silver nitrate solution is added. A white precipitate of silver chloride is formed which shows
the presence of chloride ions.

Ionic equation: Ag+(aq) + Cl-(aq) → AgCl(s)

Test for Bromides (Br-):To an aqueous solution of the sample, dilute nitric acid followed by
silver nitrate solution is added. A cream precipitate of silver bromide is formed which shows
the presence of bromide ions.

Ionic equation: Ag+(aq) + Br-(aq) → AgBr(s)

Test for Iodides (I-):To an aqueous solution of the sample, dilute nitric acid followed by silver
nitrate solution is added. A yellow precipitate of silver iodide is formed which shows the
presence of iodide ions.

Ionic equation: Ag+(aq) + I-(aq) → AgI(s)

Study Table 18.2 from Pg-196.

4
Question Answers (Pg-196):

1.(a) chlorine (b) ammonia (c) carbon dioxide (d) hydrogen (e) oxygen

2. (a) A nichrome or platinum wire is cleaned by dipping it into concentrated hydrochloric acid
and then into a flame until no colour shows. The wire is then moistened with concentrated
hydrochloric acid, dipped into the solid, and then put into non-luminous flame. Lithium ions
give a red flame colour.

(b) The solid is warmed very gently with sodium hydroxide solution. Any gases that is given
off is tested with a piece of damp red litmus paper. If it turns blue, ammonia is being given off
from an ammonium compound.

(c) To an aqueous solution of the sample, few drops of dilute hydrochloric acid followed by
barium chloride solution is added. A white precipitate of barium sulfate is produced which
indicates the presence of sulfate ions, SO42-.

(d) To a solid sample, few drops of dilute hydrochloric acid is added. A colourless gas is
produced which is bubbled through limewater. The limewater turns cloudy due to the formation
of white precipitate of calcium carbonate. This shows that the gas is carbon dioxide and the
sample contains carbonate ions, CO32-.

(e) To an aqueous solution of the sample, dilute nitric acid followed by silver nitrate solution
is added. A yellow precipitate of silver iodide is formed which shows the presence of iodide
ions.

3.(a) A= iron(III)chloride B= iron(III)hydroxide

C= silver chloride

(b) B: FeCl3(aq) + 3NaOH(aq) → Fe(OH)3(s) + 3NaCl(aq) OR

Fe3+(aq) + 3OH-(aq) → Fe(OH)3(s)

C: Ag+(aq) + Cl-(aq) → AgCl(s) OR

3AgNO3(aq) + FeCl3(aq) → 3AgCl(s) + Fe(NO3)3(aq)

4.(a) D= iron(II)sulfate E= iron(II)hydroxide F= barium sulfate

(b) E: FeSO4(aq) + 2NaOH(aq) → Fe(OH)2(s) + Na2SO4(aq) OR

Fe2+(aq) + 2OH-(aq) → Fe(OH)2(s)

5
F: Ba2+(aq) + SO42- (s) → BaSO4(s) OR

BaCl2(aq) + MgSO4(aq) → BaSO4(s) + MgCl2(aq)

5.(a) G= potassium carbonate H= potassium nitrate I= carbon dioxide

(b) K2CO3(s) + 2HNO3(aq) → 2KNO3(aq) + CO2(g) + H2O(l) OR

CO32- (s) + 2H+(aq) → CO2(g) + H2O(l)

6.(a) J= ammonium bromide K= ammonia L= silver bromide

(b) Ag+(aq) + Br-(aq) → AgBr(s)

8.(a) A few drops of the liquid is added to some anhydrous copper(II)sulfate. If the colour
changes from white to blue, then water is present.

(b) The boiling point or the freezing point/melting point of the liquid must be measured. If the
liquid boils at exactly 100oC or freezes/melts at exactly 0 oC, then it is pure water.

The End!