Revision Notes for Detection of Gases and Dry Test for Salts Class 10 Chemistry ICSE

ICSE Revision Notes for Detection of Gases and Dry Test for Salts Class 10 Chemistry

Chapter Name	Practical Work
Topics Covered	Detection of Some Common Gases  Dry Tests for Salts Identification of Ions
Related Study	Selina Chemistry ICSE Solutions for Class 10 Frank ICSE Solutions for Class 10 Chemistry

Detection of Some Common Gases 

Test for hydrogen (H₂)

Hydrogen gas is liberated when active metals such as Na, K, Mg react with dilute acids.

M + H₂SO₄  → MSO₄ + H₂

M + HCl → MCl + H₂

[M = Ca, Mg, Zn, etc.]

Few characteristics of this gas are as follows:

1. It is a colourless and odourless gas.
2. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that hydrogen gas is neutral to litmus.
3. A burning wooden splint, when brought near this gas, gets off and burns with a pale blue flame producing a pop sound.
   2 H₂ + O₂ → 2 H₂O 

Test for oxygen (O₂):

Oxygen gas is liberated on heating metal nitrates, potassium chlorate, potassium dichromate, potassium permanganate, hydrogen peroxide, barium peroxide and oxides such as HgO, PbO₂, Pb₃O₄. 

Few characteristics of this gas are as follows:

1. It is a colourless and odourless gas. 
2. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that oxygen gas is neutral to litmus. 
3. A burning wooden splint when brought near this gas re-lights brightly which shows that it is a supporter of combustion.

Test for water vapour (H₂O): 

Water vapour is liberated on heating salts containing water of crystallisation, metallic hydroxides and metallic hydrogen carbonates. 

Few characteristics of this gas are as follows:

1. It is a colourless and odourless gas. 
2. When this gas is allowed to pass through a moist red or blue litmus paper, the colour of the paper does not change. This shows that water vapour is neutral to litmus. 
3. It turns anhydrous copper sulphate to blue.
   CuSO₄ + 5 H₂O → CuSO₄. 5H₂O
4. It turns blue copper chloride to pink.
   CoCl₂ + 2H₂O → CoCl₂. H₂O 

Test for ammonia (NH₃): 

Ammonia is liberated by heating ammonium salts with alkalies and treating metallic nitrides with warm water. 

2 NH₄Cl + Ca(OH)₂ → CaCl₂ + 2 H₂O + 2 NH₃

Few characteristics of this gas are as follows:

1. It is a colourless gas with a strong biting odour that brings tears to eyes. 
2. When this gas is allowed to pass through a moist red litmus paper, the colour of the paper changes to blue. This shows that ammonia is basic in nature. 
3. Dense white fumes are formed when a rod dipped in HCl is brought near this gas.
   NH₃ + HCl → NH₄Cl
4. It turns Nessler's reagent (K₂HgI₄) brown.
5. It forms a pale blue precipitate when passed through copper sulphate solution. This precipitate is soluble in excess of the gas and the solution turns dark blue in colour.

Test for carbon dioxide (CO₂): 

Carbon dioxide is liberated by strong heating of metallic carbonates and hydrogen carbonates. It is also liberated when dilute mineral acids are treated with carbonates and hydrogen carbonates of metals. 

CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Few characteristics of this gas are as follows:

1. It is a colourless and odourless gas. 
2. It turns lime water milky.
   Ca(OH)₂ + CO₂ → CaCO₃ + H₂O 
3. When this gas is allowed to pass through a moist blue litmus paper, the colour of the paper changes to light red. This shows that carbon dioxide is acidic in nature. 
4. A burning wooden splint when brought near this gas goes off which shows that it is not a supporter of combustion. 
5. It has no effect on filter paper dipped in acidified potassium dichromate (K₂Cr₂O₇) or potassium permanganate (KMnO₄) solution. 

Test for sulphur dioxide (SO₂): 

Sulphur dioxide is liberated by strong heating of metallic sulphites and hydrogen sulphites. It is also liberated when dilute mineral acids are treated with sulphites and hydrogen sulphites of metals.

Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂

2NaHSO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2SO₂

Few Characteristics gas are as follows:

1. It is a colourless gas having suffocating odour.
2. It turns lime water milky.
   Ca(OH)₂ + SO₂ → CaSO₃ + H₂O
3. When this gas is allowed to pass through a moist blue litmus paper, the colour of the paper changes to red. This shows that sulphur dioxide is acidic in nature. 
4. It turns potassium permanganate solution colourless.
   2 KMnO₄ + 2H₂O + 5 SO₂ → K₂ SO₄ + 2MnSO₄ + 2 H₂SO₄
   It changes the colour of acidified potassium dichromate from orange to green.
   K₂Cr₂ O₇ + H₂SO₄ + 3SO₂ → Cr₂ (SO₄)₃ + K₂SO₄ + H₂O 

Test for hydrogen sulphide (H₂S): 

Hydrogen sulphide is liberated by the action of dil. HCl or dil. H₂SO₄ on metallic sulphides. 

FeS + H₂SO₄  → FeSO₄ + H₂S 

Few characteristics of this gas are as follows:

1. It is a colourless gas having rotten egg like smell. 
2. It turns lead acetate solution silvery black.
   (CH₃ COO)₂ Pb + H₂S → PbS + 2CH₃COOH
3. It turns moist blue litmus paper red. This shows that it is acidic in nature. 
4. It turns lead nitrate solution black in colour.
   Pb(NO₃)₂ + H₂S → PbS + 2 HNO₃ 

Test for nitrogen dioxide (NO₂): 

Nitrogen dioxide is liberated by heating metal nitrates. 

Few characteristics of this gas are as follows:

1. It is a reddish-brown in colour. 
2. It has pungent and irritating odour. 
3. It turns moist blue litmus paper red. This shows that it is acidic in nature.
4. It turns moist potassium iodide paper brown.
   2 KI + 2 NO₂ → 2 KNO₂ + I₂ 
5. It turns acidified ferrous sulphate solution from green to brown. 

Test for chlorine (Cl₂):

Chlorine is liberated by the action of conc. HCl on oxidising agents like Pb₃O₄, PbO₂, MnO₂, etc.

MnO₂ + 4 HCl → MnCl₂ + 2 H₂O + Cl₂ 

Few characteristics of this gas are as follows:

1. It is a greenish-yellow in colour.
2. It has sharp pungent choking odour.
3. It turns moist blue litmus paper red followed by bleaching it. This shows that it is acidic in nature. 
4. It turns moist starch iodide paper blue black.
   Cl₂ + 2KI → 2KCl + I₂
   I₂ + Starch → Blue – Black colour
5. It forms a white precipitate when passed through silver nitrate solution.
   3 Cl₂ + 5AgNO₃ + 3H₂O → 5 AgCl + HClO₃ + 5 HNO₃

Test for hydrogen Chloride (HCl) : 

NaCl + H₂SO₄ → NaHSO₄ + HCl

Few characteristics of this gas are as follows: 

1. It is colourless.
2. It has pungent choking odour.
3. It turns moist blue litmus paper red. 
4. It produced dense white fumes when a rod dipped in ammonia solution is brought near the gas.
   NH₃ + HCl → NH₄Cl 
5. It forms a white precipitate when passed through silver nitrate solution. This precipitate is soluble in excess of ammonium hydroxide solution.
   HCl + AgNO₃ → AgCl + HNO₃

Dry Tests for Salts

The Preliminary tests for the qualitative analysis of an inorganic salt involve the observation of the following physical properties:

 1. Colour and Odour

Physical Property	Experiment	Observation	Inference
Colour	Observe colour of the salt.	Pink Blue Light green Dark Brown Flesh colour White	Co2+ Cu2+ Fe2+ Fe3+ Mn2+ Pb2+, Zn2+, Ca2+, Na+, K+, NH+4
Odour	Rub a pinch of salt between the fingres with a drop of water	Ammoniacal smell Vinegar like smell Rotten egg-like smell Smell of sulphur dioxide gas	NH4+ CH3COO- S2- SO2-3

2. Dry heating Test 

	Observation/gas evolved	Inference
1.	CO2 gas:- A colourless and odourless gas which turns like lime water milky.	CO32- or C2O42-
2.	H2S gas:- Colourless gas with smell like a rotten egg turns lead acetate paper black.	S2-
3.	SO2 gas:- Colourless gas with smell like burning sulphur, turns acidified potassium dichromate paper green.	SO32-
4.	HCl gas:- Colourless gas with a pungent smell forms white fumes with white ammonia and white ppt. with silver nitrate.	Cl-
5.	Colourless gas with vinegar like smell	CH3COO-
6.	NH3 gas :- Colourless gas with characteristic smell turns Nessler’s reagent brown.	NH4+
7.	NO2 gas :- Reddish brown gas turns ferrous sulphate solution black.	NO2– or NO3-
8.	Br2 gas :- Reddish brown vapours.	Br
9.	I2 gas :- Dark violet vapours.	I-
10.	O2 gas :- Supports combustion, glowing wooden splinter burns.	O2-
11.	H2O vapours :- Droplets of water on the cooler part of the test tube	Hydrated salt

3. Flame test

In order to perform the flame test, the paste of salt with conc. HCl is introduced into the flame with the help of platinum wire and the colour of the flame is observed.

	Colour of flame	Inference
1.	Brick red	Calcium
2.	Crimson red	Strontium
3.	Grassy-green	Barium
4.	Bright-bluish green	Copper
5.	Green flashes	Zn or Mn
6.	Bull bluish	Lead

4. Solubility test

The following table represents solubility of various salts in water. 

Identification of Ions

Salts are composed of cations and anions. They are dissolved in the following solvents to make solutions: 

• Water
• Nitric acid (if the salt does not dissolve in water)

Identification of cations:

• by using of sodium hydroxide solution
• by using of potassium hydroxide solution
• by using of ammonia solution

Metal	Sodium Hydroxide solution		Ammonium Solution
	Colour of ppt	With Excess Sodium Hydroxide Solution	Colour of ppt	With Excess Ammonium Hydroxide Solution
Ca	White curdy	Insoluble	No ppt.	No change
Pb	White chalky	Soluble	White Chalky	Insoluble
Zn	White gelatinous	Soluble	White gelatinous	Soluble
Cu	Pale blue	Insoluble	Pale blue	Soluble
Fe (II)	Pale green turning brown	Insoluble	Pale green turning brown	Insoluble
Fe(III)	Reddish brown	Insoluble	Reddish brown	Insoluble

Identification of anions:

• by using dilute sulphuric acid
• by using concentrated sulphuric acid
• by using nitric acid and barium chloride

Dil. H₂SO₄ Tests

	Experiment	Observation	Inference
1.	Salt + dil. H2SO4	Brisk effervescence Colourless gas Odourless gas Does not support combustion Turns moist litmus red  Turns lime water milky	The gas evolved is carbon dioxide Salt contains carbonate
2.	Salt + dil. H2SO4	Rotten egg smelling gas  Turns moist blue litmus paper red Turns moist lead acetate paper black	The gas evolved is hydrogen sulphidare salt contains sulphide (S2-)
3.	Salt + dil. H2SO4	Suffocating gas Turns golden yellow or orange coloured filter paper moist with acidified potassium dichromate green	The gas evolved is sulphur dioxide salt contains sulphite

Conc.H₂SO₄ Tests

	Experiment	Observation	Inference
1.	Salt + conc. H2SO4 + Heat	Colourless pungent gas (HCl) Produces dense white fumes when a glass rod dipped in ammonia solution is brought near it. Greenish yellow gas evolves that turns moist starch iodine paper blue-black when pinch of manganese is added to solution	Chloride ion confirmed (Cl-)
2.	Salt + conc. H2SO4 + Heat	Reddish brown fumes that turn thick on adding copper turnings A brown ring form at the junction of two liquids when iron (II) sulphate solution followed by conc. sulphuric acid is added to salt (Brown ring test)	The gas evolved is nitrogen dioxide Salt contains nitrate

Test for Sulphate Ion

	Experiment	Observation	Inference
1.	Salt + Nitric acid + barium chloride solution	White ppt insoluble in mineral acid When solution is treated with acetic acid and lead acetate solution, white ppt is obtained which is soluble in excess of ammonium acetate solution	Sulphate ion confirmed

Test to Differentiate Black Copper Oxide from Black Manganese Dioxide

Experiment	Copper Oxide	Manganese Dioxide
Sample (black powder) + conc. HCl + Heat	No evolution of chlorine gas	Evolution of greenish yellow coloured chlorine gas
Filter	Blue coloured filtrate	Brown coloured filtrate
Filtrate + Ammonium hydroxide	Formation of blue ppt. which is soluble in excess of ammonium hydroxide	No ppt.