Test for Gases

Question 1

Describe the test to identify carbon dioxide gas. State the expected observation if carbon dioxide is present.

Answer 1

Bubble the gas evolved through limewater.

Brisk effervescence of a colourless, odourless gas that turns damp blue litmus paper red. A white precipitate is also formed with limewater.
CO2(g) + Ca(OH)2 (aq) -> CaCO3(s) + H2O(l)

Question 2

Describe the test to identify NH3. State the expected observation if NH3 is present.

Answer 2

Place a damp red litmus paper at the mouth of the test tube containing the gas.

Evolution of a colourless, pungent gas that turned damp red litmus paper blue.

Question 3

Describe the test to identify Cl2. State the expected observation if Cl2 is present.

Answer 3

Insert damp blue litmus paper into a test tube containing the gas.

Evolution of a greenish-yellow, pungent gas that turned damp blue litmus paper red, before bleaching it.

Question 4

Describe the test to identify hydrogen. State the expected observation if hydrogen is present.

Answer 4

Insert a lighted splint into a test tube containing the gas.

Effervescence of a colourless, odourless gas that extinguishes the lighted splint with a ‘pop’ sound.

Question 5

Describe the test to identify oxygen. State the expected observation if oxygen is present.

Answer 5

Insert a glowing splint into a test tube containing the gas.

Effervescence of a colourless, odourless gas that relights/rekindles the glowing splint.

Question 6

Describe the test to identify sulfur dioxide(SO2). State the expected observation if SO2 is present.

Answer 6

Test the gas evolved with acidified potassium dichromate(VI).

Evolution of a colourless, pungent gas that changes the colour of orange K2Cr2O7 (acidified potassium dichromate) to green.

OR

Test the gas evolved with acidified potassium manganate(VII).

Evolution of a colourless, pungent gas that changes the colour of purple potassium manganate to colourless.

Question 7

State the appearances of the following gases.
1: ammonia
2: carbon dioxide
3: chlorine
4: hydrogen
5: oxygen
6: sulfur dioxide
7: nitrogen dioxide (NO2)

Answer 7

1: colourless, pungent
2: colourless, odourless
3: pale green, pungent
4: colourless, odourless
5: colourless, odourless
6: colourless, pungent
7: brown, pungent

Question 8

State the likely sources that produce the following gases.
1: NH3
2: CO2
3: Cl2
4: O2
5: SO2
6: NO2

Answer 8

1: ammonium salt + alkali (NH4^+ + OH-), gas evolved on warming
2: acid + carbonate OR heating Group II carbonates
3: hydrochloric acid + oxidising agents
4: H2O2 + oxidising agents
5: acid + sulfite (SO3^2-)
6: acid + nitrite (NO2^-)