Test ions

Question 1

Test for CARBONATE ION

Answer 1

• Add dilute acid (HNO3) to the sample
• Carbon dioxide will be produced

To test for CO2:
- Bubble the gas produced into lime water, Ca(OH)2(aq)
- CO2 and calcium hydroxide will react to produce calcium carbonate and water.
- The limewater turns CLOUDY.

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

Question 2

Test for SULFATE ION

Answer 2

• Add a reagent containing Ba2+(aq)

Examples ~ BaCl2(aq) or Ba(NO3)2(aq)

• The Ba2+ forms a highly soluble WHITE PRECIPITATE when it reacts with SO42-(aq)

Question 3

Ammonia Solution

Answer 3

• The test for the ammonium ion , NH4+ is based on producing AMMONIA GAS , NH3.
• Ammonia gas is one of the few gases which DISSOLVES IN WATER to make an ALKALI (ammonium hydroxide):

NH3(g) + H2O(l) ———– NH4+(aq) + OH-(aq)

• DAMP LITMUS PAPER is used to test for ammonium hydroxide (ammonium solution).
• Turns BLUE

Question 4

Test for AMMONIUM ION

Answer 4

• Sample containing ammonium ion is WARMED with sodium hydroxide
• AMMONIA GAS is given off which turns damp litmus paper BLUE.

NH4+(aq) + OH-(aq) ———– NH3(g) + H2O(l)

Question 5

Sequence for testing for ANIONS

Answer 5

TEST 1 ~ CARBONATE ION:
- Add dilute acid
- effervescence produced

TEST 2~ SULFATE ION:
- Add Ba2+(aq)
- White precipitate produced

TEST 3 ~ HALIDE ION
- Add AgNO3(aq) and NH3(aq)
- Chloride ~ white ppt & dissolves in dil NH3
- Bromide ~ cream ppt & dissolves in c.NH3
- Iodide ~ yellow ppt & insoluble in c.NH3

Question 6

Why do ANIONS have to be tested in a specific ORDER

Answer 6

• Carbonate & sulfate ions can form a precipitate when AgNO3 is added.
• Ag2CO3(s) is a yellow precipitate
• Ag2SO4(s) is a white precipitate
• This will give INCORRECT ANALYSIS

Question 7

Sequence for testing for anions when given a SINGLE SAMPLE of a mixture

Answer 7

TEST 1 ~ HNO3(aq)
- HCl would introduce chloride ions
- H2SO4 would introduce sulfate ions
- Both giving a false positive result

TEST 2 ~ Ba(NO3)2(aq)
- BaCl2 would introduce chloride ions giving a false positive result.

• Filter solution to remove precipitate before carrying out test 3 as normal.

Question 8

Flame test colours & how it works

Answer 8

• Energy from the bunsen flame excites electron to a higher energy shell.
• Electron returns to normal lower energy level after excitation.
• Due to the energy gap, some elements give out light in the visible part of the electromagnetic spectrum.

Na+ ~ yellow/orange
K+ ~ lilac
Cu2+ ~ blue/green
Sr2+ ~ crimson
Ca2+ ~ brick red
Ba2+ ~ light green
Li+ ~ red
Zn2+ ~ no colour
Mg2+ ~ no colour
Cu2+ ~ blue