Unit 3

Question 1

If J contained a carbonate anion, what would be the observation on the addition of dilute HCl?

Answer 1

Effervescence, fizzing, bubbling

Question 2

How to detect HCO3- (carbonate ions) / CO32- (hydrogencarbonate ions)

Answer 2

• When they react with acids, they release CO2 gas
• If the ions are present in a solution or solid, fizzing / effervescence / bubbling will occur in addition to acid

Question 3

When dilute nitric acid was added to a sample of solid x, no reaction occured. Suggest the name or formula of an anion that oculd not be present in x.

Answer 3

CO32-

Question 4

Suggest the steps in carrying out a flame test

Answer 4

1. Use a platinum wire, nichrome wire, or silica rod
2. Clean the wire with conc. HCl
3. Add few drops of conc. HCl to a test tube containing the sample and mix it well to dissolve the metal
4. Dip the wire into the sample
5. Transfer the wire to the non-luminous part of the flame and observe the colour produced

Question 5

Explain the origin of colour in flame test

Answer 5

• Electrons absorb energy from the heat and transition to higher energy levels
• Electrosn then return to ground state
• Photon is emitted with wave length corresponding to a colour in the visible region of the electromagnetic spectrum

Question 6

Explain why some cations show no flame colour in flame test

Answer 6

• They emit photons whose wavelength is not in the visible region of the electromagnetic spectrum

Question 7

Flame test colour for Lithium

Answer 7

Red

Question 8

Flame test colour for Sodium

Answer 8

Yellow

Question 9

Flame test colour for Potassium

Answer 9

Lilac

Question 10

Flame test colour for Rubidium

Answer 10

Red / purple

Question 11

Flame test colour for Caesium

Answer 11

Blue / Violet

Question 12

Flame test colour for Berylium

Answer 12

No colour

Question 13

Flame test colour for Magnesium

Answer 13

No colour

Question 14

Flame test colour for Calcium

Answer 14

Brick-red

Question 15

Flame test colour for Strontium

Answer 15

Crimson red

Question 16

Flame test colour for Barium

Answer 16

Apple green

Question 17

Suggest a test to test for the presence of ammonium cation

Answer 17

1. first method
   - add NaOH and warm
   - gas is evolved that turns damp red litmus paper blue (due to formation of ammonia gas)
2. second method
   - put conc. HCl in a test tube and hold it towards the test tube evolving ammonia
   - HCl gas will combine with ammonia to form white smoke (ammonium chloride)

note that ammonia is the only alkaline gas

Question 18

Flame test colour for strontium

Answer 18

red