4.1 5 Flame Tests And The Test For Ammonium Ions

Question 1

How to carry out a flame test

Answer 1

Wear safety glasses and lab coat within a fume cupboard light a Bunsen burner
Using a dropper add a few drops on concentrated hydrochloric acid and mix together so the metal compound begins to dissolve . Use to convert any metal compound into a chloride chlorides more volatile so give better results )
Dip a clean metal wire (platinum or nichrome ) or a silica dos into mixture to obtain a sample
Hold end of wire or rod in flame and observe the colour

Question 2

Problems with a flame test

Answer 2

Many compounds contain small amounts of sodium compounds as impurities so intense colour of sodium can mask others
Describing colours is subjective

Question 3

Lithium

Answer 3

Li+ red

Question 4

Sodium

Answer 4

Na+ yellow/oraneg

Question 5

Potassium

Answer 5

K+ lilac

Question 6

Rubidium

Answer 6

Rb+ red/purple

Question 7

Caesium

Answer 7

Cs + blue /violet

Question 8

Beryllium

Answer 8

Be2+ no colour

Question 9

Magnesium

Answer 9

Mg2+ no colour

Question 10

Calcium

Answer 10

Ca 2+ brick red

Question 11

Strontium

Answer 11

Sr2+ crimson red

Question 12

Barium

Answer 12

Ba2+ apple green

Question 13

Copper compounds

Answer 13

Blue-green

Question 14

What causes the colours in the flame tests

Answer 14

Electrons can absorb energy and move to higher energy levels movement immediately followed by return of electron back to its original energy level if correspond to radiation on visable light spectrum

Question 15

Ammonium ions

Answer 15

Cation does not give a colour in the flame test
Add to sodium hydroxide solution and warm the mixture
NH4+ + OH- = NH3 + H2O
Recognize ammonia gas by smell or use damp red litmus paper turns blue
Or hydrogen chloride gas reacts with ammonia to form white fumes of ammonium chloride
NH3+ HCl= NH4Cl