Flame tests Flashcards
(15 cards)
1
Q
Flame test steps?
A
- Clean nichrome wire by dipping & burning in conc. HCl & flame
- dip nichrome wire in conc. HCl
- dip into sample
- place loop into blue flame & observe colour
2
Q
2 Problems w flame test?
A
- many compounds contain small amount of sodium compounds as impurities so - intense colour of sodium can mask other colours
- describing colours w words is subjective - ppl hv diff levels of colour vision - may mean diff colours to diff ppl
3
Q
Li+
A
red
4
Q
Na+
A
yellow
5
Q
K+
A
lilac
6
Q
Rb+
A
red
7
Q
Cs+
A
blue
8
Q
Be2+ & Mg2+
A
no colour
9
Q
Ca2+
A
red (brick)
10
Q
Sr2+
A
red (crimson)
11
Q
Ba2+
A
green (apple)
12
Q
What causes the colours in flame tests?
A
- electron absorb energy from flame
- causes it to move to higher energy level
- aka from ground state to excited state
- electron returns to its ground state - releasing energy
(if energy corresponds to radiation in visible spectrum - characteristic colour appears/electron transition corresponds to wavelength) (electron trans in Mg corresponds to wavelength outside spectrum so - no colour)
13
Q
*ground state v excited state?
A
- atom w all its electrons in their lowest possible energy levels v…
- if electron moves to a higher energy level
14
Q
Test for ammonium ions?
A
*no colour in flame test
- add NaOH sol
NH4+ + OH- –> NH3 + H2O
- & warm mixture - releases ammonia gas
- NH3 gas can be recognised by smell but test:
- use damp red litmus paper –> turns blue (ammonia only common alkaline gas) OR…
- hydrogen chloride gas reacts w NH3 to form white fumes of ammonium chloride…
NH3 + HCl —> NH4Cl
15
Q
Why use conc. HCl in flame tests?
A
to convert any metal compound to a chloride - chlorides r more volatile than other salts so - more likely to give better results
