Topic 10 - Tests for Ions Flashcards
What are flame tests used for
To identify metal ions in substances
How do you carry out a flame test?
- light a Bunsen burner and open the air hole to give a hot blue flame
- pick up a small sample of the test substance using a wire loop
- Hold the sample at the top of the flame and observe the flame colour
What is the wire loop cleaned in?
Hydrochloric acid
What metal is the wire loop and why?
Platinum wire -> it has a high melting point and it is unreactive -> It also gives no colour to the flame
What is a cheaper wire that could be used?
nichrome alloy -> although it produces its own orange colour it is much cheaper
What are the cations that are tested?
- lithium
- potassium
- sodium
- calcium
- copper
What colour does the flame go for the cation - lithium?
Red
What colour does the flame go for the cation - sodium?
Yellow
What colour does the flame go for the cation - potassium?
Lilac
What colour does the flame go for the cation - calcium?
Orange-red
What colour does the flame go for the cation - copper?
Blue-green
How does using scientific instruments to observe flame test improve the experiment?
- sensitivity (they can detect much smaller amounts)
- accuracy (they give values closer to the true values)
- Speed
Flame photometer
Measure the light intensity of the flame colours produced by metal ions
-> it’s data is used to determine the concentration of a metal ion in a dilute solution
How does a flame photometer create accurate results of colours?
It separates these colours to produce a spectrum of the light emitted by each metal ion
-> different metal ions produce different emission spectra
-> a metal ion in an unknown solution could be identified by matching its spectrum to the spectrum from a known metal ion
precipitation reactions involving sodium hydroxide
they form the basis of a text to identify dissolved metal ions
