Chemical Analysis (tests) Flashcards
(26 cards)
How do you test for hydrogen?
- test tube with unknown gas
- remove bung and insert burning splint
- hydrogen gas burns rapidly and produces a pop sound
How do you test for oxygen?
- test tube with unknown gas
- remove bung
- place glowing split into the test tube
- if it’s oxygen, it should relight
How do you test for carbon dioxide?
- test tube of unknown gas and the test tube of limewater
- drew some of the gas into a plastic pipette
- bubble the gas through limewater
- repeat this several times, the limewater will turn cloudy.
What is limewater?
Aqueous solution of calcium hydroxide.
How do you test for chlorine?
- insert damp litmus paper into the mouth of the test tube with the gas inside
- chlorine bleaches the litmus paper and turns it white
Describe how a flame test works:
- place a small amount of the chemical onto wire mounted in a handle
- place the end of this onto a blue bunsen burner flame
- colour of flame identifies the metal ion present
Flame test for lithium?
Crimson flame
Flame test for sodium?
Yellow flame
Flame test for potassium?
Lilac flame
Flame test for calcium?
Orange-red flame.
Flame test for copper?
Green flame
What are the disadvantages of flame tests?
- colour of the flame can be difficult to distinguish
- especially is there is a low concentration of the compound
- sample could contain a mixture of metal ions
- masks the colour of the flame
Describe how flame emission electroscope works:
- a sample of the metal ion in solution is placed into a flame
- the light given out is then passed into a spectroscope
- the spectroscope converts the light into a line spectrum
- the positions of the lines in the spectrum are specific for a given metal ion
- lines become more intense at higher concentrations
What are the advantages of instrumental methods?
- more rapid than flame tests
- sensitive so they will work even on a tiny sample of the metal compound
- accurate so they are more likely to identify the metal ion correctly than flame tests
What happens if we add sodium hydroxide solution to a solution of calcium/magnesium/aluminium ions?
White precipitate is formed.
What happens if you add excess sodium hydroxide solution to these solutions?
The aluminium precipitate will redissolve.
How do you work out which one of the precipitates are calcium?
Flame test
What happens if you add sodium hydroxide solution to a solution of copper (II) ions?
A blue precipitate of copper (II) hydroxide forms.
What happens if you add sodium hydroxide solution to a solution of iron (II) ions?
A green precipitate of iron (II) hydroxide is formed.
What happens if you add sodium hydroxide solution to a solution of iron (III) ions?
A brown precipitate of iron (III) hydroxide is formed.
How do you test for carbonates?
- Add dilute acid to the sample.
- The acid will react with the carbonates to make carbon dioxide gas. Fizzing. This doesn’t prove the presence of carbon dioxide.
- Bubble the gas through limewater. If it goes cloudy, then this proves that we have carbon dioxide.
- That means we started with the carbonate ion.
How do you test for halide ions?
- Add dilute nitric acid to the sample.
- Add silver nitrate solution.
- Halide ions produce a precipitate of the silver halide. Each halide makes a different coloured precipitate.
What do the chloride ions produce?
A white precipitate of silver chloride.
What do the bromide ions produce?
A cream precipitate of silver bromide.
