Identufying Substances Flashcards
What are metal ions also known as?
Cations
What are the two methods to test for positive ions (cations)?
Flame test
Sodium hydroxide
What is the flame test?
- Dip nichrome loop in concentrated hydrochloric acid then heat in Bunsen burger flame to clean it
- Dip in acid again then in the metal compound
- Put loop in blue flame and observe colour
Lithium = Crimson , Sodium = Golden/yellow , potassium = lilac , calcium = brick red , copper = green
What colour flame does calcium produce - Ca2+?
Brick Red
What colour flame does copper produce - Cu2+?
Green
What colour flame does lithium produce - Li+?
Crimson/bright red
What colour flame does sodium produce - Na+?
Yellow
What colour flame does potassium produce - K+?
Lilac
What’s the sodium hydroxide test?
- Dissolve half a spatula of powder (you are testing) in 1cm3 of distilled water
- Add 2cm3 of NaOH - sodium hydroxide
- Swirl and observe for precipitate:
White, Brown, green-brown,blue
If still white add more NaOH
If the precipitate is white and extra sodium hydroxide dissolves the precipitate what cation will be present?
Al3+ -> Aluminim
If the precipitate is white and doesn’t dissolve with extra sodium hydroxide what cations will be present?
Mg2+ -> Magnesium
Ca2+ -> Calcium
This can be distinguished using a flame test - if there is no colour magnesium is present but if there is a orange red flame calcium is present
If the precipitate is light blue what cations will be present ?
Cu2+ -> copper
If the precipitate formed is reddish-brown what cation is present ?
Fe3+. -> Iron
If the precipitate formed is green what cation is present?
Fe2+. -> Iron
Why does iron hydroxide turn from green to brown in an open test tube (sodium hydroxide test)?
The iron 2+ has been oxidised to make iron 3+ which results in the brown colout
How do you test for halide ions?
- add a few drops of your compound
- Add nitric acid - removes carbonate ions
- Add a few drops of silver nitrate
Chloride ions = white, Bromide ions = cream, Iodide ions = pale yellow
When silver nitrate and nitric acid react with chloride ions what colour precipitate forms?
Chloride ions = white precipitate
When silver nitrate and nitric acid react with bromide ions what colour precipitate forms?
Bromide ions = cream precipitate
When silver nitrate and nitric acid react with iodide ions what colour precipitate forms?
Iodide ions = yellow precipitate
How do you test for carbonate ions?
Add dilute acid (hydrochloric) to compound
Positive result = bubbles of carbon dioxide gas are seen or limewater turns cloudy
What’s the positive result of a carbonate ions presence?
Bubbles are produced or limewater turns cloudy
How do you test for sulfate ions?
Add half a spatula of powder in 1 cm3 of distilled water
Add 1cm3 of hydrochloric acid
Add 1cm3 of barium chloride and agitate
What’s a positive result for the presence of sulphate ions?
A white precipitate is formed
Ba2+ + SO4 2- —> BaSO4
What are instrumental analysis methods?
Gas chromatography
Mass spectroscopy
Flame emission spectroscopy
What is instrumental analysis used in?
Industry (quality control)
Law enforcement forensics
Advantages of instrumental analysis?
Precise, accurate, sensitive
Quick
Can test very small samples
Disadvantages of instrumental analysis?
Expensive
Need special training to operate machines
Need reference data
What does flame emission spectroscopy do?
Analyse metal ions in a solution
What does spectroscopy do?
Measure specific wavelengths of the light emitted to identify the metal ion and it’s concentration on the lined spectrum.
Every ion emits light at a specific wavelength that is unique to that ion
