Chemical Analysis Flashcards
Test for Hydrogen
- Hold a lighted splint at the end of an open test tube
- If a popping sound it produced, hydrogen is present
Test for Oxygen
- Insert a glowing splint into the mouth of a flask
- If the splint relights, oxygen is present
Test for Carbon Dioxide
- Bubble carbon dioxide gas through limewater
- If the limewater turns cloudy, carbon dioxide is present
Test for Chlorine
- Hold a piece of damp blue litmus paper to the mouth of a boiling tube
- If the paper turns red then white, chlorine is present
Why does the litmus paper turn red before turning white?
- Chlorine dissolves in the water on the litmus paper
- Reacts with it to form HCL
- HCL is acid so it turns the paper red
Test for Cations (Flame Test)
- Take a nichrome wire loop and dip it in the unknown solution
- Hold the loop over a blue flame
- Observe what colour the flame turns
What colour does Lithium (Li+) produce?
Crimson
What colour does Sodium (Na+) produce?
Yellow
What colour does Potassium (K+) produce?
Lilac
What colour does Calcium (Ca2+) produce?
Orange-Red
What colour does copper (Cu2+) produce?
Green
Test for Cations (Metal Hydroxide Test)
- Add sodium hydroxide to the metal ion
- If a coloured precipitate forms it could be 1 one of 3 cations
- If a white precipitate forms, add excess sodium hydroxide
- If it dissolves, Al3+ is present, if not its Mg2+ or Ca2+
Metal Hydroxide Precipitate Colours
- Fe3+ - Brown
- Fe2+ - Green then turns brown
- Cu2+ - Light blue
Test for Carbonates
- Add dilute HCL to carbonates
- Run any gas produced in limewater through a tube
- If carbonates are present then the limewater will turn cloudy
Test for Sulfates
- Add barium chloride to dilute HCL
- If a white precipitate forms sulfate is present
Test for Halides
- Add silver nitrate to dilute nitric acid
- If a coloured precipitate forms, a halide is present
Halide Precipitate Colours
- Iodide (I-) - Yellow
- Bromide (Br-) - Cream
Chloride (Cl-) - White
What is a pure substance
- All the particles are the same
- Contains only one element/compound
- Has a fixed melting and boiling point
What is the difference between the term pure in advertising and a pure substance?
- Pure in advertising means the product has nothing added to it and its in its natural state
- A pure substance is made up of only one element or compound
What is a formulation?
A mixture of different components each with different jobs that work together to give the formulation specific properties
What is an emulsion?
A mixture of 2 things which won’t usually mix together and a special chemical called an emulsifier that makes them stick together, e.g. mayonnaise
How can you tell when a substance isn’t pure?
When it’s melting and boiling points are a range instead of a specific value
Physical Test
When you test the physical properties of a substance
What do impurities do to the melting and boiling points of substances?
- Decrease melting points
- Increase boiling points
Paper Chromatography
Used to separate mixtures of soluble substances
Paper Chromatography Method
- Draw a base line in PENCIL at the bottom of a sheet of filter paper
- Draw a dot of the ink sample on the base line
- Fill a beaker with a shallow amount of solvent (usually water or ethanol) making sure the ink and the base line don’t come into contact with it
- Place a lid on the beaker so the solvent doesn’t evaporate
- As the solvent seeps up the paper the present dyes dissolve in it and move up the paper
Chromatogram
The result of separating mixtures by chromatography
Mobile Phase
- A substance molecules can move in
- Is always a liquid or a gas
- In paper chromatography it’s the solvent
Stationary Phase
- A substance molecules can’t move in
- Is a solid but sometimes a thick liquid
- in paper chromatography it’s the paper
Where do soluble chemical spend more time?
The mobile phase (they move faster)
Where do insoluble chemical spend more time?
The stationary phase (they move slower)
RF Value
Distance travelled by substance/Distance travelled by solvent
What does the chromatogram of a pure substance look like?
- Doesn’t separate as it only contains one substance
- Leaves a single dot on the filter paper
- Different papers and solvents produce different RF values
Flame Emission Spectroscopy
- When metal ions are heated they emit light
- The wavelength of light emitted (and the colour emitted as a result) is specific for certain metal ions
Advantages of Flame Emission Spectroscopy
- If a sample contains multiple metal ions, the spectrum shows the wavelength of all of them
- Thickness of the lines show the concentration of the ion
- More accurate
- More sensitive
- Faster than regular flame test
Manual Tests vs Instrumental Methods
- Manual - Basic but cheaper
- Instrumental - Expensive but more accurate results
