C12 - Chemical Analysis Flashcards
What is a pure substance
A pure substance is one that is made up of only one substance. It can either be an element or a compound
Examples of pure substances
- Distilled water
- Helium
- Pure ethanol
What is special about a pure substances melting and boiling points?
They are fixed and exact
What do impurities do to a substance?
They generally lower the melting point and raise the boiling
How can you identify an impure substance?
They don’t have a set melting/ boiling point; they melt/boil over a range
What is a formulation
A formulation is a mixture that has been designed to give a useful product
Examples of formulations
- Medicinal drugs
- Paint
- Washing up liquid
- Cosmetics
- Fertilisers
What are the two different phases in chromatography
- The mobile phase (solvent)
- The stationary phase (usually paper )
What does a high affinity to the mobile phase do
The substance moves further up
What does a high affinity to the stationary phase do
The substance doesn’t move up as far
How to calculate the Rf (retention factor)
Distance moved by substance / distance moved by solvent
Test for hydrogen
Lightened splint will make a squeaky pop with hydrogen
Test for oxygen
- Use hydrogen peroxide to make oxygen
- Glowing splint will be relit
Test for carbon dioxide
Limewater (calcium hydroxide) goes cloudy due to carbonate precipitate forming
Test for chlorine
Damp blue litmus paper will turn white (get bleached)
How to carry out a flame test
- Dip nichrome wire in acid and flame to clean and in acid again
- Dip in metal
- The flame may change colour
Why is it hard to identity some colours in a flame test
Colours like potassium are weak colours and can be masked by stronger colours such as sodium
What ions form white precipitates with sodium hydroxide
- Aluminium ions
- Magnesium ions
- Calcium ions
What happens to an aluminium precipitate in excess NaOH
It dissolves unlike a calcium precipitate or a magnesium precipitate
What colour precipitate do copper (II) ions form in NaOH
Light blue
What colour precipitate do iron (II) ions form in NaOH
Green
What colour precipitate do iron (III) ions form in NaOH
Brown
Test for carbonates
- Add dilute HCl to sample
- It will fizz (due to CO₂) if it is a carbonate
- Gas can then be bubbled through limewater
Test for halides
- Pour the sodium halide into test tube
- Add nitric acid to remove impurities
- Add silver nitrate
Colour of precipitate that chlorine ions give in the halide test
White
Colour of precipitate that bromine ions give in the halide test
Cream
Colour of precipitate that iodine ions give in the halide test
Yellow
Test for sulphates
- Pour sulphate into test tube
- Add HCl to remove impurities
- Add barium chloride (if sulphates present, it will go white deu to white precipitate forming)
What colour flame do magnesium ions cause
No colour
Advantages of modern instrumental analysis methods
- Highly accurate and sensitive
- Quicker
- Only a small amount of sample needed
Disadvantages of modern instrumental analysis methods
- Usually very expensive
- Require special training to use
- Specialist training needed to interpret results
- Results can sometimes only be compared to a data book value
What happens during flame emission spectroscopy
- Energy provided by flame excites electrons
- They jump into higher energy levels
- When they go back down, they release light energy
Test for water vapour
- Use blue cobalt chloride paper
- IF water vapour is present, it will turn pink (hydrated)
Why is the start line drawn in pencil in chromatography
- If we drew it using ink, the ink would move up with the solvent
- This would affect the results and make them unreadable
How to use paper chromatography to identify an unknown substance
- Set up chromatography in standard way with the unknown chemical on the pencil start line
- Draw a line where the solvent got to (solvent front)
- Measure the distance moved by the unknown chemical
- Measure the distance moved by the solvent
- Use these values in the Rf value equation
- Look this Rf value up in a database but it may not be on there if the substance has never been analysed
- This may need to be repeated with different solvents
Why does the Rf value not have any units
Because it is a ratio of the distance moved by the substance compared to the distance moved by the solvent
