chemical analysis Flashcards
What is a pure substance in chemistry?
- In chemistry, a pure substance is a single element or compound, not mixed with any other substance.
How can we distinguish pure substances from mixtures?
- Melting point and boiling point data can be used to distinguish pure substances from mixtures
- This is because pure elements and compounds melt and boil at specific temperatures
How can you test the purity of a sample?
- By measuring its melting point and boiling point and comparing it to that of a pure substance
- The closer the measure value to the actual m.p/b.p the purer the sample
What happens when there are impurities in a sample?
- It will lower the melting point and increasing the melting range of the substance
- It will also increase the boiling point and may result in the sample boiling over a rang of temps
What is a pure substance in everyday language?
- In everyday language, a pure substance can mean a substance that has had nothing added to it, so it is unadulterated and in its natural state
e.g. pure milk.
What is a formulation?
- A formulation is a mixture (with a precise purpose) that has been designed as a useful product.
- Many products are complex mixtures in which each chemical has a particular purpose.
How are formulations made?
- Formulations are made by mixing the components in carefully measured quantities to ensure
that the product has the required properties.
Give some examples of formulations
- Formulations include fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
Why are formulations important in the pharmaceutical industry?
- By altering the formulation of a pill, chemists can make sure that it delivers the drug to the correct part of the body at the right concentration, that it’s consumable and has a long shelf life
What information shows use a product’s formulation?
- Ratio or percentage of each component
What can chromatography be used for?
- Chromatography can be used to separate mixtures and can give information to help identify substances.
What does separation depend on in chromatography?
- Separation depends on the distribution of substances between the phases.
What are the two phases in chromatography?
- Chromatography involves a stationary phase and a mobile phase
What is the mobile phase?
- Where molecules can move - this is always a liquid or a gas
- this is the solvent
What is the stationary phase?
- Where the molecules can’t move - this can be a solid or a really thick liquid
- this is the chromatography paper
Explain how chromotography works
1) During chromatography, the substances in the sample constantly move between the mobile and stationary phase - and equilibrium is formed between the two phases
2) The MP moves through the SP and anything dissolved in the MP moves with it
3) How quickly a chemical moves depends on how it’s distributed between the two phases
4) The chemicals that spend more time in the MP than SP will move further in the SP - the more soluble a substance is the further up the paper it goes
5) The components in a mixture will normally separate through the stationary phase so long as all the components spend different amounts of time in the mobile phase
6) The separated components form spots
7) A pure substance will only for one spot in any solvent since there is only one substance in the sample
What does the amount of time the molecules spend in each phase of paper chromatography depend on?
- How soluble they are in the solvent
- How attracted they are to the paper
How does solubility and attractation to paper affect chromatography results?
- Molecules with a higher solubility in the solvent and which are less attracted to the paper will spend more time in the mobile phase and will be carried further up the paper
What do you call the result of a chromatography analysis?
- a chromatogram
Describe an Rf value
- the ratio between the distance travelled by the dissolved substance (the solute) (Centre of spot from origin) and the distance travelled by the solvent
State the equation used to find the retention factor (Rf)
Rf = distance moved by substance/
distance moved by solvent
What’s the relationship between the Rf value and the distance travelled in the stationary phase?
- The further the stationary phase a substance moves, the larger the Rf value
How can we identify compounds using Rf values?
- Different compounds have different Rf values in different solvents, which can be used to help identify the compounds.
- The compounds in a mixture may separate into different spots depending on the solvent but a pure compound will produce a single spot in all solvents.
Explain how paper chromatography separates mixtures
explain how paper chromatography separates mixtures
Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances
- If one spot shows then the substance is pure
How do we test for hydrogen + positive result?
- The test for hydrogen uses a burning splint held at the open end of a test tube of the gas.
- Hydrogen burns rapidly with a pop sound. (the noises comes from the hydrogen burning quickly in the oxygen in the air to form water)
How do we test for oxygen + positive result?
- The test for oxygen uses a glowing splint inserted into a test tube of the gas.
- The splint relights in oxygen.
How do we test for carbon dioxide + positive result?
- The test for carbon dioxide uses an aqueous solution of calcium hydroxide (lime water).
- When carbon dioxide is shaken with or bubbled through limewater the limewater turns milky (cloudy)
How do we test for chlorine + positive result?
- The test for chlorine uses litmus paper.
- When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white. (this is because a solution of chlorine is acidic)
What can flame tests be used for?
- Flame tests can be used to identify some metal ions (cations).
- Lithium, sodium, potassium, calcium and copper compounds
produce distinctive colours in flame tests
How do you conduct a flame test?
- Clean a nichrome wire loop by rubbing with fine emery paper and then holding it in a blue flame
- Dip the nichrome loop into the sample you want to test and put it in the flame
- Record the colour of the flame
- You can use the colours to detect and identify different ions
How do you know the nichrome loop is clean?
- When you hold it in a blue flame and it doesn’t change colour
What flame colour is shown when lithium burns?
- Lithium compounds result in a crimson flame
What flame colour is shown when sodium burns?
- Sodium compounds result in a yellow flame
What flame colour is shown when potassium burns?
- Potassium compounds result in a lilac flame
What flame colour is shown when calcium burns?
- Calcium compounds result in an orange-red flame
What flame colour is shown when copper burns?
- Copper compounds result in a green flame
What happens when there is a sample containing a mixture of ions in a flame test?
- If a sample containing a mixture of ions is used some flame colours can be masked.
How do you test for (some) metal ions? (cation)
- By adding a few drops of sodium hydroxide solution - this may form a coloured insoluble hydroxide
- The sodium hydroxide solution can be used to identify some metal ions (cation)
What metal solutions form white precipitates when sodium hydroxide solution is added?
- Solutions of aluminium, calcium and magnesium ions form white
precipitates when sodium hydroxide solution is added
What metal solutions form coloured precipitates when sodium hydroxide solution is added?
- Copper(II), iron(II) and iron(III) ions form coloured precipitates when sodium hydroxide solution is added.
What does copper(II) give in the presence of NaOH?
- A blue precipitate is formed
What does iron(II) give in the presence of NaOH?
- A green precipitate is formed
What does iron (III) give in the presence of NaOH?
- A brown precipitate is formed
What does magnesium give in the presence of NaOH?
- a white precipitate is formed
What does aluminium give in the presence of NaOH?
- a white precipitate is formed at first, but then redissolves in excess NaOH to form a colourless solution
What does calcium give in the presence of NaOH?
- a white precipitate is formed
Which precipitate dissolves in excess sodium hydroxide solution?
- Only aluminium hydroxide precipitate dissolves in excess sodium
hydroxide solution.
How can you test for carbonate ions? (anion)
- Using a pipette to add a few drops of dilute acid - this causes the carbonates to react with the dilute acid to form carbon dioxide gas
How can carbon dioxide be identified?
- CO2 can be identified with limewater
- If carbonate ions are present, the reaction with acid will release CO2 which turns limewater cloudy
How do you test for sulfates? (anion)
- Use a pipette to add a couple of drops of dilute hydrochloric acid, followed by a few drops of barium chloride solution
How can sulfates be identified?
- If sulfates are present, a white precipitate of barium sulfate will form
- This is because sulfate ions in a solution produce a white precipitate with barium chloride solution in the presence of dilute HCl
When testing for sulfates, why is HCl added?
- To get rid of any traces of carbonate ions as this would also produce a precipitate and confuse the results
How do you test for halides?
- To identify a halide ion, add a few drops of dilute nitric acid followed by a few drops of silver nitrate solution
How do you identify a halide?
- Halide ions in solution will produce precipitates with silver nitrate solution in the presence of dilute nitric acid.
What does chloride produce in the presence of silver nitrate and dilute nitric acid?
- A white precipitate of silver chloride
What does bromide produce in the presence of silver nitrate and dilute nitric acid?
- A cream precipitate of silver bromide
What does iodide produce in the presence of silver nitrate and dilute nitric acid?
- A yellow precipitate of silver iodide
How can elements and compounds be detected?
- Elements and compounds can be detected and identified using instrumental methods.
What are the advantages of instrumental methods?
- Instrumental methods are:
- accurate - reliably identify elements and compounds
- sensitive - can detect very small amounts of a substance in a small mount of sample
- rapid
What are the disadvantages of instrumental methods?
- expensive
- less readily available
State advantages of instrumental methods compared with chemical tests
- More accurate
- More sensitive
- Faster
Define flame emission spectroscopy
- Flame emission spectroscopy is an example of an instrumental method used to analyse metal ions in solutions
- It can be used to identify different ions in mixtures
What happens during flame emission spectroscopy ?
- The sample is put into a flame and the light given out is passed through a spectroscope.
- As the ions heat up, their electrons move to higher energy levels
- When they drop back to their original energy levels, t hey release energy as light
- The light passes through a spectroscope which can detect different wavelngths of light to produce a line spectrum
What can the output of a flame emission spectroscopy tell us?
- The output is a line spectrum that can be analysed to identify the metal ions in the solution and measure their
concentrations.
What do the wavelengths emitted by an ion depend on?
- The combination of wavelengths emitted by an ion depends on its charge and its electron arrangement
What does the intensity of the spectrum indicate?
- The concentration of that ion in the solution
