Chemical Analysis Flashcards
In chemistry, what is meant by a pure substance?
A single element or compound, not mixed with any other substance.
In everyday language, what is meant by a pure substance?
A substance that has had nothing added to it, so is in its natural state, e.g pure milk or beeswax.
How can we distinguish pure substances from mixtures?
By using melting point and boiling point data.
How can looking at melting and boiling point data help us in distinguishing pure substances from mixtures?
Pure elements and compounds melt and boil at specific temperatures, whereas impure substances or ‘mixtures’ melt and boil over a wide range of temperatures.
The closer a measured value is to the actual melting or boiling point of a substance, the _____ the sample is.
purer
What happens to the melting point of a substance if it contains impurities?
Impurities will lower the melting point of the substance.
For example, salt water has a melting point of -2°C rather than 0°C.
However, they will increase the melting range.
What happens to the boiling point of a substance if it contains impurities?
Impurities will increase the boiling point of the substance.
For example, salt water has a boiling point of 100.5°C rather than 100°C
They will also increase the boiling range.
What is a formulation?
A mixture that has been designed as a useful product.
What are many products? (in chemistry terms)
Complex mixtures in which each chemical has a particular purpose.
How are formulations made?
By mixing the components in carefully measured quantities to ensure that the product has the required properties.
Give examples of formulations
-Fuels
-Cleaning agents
-Paints
-Medicines
-Alloys
-Fertilisers
-Foods
-Drinks
What is paint?
A formulation composed of a pigment for colour, a solvent to alter viscosity, a binder to hold the pigment in place, and additives to further change the paint’s physical and chemical properties.
In real life, how can we tell if a product is a formulation?
-There will be information about its composition on the packaging (which could be in ratios or percentages)
-This allows us to choose formulations with the right compositions for a particular use
Explain how formulations are important in the pharmaceutical industry.
e.g. chemists altering the formulation of a pill to ensure it:
-delivers the drug to the correct part of the body
-is at the right concentration
-is consumable
-has a long enough shelf life
What is chromatography used for?
-To separate mixtures of different dyes in an ink
-This can help us to identify substances
What is the test for hydrogen?
A lit splint is held at the open end of a test tube of the gas.
Hydrogen burns rapidly with a squeaky pop sound.
(lit splint - squeaky pop)
What is the test for oxygen?
A glowing splint is inserted into a test tube of the gas.
The splint relights in oxygen.
(glowing splint - relights)
What is the test for carbon dioxide?
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 cloudy, or ‘milky’.
(Bubble through limewater - cloudy)
What is the test for chlorine gas?
The test for chlorine uses litmus paper.
When damp litmus paper is put into chlorine gas the litmus paper is bleached and turns white.
(Damp blue litmus paper - bleached white)
When testing for chlorine, the litmus paper might briefly turn red. Explain why.
The chlorine gas might dissolve in the water on the paper to form hydrochloric acid.
Litmus turns red in acidic solutions.
What manual methods can be used to identify metal ions? (cations because they’re positive)
-Flame tests
-Reacting them with sodium hydroxide solution
Which compounds produce distinctive colours in flame tests?
Lithium, sodium, potassium, calcium and copper.
Lithium ions (Li⁺) burn with a…
crimson flame
Sodium ions (Na⁺) burn with a…
yellow flame
Potassium ions (K⁺) burn with a…
lilac flame
Calcium ions (Ca²⁺) burn with an..
orange-red flame / ‘brick red’
Copper ions (Cu²⁺) burn with a…
green flame.
What is ‘species’ another word for in chemistry?
The chemical substance (Atom, molecule, or ions).
What happens in a flame test when a sample containing a mixture of ions is used?
Some flame colours can be masked. (hidden by others)
How do we carry out flame tests?
-Clean a platinum wire loop by dipping it in some dilute HCl and then holding it in a blue bunsen burner flame until it burns without any colour
-Then, dip the loop into the sample you want to test and put it back in the flame
-Record the colour of the flame
How to carry out the sodium hydroxide test for cations?
Add a few drops of sodium hydroxide solution to your sample.
A coloured insoluble hydroxide tells you which metal was in the compound.
Which solutions form white precipitates when sodium hydroxide solution is added?
Solutions of aluminium, calcium and magnesium ions.
Which solutions form coloured precipitates when sodium hydroxide solution is added?
Solutions of copper(II), iron(II) and iron(III) ions.
What colour do solutions containing copper (II) ions form when reacted with sodium hydroxide solution?
Blue (due to a blue precipitate)
What colour do solutions of iron(II) ions form when reacted with sodium hydroxide solution?
green (due to a green precipitate)
What colour do solutions of iron(III) ions form when reacted with sodium hydroxide solution?
Brown (due to a brown precipitate)
Why do we use the blue part of the flame in the flame tests?
It’s the hottest part and so we can easily see the colour.
Write the balanced half equation for the reaction of copper(II) ions and hydroxide ions.
Cu²⁺(aq) + 2OH⁻(aq) ➔ Cu(OH)₂ (s)
Write the balanced half equation for the reaction of calcium ions and hydroxide ions.
Ca²⁺ (aq) + 2OH⁻(aq) ➔ Ca(OH)₂(s)
Write the balanced half equation for the reaction of iron(II) ions and hydroxide ions.
Fe²⁺ (aq) + 2OH⁻ ➔ Fe(OH)₂ (s)
Write the balanced half equation for the reaction of iron(III) ions and hydroxide ions.
Fe³⁺(aq) + 3OH⁻(aq) ➔ Fe(OH)₃ (s)
Write the balanced half equation for the reaction of magnesium ions and hydroxide ions.
Mg²⁺(aq) + 2OH⁻(aq) ➔ Mg(OH)₂(s)
Write the balanced half equation for the reaction of aluminium ions and hydroxide ions.
Al³⁺(aq) + 3OH⁻(aq) ➔ Al(OH)₃ (s)
What do carbonates react with dilute acids to form?
Carbon dioxide gas, which can be identified with limewater.
What are the three types of anions (negative ions) we need to know how to test for?
-Halides
-Sulfates
-Carbonates
What is the test for halides?
-Add a few drops of dilute nitric acid to remove impurities such as carbonate ions
-Add a few drops of silver nitrate solution
What is the symbol for silver nitrate solution?
AgNO₃
Halide ions in solution produce precipitates when reacted with what?
silver nitrate solution
What colour is silver chloride - AgCl (s)?
white
What colour is silver bromide - AgBr (s)?
cream
What colour is silver iodide - AgI (s) ?
Yellow
What is the test for sulfates?
-Use a dropping pipette to add a few drops of dilute HCl t o removes impurities like carbonate ions
-Add a few drops of Barium chloride solution (BaCl₂)
-If sulfate ions are present, a white precipitate of barium sulfate will form
Give the half equation for the reaction of sulfates with barium chloride
Ba²⁺ (aq) + SO₄²⁻ (aq) → BaSO₄ (s)
How can elements and compounds can be detected and identified?
-Manual methods
-Instrumental methods
What is meant by ‘instrumental methods’?
Methods that use machines to analyse substances, rather than conducting tests.
State the advantages of instrumental methods compared with the chemical tests in this specification.
-They are very sensitive, so can detect the tiniest amounts of a substance
-They are accurate
-They are rapid
-(They can be automated)
What is flame emission spectroscopy?
An instrumental method used to analyse metal ions in solutions.
Are all metal ions cations?
Yes
Are all cations metal ions?
No e.g. the ammonium ion, NH₄⁺
Explain the process of flame emission spectroscopy.
-The sample is put into a flame
-The light given out is passed through a spectroscope, which detects different wavelengths of light to produce a line spectrum
-This can be analysed to identify the metal ions in the solution and measure their concentrations.
How does a sample emit light in flame emission spectroscopy?
As the ions heat up, their electrons become excited and move up energy levels.
They then drop back to their original levels, transferring the energy gained as light.
What does the combination of wavelengths of light emitted by an ion depend on?
-Its charge
-It’s electron arrangement
Why do different ions emit different wavelengths of light?
Because no two ions have both the same charge and electron arrangement.
Because different ions emit different wavelengths of light…
…each ion has a different line spectrum.
How do we tell the concentration of an ion in a solution?
Look at the intensity of the spectrum (rainbow lines).
From line spectrums, how can we decide what ions are present in a sample?
See which ones match
Why might it be necessary to use flame emission spectroscopy instead of flame tests to identify metal ions?
Flame emission spectroscopy van be used to identify the metal ions in mixtures whereas flame tests can only be used for substances that contain a single metal ion.
What is the end result of a paper chromatography experiment?
A pattern of spots called a chromatogram.
Why do you draw the baseline in pencil for chromatography?
Pencil marks are insoluble so won’t dissolve in the solvent.
Explain why you shouldn’t use a pen to draw a line on the filter paper for paper chromatography.
The pen ink might dissolve in the solvent and rise up the filter paper.
What does the solvent used in chromatography depend on?
What’s being tested.
Some compounds dissolve well in water but some don’t.
In which case other solvents like ethanol are needed.
Formula for rf values
distance travelled by component ÷ distance travelled by solvent
n chromatography, what phase is the paper?
the stationary phase
In chromatography, what phase is the solvent?
the mobile phase
What is chromatography used to separate?
Substances in a mixture.
(You can then use the information to help identify substances).
Mobile phase
Where the molecules can move. This is always a liquid or a gas.
Stationary phase
Where the molecules can’t move.
This can be a solid or a really thick liquid.
Why do some chemicals move further up the filter paper than others in chromatography?
They spend more time in the mobile phase than the stationary phase.
What does separation depend on in chromatography?
The distribution of substances between the phases.
What is a retardation factor (Rf) value actually showing us?
The ratio of the distance moved by a compound (origin to centre of spot) to the distance moved by the solvent.
Explain how paper chromatography separates mixtures.
Anything dissolved in the mobile phase (solvent) moves with it through the stationary phase (i.e. up the filter paper).
Different chemicals move different distances with the solvent.
Chemicals that spend more time in the mobile phase than the stationary phase will move further.
So they separate.
Substances in a mixture constantly move between the mobile and stationary phases in chromatography.
What is formed between the two phases?
An equilibrium
Under what circumstances do the components in a mixture separate through the stationary phase?
They all spend different amounts of time in the mobile phase.
The components in a mixture will normally separate through which phase? (stationary/mobile)
The stationary phase
How QUICKLY a chemical moves in chromatography depends on what?
Whether it spends more time in the stationary or mobile phase.
Suggest how chromatographic methods can be used for distinguishing pure substances from impure substances.
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
What do you do if the sports are colourless in chromatography?
Use a locating agent to show where the spots of the mixture are.
If you change the solvent in chromatography, what might change as a result?
The Rf value for the substance.
How do we know that it’s likely the reference compound is present in the mixture.
The mixture and reference will match Rf values in a number of solvents.
What two things does the length of time spent in each phase depend on?
-How soluble the molecules are
-How attracted the molecules are to the paper
Molecules with a higher solubility in the solvent spend more time in the…
mobile phase
Molecules which are less attracted to the paper will spend more time in the…
mobile phase.
How do we test that a white precipitate is aluminium?
Add more sodium hydroxide to the solution so that it’s in excess. If the precipitate redissolves to form a clear and colourless solution, the cation formed is aluminium.
