C12: Chemical analysis Flashcards
What is a pure substance?
A single element or compound containing only one type of particle
How can melting and boiling points be used to determine the purity of a substance?
A pure substance has a fixed m.p./b.p.
An impure substance melts and boils over a range of temperatures, and are overall higher
What is a formulation?
A complex mixture that’s been designed as a useful product. The quantities of each component are carefully measured so that the product has the properties that we need.
e.g. fuels, cleaning products, paint, medicine, alloys, fertilisers and food.
What is paper chromatography?
A physical separation technique that allow substances to be separated based on their solubilities.
What is the stationary phase of chromatography?
The paper because it doesn’t move
What is the mobile phase of chromatography?
The solvent because it moves
What happens to more soluble substances during chromatography?
Travel further up the paper
Why is the starting line drawn in pencil?
Pen ink is soluble so it’ll move up with the solvent
What is the formula for calculating the rf value?
Rf (no units) = distance moved by substance / distance moved by solvent.
Look up rf value in a database to identify the substance
What do you do if 2 substances have the same Rf value?
Use a different solvent
Describe the test for hydrogen
Insert a burning splint - a pop sound is produced when near hydrogen
Describe the test for oxygen
Insert a glowing/smouldering splint - it relights when near oxygen
Describe the test for carbon dioxide
Use limewater (aqueous calcium hydroxide). Bubble co2 gas through limewater, if present, limewater turns cloudy
Describe the test for chlorine
Insert damp litmus paper into test tube - presence of chlorine bleaches the paper
What is the flame colour of lithium ions?
Crimson
What is the flame colour of sodium ions?
Yellow
What is the flame colour of potassium ions?
Lilac
What is the flame colour of calcium ions?
Orange-red
What are the flame colours for copper and beryllium?
Green
What are problems with using flame tests?
Colour can be difficult to distinguish (esp. if there is a low concentration of the compound)
Sample may contain impurities/ a mixture of ions and mask the colour of the flame
What else can scientists use, apart from flame tests?
Flame emission spectroscopy
What is flame emission spectroscopy?
A sample of a metal ion in a solution is placed into a flame, the light given out is passed through a spectroscope, which converts the light into a line spectrum. The positions are specific to the metal ion. It can also tell the concentration of a metal ion - lines become more intense with higher conc.
What is an instrumental method?
A method carried out by a machine
Why are instrumental methods good?
They are rapid - more so than flame tests
Very sensitive - work on a tiny sample of a metal compound
Accurate
What 3 ions do you need to know that react with NaOH?
Magnesium, Calcium, Aluminium, Iron, Copper
What happens when you add NaOH to the 3 aforementioned ions?
White precipitate forms
What happens when you add NaOH to iron?
Brown precipitate forms (3+)
Green precipitate forms (2+)
What happens when you add NaOH to copper?
Blue precipitate forms
How do you distinguish between Mg, Al and Ca precipitates?
Flame tests for calcium
Describe the test for the carbonate ion
Add dilute acid to sample.
It will react to form a gas and we see fizzing.
Bubble gas through lime water to test for CO2
Describe the test for the halide ions
Add dilute nitric acid to the sample.
Add dilute silver nitrate solution
Halide ions produce a precipitate of the silver halide. Each one makes a different colour precipitate.
Chloride - white Bromide - cream Iodide - yellow
