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Topic 8: Chemical analysis - Coggle Diagram
Topic 8: Chemical analysis
Purity
Chemistry definition: A substance that only contains one compound or element throughout. eg. Diamond
Every-day definition: A substance with nothing added to it. eg. Pure milk or beeswax
This can be tested by boiling or melting the substance:
The closer the melting/boiling point is to that in a data book, the purer the substance is.
Impurities can:
Increase the boiling point and the boiling range of the substance
Lower the melting point and increase the melting range of the substance.
Formulations
Formulations are mixtures of chemicals that have a
specific
use. They are made using a formula.
The formula must be precise to ensure the formulation has the correct properties.
Formulations are found in: paints, fuels, cleaning products, fertilisers and pharmaceuticals
Chromatography
A way to separate mixtures
Separation depends on the distribution of the substances between the two phases.
Chromatography has two phases:
Mobile phase: Where the molecules can move. This is always a solvent.
Stationary phase: Where the molecules cannot move. This can be a solid or a thick liquid.
Chromatograms: These are the result of the chromotography that can be analysed
RF values: The ratio between the distance travelled by the solute and the distance travelled by the solvent
This can be calculated by the equation=
Distance travelled by the substance
(distance between the baseline and centre spot)
/ Distance travelled by solvent
(distance from baseline to solvent front)
Tests for gases
Chlorine: Bleaches damp litmus paper, turning it white
Oxygen: If a glowing splint is placed inside a test tube, the oxygen will relight the glowing splint
Carbon dioxide: Bubble carbon dioxide with limewater, the carbon dioxide will turn the solution cloudy
Hydrogen: A lit splint at the end of a test tube of gas will trigger a 'squeaky pop'
Flame tests
Compounds of some metals burn with a colour
Lithium ions (Li+): Crimson flame
Sodium ions (Na+): Yellow flame
Potassium ions (K+): Lilac flame
Calcium ions (Ca2+): Orange/red flame
Copper ions (Cu2+): Green flame
A flame test is performed by: Firstly, sterilise a wire loop using dilute HCL and a bunsen flame. Next, dip the wire loop into the sample being tested. After that, put the loop on the edge of a blue bunsen flame. Then, observe the colour and note it down. Finally, re-sterilise the loop and repeat again for accuracy.
Tests for metal ions
Add sodium hydroxide to unknown metal compounds to produce coloured insoluble hydroxides. This often allows you to identify which metal was in the compound
Calcium (Ca2+) White ppt - no change in excess sodium hydroxide solution
Magnesium (Mg2+) White ppt - no change in excess sodium hydroxide solution
Copper (Cu2+) Blue ppt
Iron(II) (Fe2+) Green ppt
Iron(III) (Fe3+) Brown ppt
Aluminium (Al3+): White ppt - dissolves in excess sodium hydroxide solution
Tests for Halides
Add a couple of drops of dilute nitric acid, followed by a couple of drops of silver nitrate solution.
This produces coloured nitrates depending on the halide:
Silver chloride is white
Silver bromide is cream
Silver iodide is yellow
Tests for Sulfates
Add a couple of drops of dilute hydrochloric acid followed by a few drops of barium chloride solution
If sulfate ions are present, a white precitipate will form:
Ba2+ (aq) + SO42- (aq) => BaSO4 (s)
.
Test for carbonates
Add a couple of drops of hydrochloric acid and connect the test tube to limewater. Then, bubble the solution through the limewater.
Due to carbon dioxide being produced, the limewater will turn cloudy: Na2CO3(aq) + 2HCl(aq) --> CO2(g) + 2NaCl(aq) + H2O(l)
Instrumental analysis
The use of machines instead of conducting tests
Advantages:
Very sensitive
Very fast (and automated)
Very accurate
Disadvantages
Expensive
Inaccessible (requires a lot of training to use)
Flame emission spectroscopy
The sample is placed in a flame and its electrons give light out, which is passed through a spectroscope. This can be analysed to identify the ions in a solution and identify the concentrations.
Can also be used to identify different metal ions in mixtures. This is because, unlike flame tests, the flames don't mask each other.
