Chemistry Topic 8

Pure - a pure substance is something that only contains one compound or element throughout

A pure substance will melt or boil at a specific temperature. You can test the purity of a sample by measuring its melting or boiling point and comparing it with the melting or boiling point of the pure substance, the closer the melting or boiling point is to the actual substance the purer the sample is.

Impurities in a sample will lower the melting point and increase the melting range of a substance.  

Impurities in a sample will increase the boiling point and result in the sample boiling over a range of temperatures.

Formulations - mixtures with exact amounts of components.

Formulations can be found in paints, cleaning products, fuels, cosmetics, fertilisers and metal alloys. Formulations are vital in the pharmaceutical industry, by altering the formulation of a pill, chemists can make sure it delivers the drug to the correct part of the body at the right concentration, that it's consumable and has a long enough shelf life.

Chlorine - bleaches damp litmus paper

Oxygen - put a glowing splint inside a test tube containing oxygen, the oxygen will relight the glowing splint

Carbon Dioxide - bubbling carbon dioxide through limewater causes the solution to turn cloudy

Hydrogen - hold a burning splint at the end of a test tube containing hydrogen, there'll be a squeaky pop

mobile phase - where the molecules can move, always a liquid or gas (solvent)

stationary phase - where the molecules can't move, can be solid or thick liquid (chromatography paper)

the amount of time molecules spend in each phase depends on

• how soluble they are in the solvent
• how attracted they are to the paper

higher solubility + less attraction = more time in mobile phase

the result is a chromatogram, you can calculate the Rf using this. An Rf value is the ratio between the distance travelled by the substance and the distance travelled by the solvent. The Rf value of a substance is dependent on the solvent used.

Rf = distance travelled by substance / distance travelled by solvent  

• the substances in a sample constantly move between the mobile and stationary phases - an equilibrium is formed between the two phases
• the mobile phase moves through the stationary phase along with anything dissolved in it
• how quickly a chemical moves depends on how it's distributed between the two phases
• the chemicals that spend more time in the mobile phase than the stationary phase will move further through the stationary phase
• the components in a mixture will normally separate through the stationary phase, so long as the components spend different amounts of time in the mobile phase
• the separated components form spots, the number of spots formed may change in different solvents as the distribution of the chemical will change depending on the solvent
• a pure substance will only form one spot in any solvent as there's only one substance

Anion - a negative ion

Carbonates - add dilute HCl and bubble gas through limewater. Limewater will go cloudy as when carbonates and acids react they produce carbon dioxide.

Sulfates - add dilute HCl then add barium chloride. A white precipitate is made.

Halides - add 3 drops of dilute nitric acid then 3 drops of silver nitrate solution. 

Cations - positive ion

Flame tests identify metal ions

• clean a nichrome wire with dilute HCl and hold it in a blue flame from a bunsen burner. When the flame is blue, the wire is clean. Dip loop in sample you want to test and put it back in the flame, record the colour.
• this test only works for samples that contain a single metal ion

Lithium - crimson (Li^+)
Sodium - yellow (Na^+)
Potassium - lilac (K^+)
Calcium - orange (Ca^2+)
Copper - blue (Cu^2+)

NaOH Precipitates

• add a few drops of sodium hydroxide solution to a solution of a mystery compound. You get a coloured insoluble hydroxide if the following cations are present...

calcium - white   Ca^2+ + 2OH- --> Ca(OH)2
copper - blue      Cu^2+ + 2OH- --> Cu(OH)2
iron II - green      Fe^2+ + 2OH- --> Fe(OH)2
iron III - brown    Fe^3+ + 3OH- --> Fe(OH)3
aluminum - white   Al^3+ + 3OH- --> Al(OH)3    (forms a colourless solution in excess NaOH)
magnesium - white   Mg^2+ + 2OH- --> Mg(OH)2

• during FES a sample is placed in a flame. As the ions heat up, their electrons move to higher energy levels. When the electrons drop back to their original energy levels, they release energy as light
• the light passes through a spectroscope, which can detect different wavelengths of light to produce a line spectrum
• the combination of wavelengths emitted by an ion depends on its charge and its electron arrangement
• no two ions have the same charge and electron arrangement, different ions emit different wavelengths of light so have different line spectrums
• the intensity of the spectrum indicates the concentration of that ion in solution
• line spectra can be used to identify ions in solution and calculate their concentrations

FES works for mixtures - you can compare various spectra to another spectrum to find what ions make up the solution of the origial spectrum.

Machines - chemists often use instrumental analysis (eg FES) instead of conducting manual tests. Advantages of machines: very sensitive, very fast and very accurate