experimental techniques

                    apparatus                       units

time           stop clock                       hours, minuits, seconds

temprature thermometer                degree celcius

mass          electronic balance        kg,g, milligram, tonne

volume(l)    measuring cylinder       cm^3, dm^3

                   burette                            

                   pipette

volume(g) gas cylinder                cm^3, dm^3

A pipette measures fixed liquid volumes very accurately

A burette measures liquid volumes between 0.00 - 5.00cm^3. it is very accurate having two decimal places.

2.2.1 - Criteria for purity.

Chromatography.

• Used to separate and identify a mixture of substances due to their different solubility and their attraction to the chromatography paper.
• In chromatography, a piece of paper is placed on a beaker of solvent. The solvent travels up the chromatography paper, and carries the solutes with it. Soluble substances only move. Insoluble substances remain on the datum line.
• The substance most soluble with the least attraction to the paper travels up the farthest.
• The distance moved by the solvent is called the solvent front.
• The distance moved up the paper by each solution is called a dot.
• Some substances like amino acids and sugars cannot be seen, so locating agents like ninhydrin are added. They add colour and make dots visible.

Chromatography method.

•   draw a pencil line 2 cm from the bottom of the paper and call it the datum line.
• Place concentrated spots of different samples on the datum line.
• Suspend the paper in a suitable solvent with the solvent level below the datum line.
• When the solvent front reaches near the top of the paper remove the paper from the solvent and observe the spots if coloured. The paper is now a chromatogram.
• If the spots are colourless, spray the spots with a locating agent to make them visible.
• If the chromatogram shows one dot for a sample, then that sample is pure, but if there are a number of dots per sample, then the sample is impure.
• A very soluble component travels higher up the paper.

rf values

 Rf = distance moved by the compound ÷ distance moved by the solvent

Test for purity using melting and boiling point.

Pure substances melt and boil at a particular temperature.

Example - pure ice melts at 0  ̊C always and pure water boils at a 100  ̊C.

Frozen salt water however, boils at a temperature above 100  ̊C and melts below 0.

A pure substance melts and boils at a temperature that is fixed for the substance.

An impure substance melts below and boils above the temperature range.

2.2.2…