PRE INVESTIGATION in Qualitative chemical research
In this pre-investigation part the emphasis - just as in most separation methods - is more physical than chemical.
No real chemical reactions are carried out.
The pre-investigation considers a number of substance properties, like:
It deals with the substance properties that can be determined rather directly, sometimes even at first glance (you see that immediately)); sometimes you need an instrument.
- external, visible characteristics
- what you observe at heating
- solubility in different solvents
- and more
Knowledge about melting points or boiling points of a substance is sometimes connected to 'thermic analysis'. Often it is a simple act to put some substance in a test tube and then heat it. Observe well: what happens, what do you see, smell?
Visible characteristics of a substance at room temp (25ºC) and at normal pressure (1 atm) can be directly observed. A couple of data you find in table XVII.
When a substance is solid, it obviously can make a lattice structure, so the particles are not very movable then.
In other words: particles who love it to hold each other strongly, do not stay with lots of freedom.
If they turn out to have high melting points, (easy to investigate), they probable have no molecular lattice, but a metalic of ionic lattice.
With a low melting point, the solid may have a molecular lattice with in it rather heavy molecules, regular of shape and/or a bit polar.
Sometimes a substance does not have a melting point, but a melting zone. This means that the substance may not be pure, but a mixture. The same for boiling points and boiling zones.
If a substance is not pure, then purification (separation) is needed.
A gas or a substance that easily evaporates probably consists op light molecules and/or molecules that have no or little dipole character. There is little attraction between the particles.
Liquids with a high boiling point probably are strong polar and/or have long molecules.
They also might be (poly)unsaturated.
In table XVII you find some boiling points and melting points and in table V you find boiling and melting points of all elements.
Data about solubility of salts in water can be found in table XI. goto the tables
It is possible to investigate substance properties with optical analytic methods. There observation is very important, in particular with help of special apparatus. What phenomena appear at radiation, heating, etc.? The spectrophotometer is most important here.
Spectroscopy can be divided as follows:
- Atom spectroscopy
- Absorption spectroscopy
- Emission spectroscopy
- Molecular spectroscopy
- Microwave spectroscopy
- Infrared spectroscopy
- Raman spectroscopy
- Ultraviolet spectroscopy
- Nuclear spin resonance spectroscopy
- Mass spectrometry