We can determine the purity of a substance by:
1. checking its melting and boiling point;
2. using chromatography.
Pure substance :
- has an exact and constant/fixed melting point (melt completely at one temperature)
Effect of impurities on melting point
1) lower the melting point (the greater the amount of impurities, the lower the melting of the substance
2) melting takes places over a range of temperature- has an exact and constant/fixed boiling point
Effect of impurities on boiling point
1) boiling point will increase (greater the amount of impurities, the higher the boiling point of liquid)
2) boil over a range of temperature
Effect of pressure on boiling point
1) pressure increase, boiling point increase
2) vice versa
To test the purification of a substance, we can use chromatography.
The technique of using a solvent to separate a mixture into its components is called chromatography.
Uses of chromatography
- Separate the components in a sample
- identify the number of components in a sample
- identify the components present in a sample
- determine whether a sample is pure.
Separating solid from a liquid
Decanting
This is a way to separate the water from an insoluble solid (e.g. pebbles, sand...)
Filtration
Filtration is suitable for separating mixtures whose solid components behave
differently in a particular solvent – one component must be soluble and the other
insoluble in it. A liquid which contains insoluble solid particles is called a
suspension.To separate small solid particles from a liquid. (e.g. sand, clay, dust particles, precipitates...) After filtration, the solid that remains on the filter paper is called the residue. The liquid or solution that passes through the filter paper is called the filtrate.
Evaporation to dryness
To separate soluble solid particles from a liquid. (e.g. common salt, sugar)
We evaporate water from the solution. For example, when we evaporate salt solution to dryness, we recover solid salt. This can be used to recover salt from seawater.
However, many substances decompose when they are heated strongly. For example, sugar will decompose to give water and carbon.
We evaporate water from the solution. For example, when we evaporate salt solution to dryness, we recover solid salt. This can be used to recover salt from seawater.
However, many substances decompose when they are heated strongly. For example, sugar will decompose to give water and carbon.
Crystallisation
In crystallisation, water is removed by heating the solution. Heating is stopped at the stage when a hot saturated solution is formed. If the resulting solution is allowed to cool to roonm temperature, the dissolved solid will be formed as pure crystals.
A clean glass rod can be used to test whether a solution is saturated. It is dipped into the solution and removed. There will be a small amount of solution on the rod. If small crystals form on the rood as the solution cools, the solution is saturated. We say the solution is at its saturation point of crystallisation point.
A clean glass rod can be used to test whether a solution is saturated. It is dipped into the solution and removed. There will be a small amount of solution on the rod. If small crystals form on the rood as the solution cools, the solution is saturated. We say the solution is at its saturation point of crystallisation point.
Separating Funnel
Separating funnel is used to separate two liquids which are
immiscible, e.g. organic compounds like oil and water. The mixture is
poured into the funnel and the layers allowed to separate out. The less
dense liquid collects above the denser liquid. The lower and denser
layer is run off by opening the tap. The tap is closed as the last drop of
the denser layer runs out. The tap is then opened again to drain the
lighter layer into another beaker.
Fractional Distilation
Fractional distillation is a technique used to separate two liquids which
dissolve in one another. They are said to be miscible liquids as they mix completely
in any proportion to form a homogenous layer. The separation relies on the
difference in boiling points of the two liquids. The liquid with the lowest boiling pointis distilled first. The liquid with the highest boiling point is distilled last. A mixture of
ethanol (boiling point 78°C) and water (boiling point 100°C) may be separated using
fractional distillation because both liquids are miscible and have very different boiling
points.
Paper Chromatography
Chromatography is a technique engaged extensively in researches,
experiments, and investigations due to its sensitivity and ability to pick up minute
traces of impurities and drugs. The principle involved depends upon the relative
solubilities of the solutes (in the mixture) between the stationary phase (the paper or
plate) and mobile phase (the solvent).
