More Lessons for IGCSE Chemistry
A series of free GCSE Chemistry Lessons on Separation Techniques.
In this lesson, we will learn
Elements, Compounds and Mixtures
- Elements, compounds and mixtures
- Interpreting a chemical formula
- Filtration and crystallisation
- Simple distillation
- Fractional distillation
What are elements, compounds, mixtures, and molecules?
The Periodic Table shows the elements.
There are around 100 elements.
In an element, all the atoms are the same.
Compounds contain two or more different elements chemically combined in a fixed proportion.
In a mixture, we have different elements or compounds not chemically combined together.
A molecule has any elements joined together. The elements may be the same or different.
Interpreting a chemical formula
The Periodic Table shows us the elements.
Each element has a symbol. Every symbol starts with a capital letter.
Some symbols have a second lower case letter.
In a specific compound, the number of atoms of each element is fixed.
Filtration and Crystallisation
Physical separation techniques are used to separate mixtures. They cannot be used to separate elements in a compound.
Filtration is used to separate an insoluble solid from a liquid.
Crystallisation is used to separate a soluble solid from a liquid.
Simple distillation is used to separate a liquid from a solid if we want to keep the liquid.
1. Evaporate the liquid by heating.
2. Condense the vapour by cooling.
How fractional distillation can be used to separate a mixture of liquids?
In fractional distillation, we separate a mixture of different liquids.
These liquids must have different boiling points.
If the two liquids have got very similar boiling points, then it is much harder to separate them. We might need to carry out several rounds of fractional distillation.
Using fractional distillation to purify large values of liquid eg. crude oil requires different equipments although the principle remains the same.
Paper chromatography allows us to separate substances based on their different solubilities.
The paper is the stationary phase because it does not move.
The solvent is the mobile phase because it does move.
Why do we draw our starting line in pencil?
If we drew the line in pen, the pen ink would move up the paper, with the solvent.
The Rf value of a compound can be calculated by the following formula:
Rf = distance moved by the compound ÷ distance moved by the solvent
If the chromatography has been carried out in the same way, the Rf value of a particular compound is always the same.
This means we can use chromatography to separate and identify compounds in mixtures.
If several different substances have the same Rf then we would need to repeat the experiment using a different solvent.
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