Edexcel combined science Chem CC2


Mixtures, Pure substances and melting points

Pure substance- is the same throughout, composition of pure substance cannot be changed.

*Compounds can also be pure - sucrose is made up of hydrogen, carbon and oxygen, they are chemically bonded together so the molecules are sucrose molecules. The composition of pure sucrose cannot be changed.

Mixture- elements/compounds that aren’t chemically bonded. Physical processes can separate them. Don’t have fixed composition.

Melting points- each part of a pure substance melts at the same time because every part has the same composition. Mixtures have streaks range of melting points because each of the components have different melting points.

Consequently, melting points can be used to separate mixtures.

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Filtration and crystallisation

*Filters can separate mixtures. They allow liquids or small pieces through but stop bigger pieces or insoluble substances.

*Crystallisation- solutes can be separated from solvents by evaporating the solvent. Process forms solid crystals

In the Lab:
Filter funnel lined with filter paper, solvent & solutes pass through (called filtrate).
Insoluble substances leave residue in the filter paper.
Bunsen then used to evaporate the filtrate
Once crystals start to form the solution is saturated, stop heating and allow to evaporate naturally.
Once all the filtrate is evaporated, pat dry the crystals.

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Methods of separation

Distillation- used to separate liquids from dissolved solids.

Example: purified tapwater by removing minerals. The mineral water is evaporated and only the water evaporates as the solid minerals have much higher boiling points. The steam is then pure and can be condensed and the water collected will also be pure. 

Fractional distillation- use to separate two or more liquids. Works when the liquids have different boiling points and can be evaporated at different temperatures. Done using a fractionating column.

Filtration see previous card.

Crystallisation see previous card.

Paper chromatography- can be used to find out which coloured compounds the mixture contains. When is solvent moves along the ***** of paper it carries the different substances in a mixture of different speeds so they are separated. Solvent called mobile phase, paper contains stationary phase.

Rf value=distance compound has risen/distance solvent has risen.

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