Chemistry-elements, compounds and mixtures

Topic 1b

What is an element?

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What is a compound?

A substance made from two or more elements that have reacted chemically with each other.

What is a mixture?

A combination of two or more elements or compounds that are not chemically combined.

What is a pure substance?

A single element or compound, not mixed with any other substance.

How can you distinguish a pure substance from a mixture using melting/boiling points?

Pure substances melt/boil at specific temperatures; mixtures melt/boil over a range of temperatures

What is simple distillation used for?

To separate a pure liquid from a mixture based on different boiling points.

What is fractional distillation used for?

To separate a mixture of liquids (like crude oil) into fractions with similar numbers of carbon atoms.

What is filtration used for?

To separate an insoluble solid (like a precipitate) from a liquid.

What is crystallisation used for?

To separate a soluble solid from a solution by evaporating the solvent and forming crystals.

What is paper chromatography used for?

To separate mixtures and identify substances based on how far they travel on the paper.

In paper chromatography, what does a pure substance show?

A single spot in all solvents.

What does a chromatogram tell you?

The number of components in a mixture and helps identify them using Rf values.

What is the formula for Rf value?

Rf = distance moved by substance ÷ distance moved by solvent.

How are different compounds identified in chromatography?

By comparing their Rf values in specific solvents