c2.1 purity overview

What is a pure substance?

A substance made of only one element or compound

What is an impure substance?

A substance containing more than one element or compound

Effect of impurities on melting point

Lower and broader melting point

Effect of impurities on boiling point

Higher and broader boiling point

How can melting point test purity?

Compare measured melting point to known value; lower/wider range = impure

How can boiling point test purity?

Compare measured boiling point to known value; higher/wider range = impure

What is a formulation?

A mixture designed for a specific purpose

Examples of formulations

Medicines, paints, fuels, cleaning products

Why formulations are important

Each chemical is added in precise amounts to ensure correct function

Why is a pencil line used in chromatography?

Pencil is insoluble so it does not dissolve in the solvent

Why must the solvent level be below the baseline?

So the spots do not dissolve into the solvent directly

Why must the spots be small?

To prevent them spreading and merging

Why must the beaker be covered?

To stop solvent evaporating

Why do different solvents give different chromatograms?

Solubility and attraction change depending on the solvent

How can chromatography identify an unknown substance?

Compare Rf values or spot positions with known samples

How do you calculate Rf value?

Distance travelled by substance ÷ distance travelled by solvent

What does a high Rf value show?

Substance is more soluble in the solvent and less attracted to the paper

What does one spot on a chromatogram mean?

Pure substance

What do multiple spots mean?

Mixture

What is the mobile phase?

The solvent that moves up the paper

What is the stationary phase?

The chromatography paper

How does the mobile phase affect movement?

More soluble substances move further up the paper

How does the stationary phase affect movement?

Substances that stick strongly to the paper move a shorter distance

Why do substances move different distances?

Different attractions to the mobile and stationary phases

What is filtration used for?

Separating an insoluble solid from a liquid

How does filtration work?

Liquid passes through filter paper; solid stays as residue

What is the filtrate?

The liquid that passes through the filter paper

What is the residue?

The solid left behind

What is crystallisation used for?

Separating a soluble solid from a solution

How does crystallisation work?

Heat solution to evaporate solvent; cool to form crystals

Why should you not heat to dryness?

It can damage or decompose the solid

What is simple distillation used for?

Separating a solvent from a solution

How does simple distillation work?

Heat solution → solvent evaporates → vapour condenses → pure liquid collected

Why is a condenser needed?

To cool vapour and turn it back into liquid

What is fractional distillation used for?

Separating liquids with different boiling points

Why is a fractionating column used?

Allows repeated evaporation and condensation for better separation

Which liquid is collected first?

The one with the lowest boiling point