CC2 - Mixtures

Differences between a pure substance and a mixture?

The composition of a pure substance cannot be changed and is the same throughout the substance. By contrast, a mixture does not have a fixed composition and will contain elements and/or compounds that are not chemically joined.

How can you use melting point information to decide wether a substance is pure or a mixture?

A pure substance will melt at the same fixed temperature so will have a sharp fixed melting point.

Mixtures melt over a range of temperatures and do not have sharp fixed melting points.

What happens to atoms at a pure substance’s melting point?

All the atoms gain enough energy to overcome the weak forces of attraction between them and become a liquid.

How can you use a heating curve to identify a melting point?

Look for the point at which the curve becomes flat. A pure substance has a precise melting point and so the temperature of the solid would rise until it reached the melting point and would then plateau or stay at this temperature until all the same forces of attraction were overcome and the substance changes to a liquid, at which point the temperature of the liquid would rise.

Why does the temperature not change as a pure substance melts?

All the particles have the same composition and the same forces of attraction between them, so they melt at the same temperature, until all the substance has changed state.

What sort of mixtures can be separated by filtration?

Filtration is generally used to separate insoluble solids from a liquid or solution. It can also be used to separate solids from a gas.

What sort of mixtures can be separated by crystallisation?

Solutions - mixtures of a solute and solvent.

What lab apparatus is needed for filtration and crystallisation?

For filtration you need a filter funnel, filter paper and a beaker

For crystallisation you need an evaporating basin, beaker of water on a tripod and a bunsen burner.

Explain the formation of crystals during crystallisation

Crystals form when the solvent is evaporated and becomes a saturated solution. At this point crystals start to form as the solute leaves the solution. If the solvent evaporates slowly then larger crystals form as the particles have more time to arrange themselves in a regular pattern.

How does filtration work?

Solvent and solute pass through filter paper but insoluble substances cannot get through and will be trapped in the paper leaving residue

How to reduce risk when using filtration and crystallisation?

Wear eye protection

Stop heating before basin is completely dry

Use a steam bath

How are mixtures separated using chromatography?

Some substances dissolve better than others and when a solvent moves along a piece of paper it carries the substances that are dissolved at different speeds, so they are separated.

How to identify pure substances and mixtures on chromatographs?

Pure substances will only have one spot on the chromatogram, whereas mixtures will have 2 or more spots.

How can you identify substances that are identical on a chramatograph?

Spots which travel the same distance (and have the same Rf value) are likely to be the same substance. If a spot is compared to a known substance it can be identified.

Apparatus for chromatography

Strip of chromatograph paper, a beaker with a lid, a solvent, a pencil and a ruler.

How are substances separated by chromatography?

More soluble substances are carried up the paper more quickly than less soluble ones, so the compounds separate out. The less soluble compounds are slowed down more by the paper. Thus the more soluble substances appear further up the paper than less soluble ones.

How to calculate Rf value?

Rf values are calculated by measuring the distance moved by the spot on the paper and dividing it by the distance moved by the solvent.

Simple distillation

A process of evaporation followed by condensation to separate a solvent from a solution.

Difference between fractional and simple distillation

Simple distillation is used to separate a solvent from a solute in a solution.

Fractional distillation is used to separate a mixture of two or more liquids or mixtures of gasses.

When should fractional distillation be used?

When there is a difference in the boiling points of the components in the mixture.

How is fractional distillation carried out?

The mixture is heated and liquid with the lowest boiling point is boiled first. The vapour passes into a condenser and is condensed to a liquid. The first liquid to be separated is called a fraction and then the other liquids in the mixture will start to boil and condense as temperature rises, to be collected as different fractions.

How are the products of fractional distillation linked to the boiling points of the components?

The first fraction collected has the lowest boiling point component. This is because the fraction with the lowest boiling point will boil and condense more readily and reach the top of the column first. The fractions collected after that will be collected in order of increasing boiling point, so the last fraction to boil and condense will have the highest boiling point. There is a temperature gradient on the fractionating column, where it is hottest at the bottom and coolest at the top.

Special precaution when performing distillation

Add anti-bumping granules to the still or distillation flask to make the liquid boil more smoothly.

Why must water used in chemical analysis not contain dissolved salts?

It could give incorrect results or interfere with chemical reactions when analysing samples. The dissolved salts might give unexpected precipitates during the analysis.

How can freshwater be produced from sea water?

By using simple distillation. The dissolved salts have very high boiling points (above 100C). The water boils at 100C and leaves as vapour which can be condensed back to pure water.

What steps are needed to make fresh water safe to drink?

Screening, sedimentation, filtration and chlorination

How to purify water when you know what it contains?

Remove insoluble materials by screening, sedimentation and filtration.

Remove soluble materials by distillation if needed for pure water or chemical analysis.

Use chlorine to kill harmful microorganisms.

Hazards and ways to control risk when purifying water

Avoid touching hot surfaces, such as the tripod supporting the distillation flask, which should be picked up at the base. In industry, chlorine is a toxic gas and should be handled carefully, using breathing apparatus.