Year 10 GCSE Chemistry - Separating Mixtures

Boiling point

Temperature at which a substance turns from liquid into a gas

Chromatography

Method in which a mixture of (often coloured) substances is separated, relying on a mobile and stationary chemical phase

Crystallisation

When crystals form during evaporation of a solvent from a solution

Dissolution (derivative of verb 'dissolve')

When a solvent and solute mix to form solution

Evaporation

Liquid turning into gas

Filtration

Separates solid insoluble substances from a liquid

Fractional distillation

Method for separating a mixture of liquids with different boiling points

Insoluble substance

One that will not dissolve

Melting point

Temperature at which a substance changes from the solid to liquid state

Mixture

Two or more different substances not chemically bonded together

Pure substance

Single element or compound

Simple distillation

Method to separate a solvent from a solution

Soluble substance

One that will dissolve in a given solvent

Solute

Substance that dissolves in a solvent

Solution

Mixture formed when one substance dissolves in another

Solvent

Substance that can dissolve a solute to form a solution

How filtration works

Filter paper used has tiny microscopic holes in it that allow small enough particles like water molecules to pass through but prevent too large particles passing like grains of sand

When evaporation is used as a method for separating solutions

When separating a solute from a solution, you evaporate the solvent

How you can safely carry out evaporation as a method for separating a solute from a solution

Slowly, using a water bath

Equipment used in evaporation as a method for separating a solute from a solution, bottom to top

Bunsen burner, tripod and gauze, beaker containing water and evaporating dish containing solution

Gauze

Thin, transparent fabric with a loose open weave placed on top of tripod for support e.g. of beaker in evaporation method for separating solute from solution

What crystallisation results in

Larger, well-formed crystals

Carrying out crystallisation

Evaporate water until the solution becomes saturated and then leave the solution i.e. to crystallise slowly

Saturated solution

One in which no more solute cannot dissolve

What simple distillation is used for

To separate a mixture of a solute dissolved in a solvent or one of two different liquids, importantly with different boiling points

What simple distillation allows you to do

Allows you to evaporate and then condense the liquid, rather than letting it escape into the air

Equipment in simple distillation, left to right

Bunsen burner or similar (↑ heat), round-bottom flask, thermometer, Liebig condenser with water in below water out opening and beaker with ideally pure liquid

Simple distillation diagram

Why water in opening is below water out opening in Liebig condenser (in simple distillation)

Ensure water completely fills condenser to optimise cooling efficiency

How fractional distillation is carried out

The fractionating column has a large surface area inside for improved condensation allowing each liquid to be collected separately, stopping other liquids evaporating

Equipment used in fractional distillation, bottom-left to top to right

Bunsen burner or similar (↑ heat), round-bottom flask containing mixture of liquids, fractionating column containing glass beads to increase its surface area, thermometer, condenser (and beaker to collect ideally pure liquid)

Example of mixture of two liquids that may be used in fractional distillation

Ethanol and water

Fractional distillation diagram

Two examples of ways scientists may use chromatography

To tell whether food contains traces of a harmful substance or whether a sample of paint found at a crime scene matches a suspect's car

Type of technique chromatography is

Analysis

What paper chromatography can be used for

Separating the coloured substances in ink

Stationary phase

One that does not move

Mobile phase

One that moves

Stationary phase in paper chromatography

Absorbent paper

Mobile phase in paper chromatography

Solvent such as water

Thin layer chromatography

Works in same way as paper chromatography but the stationary phase is a thin layer of silica or alumina powder spread over a sheet of glass or plastic, which the solvent travels up, carrying the solute

Chemical formula for silica - word and symbols

Silicon dioxide, SiO2

Chemical formula for alumina powder - word and symbols

Alumina oxide, Al2O3

Diagram of apparatus used for TLC (thin layer chromatography) - revise equipment shown

What Rf values are used for

Compare different spots on a chromatogram

What two spots having the same Rf value and being the same colour means

They are likely to be identical

Stationary phase in gas chromatography

Silica or alumina powder packed into a metal column

Mobile phase in gas chromatography

An unreactive carrier gas such as nitrogen that does not react with the sample

What gas chromatography is used for

Separating the components of a mixture and measuring their quantities

Revise this diagram showing the instruments for gas chromatography

'Phase' meaning in chemistry

Physically distinct form of matter

Phase in chromatography

One of two main components used

Purpose of column oven in gas chromatography

High temperature

What column contains in gas chromatography

Finely divided solid powder such as silica or aluminium oxide, the stationary phase

Explanation of gas chromatography

Sample turns into gas when injected into the column, pushed through by the carrier gas. Different components take different times to travel through the column depending on how strongly they bond to the stationary phase. A detector sends a signal to a computer as each component leaves the column and the computer produces a chromatogram in which each component is a peak plotted against travel time

Solvent front

Furthest point reached by solvent

Rf =

distance substance travelled (spot distance) / distance solvent travelled

Range of Rf values

0 to 1

What Rf stands for

Retention factor

Chromatography for white or colourless mixtures

Process is carried out in the same way but spots do not show up, unless under UV light or using a stain such as iodine

Paper chromatogram image (unfinished)

What x-axis shows on gas chromatogram

Retention time

What y-axis is directly proportional to on gas chromatogram

Quantity of substance

What you can infer from a gas chromatogram with four peaks

It measures a mixture containing four substances

Type of chromatography not mentioned elsewhere in this Quizlet

Column chromatography

Starting line in paper chromatography

Drawn in pencil so that it does not dissolve in the solvent and affect the results, this is where small spots of each sample are placed

What you do once the solvent is near the top of paper in paper chromatography

Paper is taken out of the solvent, the level of solvent is marked and the paper is left to dry

Why substances in a mixture separate

They have different attractions to the stationary phase such as paper and mobile phase such as solvent

Substances have different Rf values in different -

Solvents

Why the water level in the beaker must be below spots in paper chromatography

Prevents the sample dissolving directly into the solvent, rather than being carried up the mobile phase

Watch video? (On chromatography - add a bit if necessary)

What 'pure' refers to in everyday life

Natural substances that have not been processed or changed

What 'pure' means in science

Substance consisting of one element or compound

What mixtures are (pure or impure) and why

Impure, as they contain more than one element or compound

It is difficult to obtain completely -

Pure substances

What liquid is used in the manufacture of computer chips, and why it is not likely pure

'Ultra pure' water, although this is easily contaminated by carbon dioxide from the air

What air is a mixture of

Oxygen, nitrogen and other substances

Why many useful materials are mixtures

Different substances are often deliberately chosen to produce the desired properties

Alloy

Mixture of metals with one or more other elements

What most of the metals we use are

Alloys

Why a mixture of gold and copper is used for jewellery

Pure gold is very soft so this alloy is harder

What a melting point of a pure substance is (answer is not a number)

A single temperature

Melting point of an impure substance

Often less than that of a pure substance and melts over a range of temperatures

How melting points can be used to determine the purity of a substance

The greater the difference between the measured melting point of a substance and its accepted value, the lower the purity of the substance

Melting point of pure stearic acid

69 °C

How melting point is measured with apparatus

A solid is heated and the temperature at which it melts is measured using a thermometer. Alternatively, the temperature of a solid can be measured at regular time intervals and a graph plotted

Why a substance must be heated slowly when finding its melting point

To allow the temperature of the whole sample to increase, so that the melting point is measured accurately

Why it is important to stir the sample when finding its melting point

So it is heated evenly, so that the melting point is measured accurately

Two types of chromatography you can use to tell if a substance is pure

Paper or thin layer chromatography

Four main advantages of thin layer chromatography when determining if a substance is pure

It is quicker, more sensitive, so a smaller a sample can be used, there is a larger range of stationary phases and solvents to choose from and you can scrape an individual spot from a thin layer chromatogram for further analysis, for example by gas chromatography

How scientists discovered what our ancestors ate

By analysing the insides of ancient cooking pots to identify the food substances left behind. Using gas chromatography, they analysed wax from cabbage leaves and compared it to the chromatogram from the contents of the cooking pot and found they matched almost exactly, concluding the ancient pot had been used to cook cabbage

Separation of a mixture can be improved by using what method of chromatography

Two-dimensional chromatography

Two-dimensional chromatography

Uses two different solvents. Chromatography paper or plate is rotated by 90 ° after using the first solvent

Example of what two-dimensional thin layer chromatography can be used to separate

Different plant oils

What you must know to decide on a separation method of a mixture

Some properties of individual components in the mixture

What gas chromatography can be used for

Many different kinds of substance

Even though gas chromatography is versatile, it would not be -

A method you would use in school

Revise table showing methods you could use in school and question