Mixture and Seperations

7/29/2019 Mixture and Seperations

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1

Identify the different types of solutions

Investigate experimentally the effect of structure and

temperature on solubility of solids in water Distinguish among solutions, suspensions and

colloids Identify suitable separation techniques basedon difference in properties of the components ofmixtures

Describe the extraction of sucrose from sugar cane

Mixtures and SeparationsChapter 8

Learning Outcomes


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Copper(II) sulphate

Pure substances

A pure substance is a single substance not mixed

with anything else.

Sugar and table salt are examples of pure

substances.

Pure substances usually exist in crystal form.

For example, table salt, sugar and copper(II)

sulphate all exist as crystals. A pure substance can be an element or a

compound.

Sugar crystals

Chapter 8Mixtures and Separations


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Pure and impure substances

A pure substance has afixed melting point andboiling point.For example, pure waterboils at 100 oC, and

melts at 0 oC.b.p. 100 oC

m.p. 0 oC



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Impure substances or mixtures

A mixture on the other hand, is an impure substance. It

does not have a fixed melting point and boiling point. Itmelts or boils over a range of temperatures.

For example, sea water boils at about 102 oC and freezes

at around 2.5oC.

In general, an impurity lowers the melting point and

increases the boiling point of a substance.



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Impure substances or mixtures

In nature, most substances are impure. They consist of two

or more substances mixed together. Such substances are called mixtures.

Examples of mixtures are air, sea water, petroleum and

granite rock.

Mixtures can be purified orseparated by physical methods.



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A Mixture

Air is made up of different gases: nitrogen, oxygen, carbon dioxide andnoble gases such as argon, neon, and helium.

The gases in air can be easily separated by liquefaction followed byfractional distillation.

Air is an example of a mixture.

A mixture is made up of two or more substances

physically combined together.

Mixtures and SeparationsChapter 8


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Composition of a mixture

A mixture can be made up oftwo or more

elements.

E.g. Brass is a mixture of copper and zinc.

A mixture can be made up oftwo

or more elements or compounds.

E.g. Air is a mixture which containsboth elements and compounds.

Copper

atom

Zincatom

Air contains both elements and

compounds.


Brass

Element

Compound


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Mixtures

Homogenous

Mixtures

Heterogeneous

Solution

Suspension Colloid

Emulsion


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Homogeneous mixtures

Homogeneous mixture is uniform throughout no

difference. Characteristics of solutions:

When left to stand, the solute and solvent do not separate.

Solutes are not visible (particle size 1 nm).

All parts of the solution have the same chemical composition,chemical and physical properties.



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Homogeneous mixtures

Solubility of a solution refers to the amount of solute that isable to dissolve in a given quantity of solvent at a fixedtemperature and pressure.

The rate of dissolving of a solute depends on: Temperature (e.g. Solubility increases with temperature.)

Surface area of solute (e.g. Fine sugar dissolves faster than rock

sugar.) Rate of agitation (e.g. Stirring the solution increases the rate of

dissolving.)



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Heterogeneous mixtures

Heterogeneous mixtures have non-uniform compositionswhich can be mechanically separated.

Examples of heterogeneous mixtures are colloids andsuspensions.

Colloids include fog, smoke, shaving cream, milk, blood,styrofoam, gelatin, and cheese.

Suspensions include muddy water, paint and chalk powdersuspended in water.



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Definitions

Solution- A solution is

formed when one

substance called a

solute dissolves inanother called the

solvent.

Types of solution

State ofsolute

State ofsolvent

Example

solid liquid Sodiumchlori

de and water

solid solid Metal alloys

eg brass

gas Liquid Carbon

dioxide inbeverages

liquid liquid Alcohol and

water


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When two liquids form a solution they are said to be

miscible.

Suspension- A suspension is a type of mixtureformed when the solute does not dissolve in a

solvent. These particles settle on standing.

Liquids that do not dissolve in each other are said to

be immiscible.

A colloid is a mixture where the solute particles are

bigger than that of a solution but smaller than those

in a suspension. These particle do not settle onstanding and can pass through a filter paper.


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Emulsion- These are type of colloids where one

liquid is finely dispersed in another liquid.

Eg mayonnaise, oil dispersed in water

Summary of separation Technique

Type of mixture Separation

Method

Principle of method Example

Solid/solid Sublimation

Solvent

extraction

One substance

sublimes the other

does not

Differing solubilityin a particular

solvent.

Ammonium

chloride/sodium

chloride

Iodine/sodiumchloride


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solid/liquid

suspension

Solution

colloid

Decantation and

filtration

Evaporation/

Crystallisation

Simple

distillation

chromatography

Differing particle

sizes

Widely differing

boiling points

Differing

solubilities in apraticular

solvent leading

to differing

speed of

movement on

chromatogram

Chalk/water

Copper

sulphate/water

Screened

methyl orange

Liquid/liquid

Miscible

immiscible

Fractional

distillation

Separatingfunnel

Slightly differing

boiling points

Differingdensities

Ethanol/water

Oil/water


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Quick check 1

1. Explain why air is a mixture but wateris a compound.

2. (a), State two reasons why a mixture of iron filingsand sulphur before heating is a mixture.(b) When the mixture of iron filingsand sulphur is heated strongly, statetwo reasons why the solid formed

is a compound.3. State whether the following diagrams on the rightrepresent elements, mixtures or compounds.

Solution



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Solution to Quick check 1

1. Air is a mixture because it is made up of many gases such as oxygen,nitrogen, etc. not chemically combined together. The gases in air can beseparated by physical means. Water is a compound because it is made up

of hydrogen and oxygen chemically joined together. We cannot separatethe gases in water by physical means.

2. (a) It is a mixture because the iron and sulphur can be separated byphysical means e.g. by using a magnet. Also, no heat or light is given outwhen the iron filings and sulphur are mixed together.(b) When the mixture of iron filings and sulphur is heated strongly, a

compound is formed because a chemical reaction occurs e.g. heat andlight are given off. Also, the sulphur and iron in the compound can nolonger be separated by physical means such as by using a magnet.

3. (a) compound, (b) compound, (c) mixture, (d) mixture

Return



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Methods of separating Mixtures

Filtration

Evaporation

Crystallisation

Sublimation

Simple distillation

Fractional distillation

Use of separating funnel

Chromatography



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Filtration

Filtration is the method used to separate an insoluble solid from a liquid.

An example would be to separate a mixture of sand and water.

The liquid (water) that

has passed through

the filter paper is called

filtrate.

The solid (sand) left on thefilter paper is called residue.



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Evaporation

Evaporation is the method used to separate a solute from a solution.

It can only be used for solids which do not decompose under heat e.g.table salt (sodium chloride).

It is done by heating the solution in an evaporating dish until it is completelydry.

Crystals of salt remain after the solution is evaporated to dryness.



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Crystallisation Crystallisation is the method used to obtain pure crystals from a solution.

It is done by heating the solution in an evaporating dish until it is saturated.

The hot solution is then allowed to cool.

Crystals will be formed on cooling. They are then dried between sheets of filter

paper.



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Sublimation

Sublimation is the method used toseparate a substance which sublimesfrom a mixture.

Examples of substances whichsublime are: iodine, ammonium salts,naphthalene and dry ice.

The mixture is heated in anevaporating dish covered witha filter funnel.

The substance which sublimeswill be changed into a vapourand then formed back on theinside of the funnel.

To separate ammonium chloride

from sodium chloride, this

method can be employed.



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Simple distillation

Distillation is the method used toobtain a pure solvent from asolution. E.g. pure water from

seawater. It is done by heating the solution in

a distillation flask and collecting thevapour that boils off.

A condenseris used to condensethe hot vapour and change it to a

liquid. The condenser is cooled by the

flow of cold water (in thecondenser) from the tap.

The pure liquid collected from

distillation is called distillate.



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Fractional distillation

Fractional distillation is used to separatetwo or more miscible liquids with differentboiling points.

A fractionating column and acondenserare used.

Example: Mixture of ethanol and water

When the thermometer reaches a steady

temperature of78 oC,ethanol is collected.

Water is collected when thethermometer reaches a steadytemperature of 100 oC.


Separation of ethanol-water mixture

by fractional distillation


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Use of separating funnel

This method is used to separate a mixture ofimmiscible liquids.

The mixture is placed into a separating funneland allowed to settle into two layers.

Example: Mixture of oil and water

The tap is first opened to allow the water todrain out of the funnel.

After all the water has drained out, anotherbeaker is placed below the funnel and oil isnow drained out.

Separation of oil-water mixture

by using a separating funnel



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Chromatography

Chromatography is a method used toseparate and identify small quantities ofsubstances.

There are different kinds of chromatographytechniques, such as gas chromatography,liquid chromatography and paperchromatography.

It is based on the principle thatdifferent substances have different

solubilities in the same solvent. The more soluble substance will get

carried along faster by the solventand move further ahead than theless soluble substances.

Chromatography can be used todetect and identify very smallquantities of substances. It is usedto detect banned dyes used infood colourings, and detectillegal drugs used by athletesduring major competitions.

Paper Chromatography



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Setup for paper chromatography

-The starting line must be drawn in

pencil, not ink. This is because unlike

ink, pencil lead is insoluble in the

solvent and will not interfere with thechromatogram.

-The spots of mixtures must be placed

above the solvent level, so that they

will not immediately dissolve in the

solvent, and the solvent has time to

slowly move up.

starting line

solvent

solvent front

chromatographypaper

large

beaker

glass cover

-The solvent front must be allowed to move as far up the paper as possible to

ensure that all the dyes are separated.



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The Rfvalue of a substance

Rfvalue = distance moved by a substancedistance moved by solvent front

(7 cm)(10 cm)

E.g. Rfvalue of red dye = 7 cm = 0.7

10 cm

Sometimes the chromatogram needs to be sprayed with a locating

agent to make the colourless spots in the chromatogram appear.



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Paper chromatography

Worked example

The chromatogram shows 3 single dyes

red, green and blue, and also fourunknown samples P, Q, R and S.

Identify the dyes present in each of the

samples, P, Q, R, S.

SolutionSample P contains green dye and one unknown dye.

Sample Q contains only blue dye.

Sample R contains green, blue and red dyes.

Sample S contains green and red dyes.



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Identify the various method separation methods used in the diagram below .



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Extraction of sucrose from sugarcane

The cane sugar stalks and harvested and brought tofactory for processing

The cane stalks are cleaned by passing then alongan agitated conveyer belt which passes strong jets

of water over the cane stalks to remove dust, smallrocks and leaves.

The stalks are passed into shredder which containsrevolving knives which cut stalks into chips

The chips are passed into crusher consisting of twoor three heavily grooved rollers. At the same time asthe chips are crushed , hot water is sprayed onto thechips to dissolve the sugar in the stalks.

e u ce t at s extracte s an ac c ar green qu


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e u ce t at s extracte s an ac c, ar green quwhich contains sediments.

The next step is clarification. The juice is neutralisation of

calcium hydroxide and the addition of heat, insoluble salts

are formed by the reaction between calcium hydroxide and

sediment impurities. The non- sugar impurities are

removed by continuous filtration producing a final clarifiedjuice, which is 85% of water.

The clarified juice goes into a series of three or four evaporators.

The evaporators are under successively lower pressure so that asthe juice passes from one to the next it boils at successively low

temperatures .In this way the juice is concentrated but not charred or

caramelized by the boiling process. The juice from the last

evaporator is a thick syrup containing only 35% water.

The next step is crystallisation which take place in a single stage


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The next step is crystallisation which take place in a single stage

vacuum pan .The syrup is evaporated until saturate with sugar .As

soon as the saturation point has been exceeded, small grains of

sugar (called seed) are added to the pan to serve as nuclei for theformation of sugar crystals .As the crystals form, the remaining syrup

becomes thick and viscous and is called molasses. The mixture of

crystals and molasses form massecuite.

Molasses and sugar in the massecuite are separated by spinning in

centrifuges. Each centrifuge contains a perforated basket .The

massecuite is placed in the basket and is spun at high speed .The

sugar crystals stay behind in the basket and the molasses is forcedout through the holes.


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Quick check

1. State the method you will use toseparate the following substances.(a) calcium carbonate from table salt(b) iodine from sodium chloride(c) table salt from seawater(d) sugar from sugar solution(e) pure water from sewage water(f) ethanol from beer(g) yellow dye from durian ice cream

2. Explain the following in chromatography.

(a) Why is the starting line not drawn with ink or a ball point pen?(b) The spots of samples on the start line should be small.(c) What is the biggest advantage of chromatography?

3. A sample of ink was analysed using paper chromatography (seediagram above). Identify the dyes present in the ink.

Solution


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Solution to Quick check

1. State the method you will use to separate the following substances.(a) dissolution, followed byfiltration(b) sublimation(c) evaporation(d) crystallisation(e) distillation(f ) fractional distillation(g) chromatography

2. Explain the following in chromatography.(a) Ink contains dyes which could dissolve in the solvent and

interfere with the chromatogram.(b) So that they would not smudge the paper.(c) It can detect and identify very small amounts of substances.

3. Blueandyellow dyes

Return


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3. Hold a magnet over the mixture of

iron filings and sulphur.

The iron filings are attracted by the magnetand can be separated from the sulphur.

4. Heat the mixture of sulphur and iron

filings strongly in an evaporating dish.

Examine the substance formed carefully. A black solid is formed. It does not look

like iron filings or sulphur.

Compound of iron and sulphur

Mixture of iron and sulphur

Experiment


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5. Hold a magnet over the black solid. What happens?

The black solid is not attracted by the magnet.

The iron can no longer be separated from the sulphur in the black

solid.

6. When iron filings and sulphur are heated,

a compound called iron(II)sulphide is formed.

The equation for the reaction is:

Fe(s) + S(s) FeS(s)

Experiment


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1. http://orgchem.colorado.edu/hndbksupport/filt/filtration.html

2. http://en.wikipedia.org/wiki/Distillation

3. http://orgchem.colorado.edu/hndbksupport/dist/dist.html

4. http://en.wikipedia.org/wiki/Fractional_distillation

5. http://www.chemistrydaily.com/chemistry/Chromatography

To find out more about Methods of Purification and

Analysis, click on the links below!

http://orgchem.colorado.edu/hndbksupport/filt/filtration.htmlhttp://en.wikipedia.org/wiki/Distillationhttp://orgchem.colorado.edu/hndbksupport/dist/dist.htmlhttp://en.wikipedia.org/wiki/Fractional_distillationhttp://www.chemistrydaily.com/chemistry/Chromatographyhttp://www.chemistrydaily.com/chemistry/Chromatographyhttp://en.wikipedia.org/wiki/Fractional_distillationhttp://orgchem.colorado.edu/hndbksupport/dist/dist.htmlhttp://en.wikipedia.org/wiki/Distillationhttp://orgchem.colorado.edu/hndbksupport/filt/filtration.html

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Mixture and Seperations