C08 mixtures and separations

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Identify the different types of solutionsIdentify the different types of solutions Investigate experimentally the effect of structure and Investigate experimentally the effect of structure and

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

Identify suitable separation techniques based on Identify suitable separation techniques based on difference in properties of the components of difference in properties of the components of mixturesmixtures

Describe the extraction of sucrose from sugar caneDescribe the extraction of sucrose from sugar cane

Mixtures and SeparationsChapter 8

Learning Outcomes

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

Pure and impure substances

A pure substance has a fixed melting point and boiling point.For example, pure water boils at 100 oC, and melts at 0 oC.

b.p. 100 oC

m.p. 0 oC


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. It melts 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.


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 or separated by physical

methods.


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A MixtureA Mixture Air is made up of different gases: nitrogen, oxygen, carbon Air is made up of different gases: nitrogen, oxygen, carbon

dioxide and noble gases such as argon, neon, and helium.dioxide and noble gases such as argon, neon, and helium. The gases in air can be easily separated by liquefaction followed The gases in air can be easily separated by liquefaction followed

by fractional distillation. by fractional distillation. Air is an example of a mixture. Air is an example of a mixture.

A mixture is made up of two or more substances not chemically combined together.

Mixtures and SeparationsChapter 8

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Composition of a mixtureComposition of a mixture A mixture can be made up of A mixture can be made up of two or more two or more

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

A mixture can be made up of two or more elements or compounds. E.g. Air is a mixture which contains both elements and compounds.

Copperatom

Zincatom

Air contains both elements and compounds.


Brass

Element

Compound

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3. Hold a magnet over the mixture of iron filings and sulphur. The iron filings are attracted by the magnet and 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

ExperimentExperiment


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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)

ExperimentExperiment


Homogeneous mixtures A solution is a homogeneous mixture. 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.


Homogeneous mixtures Solubility of a solution refers to the amount of solute

that is able to dissolve in a given quantity of solvent at a fixed temperature 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.)


Heterogeneous mixtures Heterogeneous mixtures have non-uniform

compositions which can be mechanically separated. Examples of heterogeneous mixtures are colloids and

suspensions. Colloids include fog, smoke, shaving cream, milk,

blood, styrofoam, gelatin, and cheese. Suspensions include muddy water, paint and chalk

powder suspended in water.


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Quick check 1Quick check 11.1. Explain why air is a mixture but water Explain why air is a mixture but water

is a compound. is a compound.2.2. (a), State two reasons why a mixture of iron (a), State two reasons why a mixture of iron

filings and sulphur before heating is a mixture.filings and sulphur before heating is a mixture.(b) When the mixture of iron filings (b) When the mixture of iron filings and sulphur is heated strongly, and sulphur is heated strongly, state two reasons why state two reasons why the solid formed is a the solid formed is a compound. compound.

3.3. State whether the following diagrams on the State whether the following diagrams on the right represent elements, mixtures or right represent elements, mixtures or compounds.compounds. Solution


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Solution to Quick check 1Solution 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 be separated by physical means. Water is a compound because it is made up of hydrogen and oxygen chemically joined together. We cannot separate the gases in water by physical means.

2. (a) It is a mixture because the iron and sulphur can be separated by physical means e.g. by using a magnet. Also, no heat or light is given out when 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 and light are given off. Also, the sulphur and iron in the compound can no longer be separated by physical means such as by using a magnet.

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


Methods of separating Mixtures Filtration Evaporation Crystallisation Sublimation Simple distillation Fractional distillation Use of separating funnel Chromatography


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 the filter paper is called residue.


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

completely dry.

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


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.


Sublimation Sublimation is the method used to

separate a substance which sublimes from a mixture.

Examples of substances which sublime 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 the inside of the funnel.

To separate ammonium chloride from sodium chloride, this method can be employed.


Simple distillation Distillation is the method used to

obtain a pure solvent from a solution. E.g. pure water from seawater.

It is done by heating the solution in a distillation flask and collecting the vapour that boils off.

A condenser is used to condense the hot vapour and change it to a liquid.

The condenser is cooled by the flow of cold water (in the condenser) from the tap. The pure liquid collected from

distillation is called distillate.


Fractional distillation Fractional distillation is used to

separate two or more miscible liquids with different boiling points.

A fractionating column and acondenser are used.

Example: Mixture of ethanol and water When the thermometer reaches a

steady temperature of 78 oC,ethanol is collected.

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


Separation of ethanol-water mixture by fractional distillation

Use of separating funnel This method is used to separate a mixture

of immiscible liquids. The mixture is placed into a separating

funnel and allowed to settle into two layers.

Example: Mixture of oil and water The tap is first opened to allow the water to

drain out of the funnel. After all the water has drained out, another

beaker is placed below the funnel and oil is now drained out.

Separation of oil-water mixture by using a separating funnel


Chromatography Chromatography is a method used to

separate and identify small quantities of substances.

There are different kinds of chromatography techniques, such as gas chromatography, liquid chromatography and paper chromatography.

It is based on the principle thatdifferent substances have differentsolubilities in the same solvent.

The more soluble substance will getcarried along faster by the solventand move further ahead than theless soluble substances.

Chromatography can be used to detect and identify very small quantities of substances. It is used to detect banned dyes used infood colourings, and detect illegal drugs used by athletes during major competitions.

Paper Chromatography


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 the chromatogram.

-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

chromatography paper

large beaker

glass cover

-The solvent front must be allowed to move as far up the paper as possible toensure that all the dyes are separated.


The Rf value of a substance

Rf value = distance moved by a substance distance moved by solvent front

(7 cm)(10 cm)

E.g. Rf value 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.


Paper chromatographyWorked example

The chromatogram shows 3 single dyes red, green and blue, and also four unknown 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 .


Quick check1. State the method you will use to

separate 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 (see diagram above). Identify the dyes present in the ink.

Solution


Solution to Quick check

1. State the method you will use to separate the following substances.(a) dissolution, followed by filtration(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. Blue and yellow dyes

Return


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!


ReferencesReferences

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver

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C08 mixtures and separations

Science

Transcript of C08 mixtures and separations