complete chemistry for igcse chapter 1

8/14/2019 Complete Chemistry for IGCSE Chapter 1

1/20

Everything is made of particles1.1

6

Made of particlesRock, air, and water look very different. But they have one big thing incommon: they are all mad e of very tiny pieces, far too small to see.For the moment, we will call these pieces particles .

In fact everything aroun d you is made of particles and so are you!

Particles on the moveIn rock and other solids, the particles are not free to move arou nd . But inliquids and gases, they move freely. As they mov e they collide with eachother, and bounce off in all directions.

So the path of one particle, in a liquid or gas, looks like this:

The particle moves in a ran dom way, changing direction every time it hitsano ther p article. We call this random motion .

Some evidence for particlesThere is evidence all aroun d you that things are m ade of par ticles, and thatthey move arou nd in liquid s and g ases. Look at these examples.

Evidence outside the lab

1 Cooking smells can spread through the hou se. This isbecause 'smells' are caused by gas particles mixing with,and moving th rough , the air. They d issolve in moisture inthe lining of your nose.

2 You often see d ust an d smoke d ancing in the air, inbright sunlight. The d ust and smoke are clusters of particles. They d ance around because they are beingbombard ed by tiny particles in the air.

All made of particles!


2/20

Evidence in the lab

1 Place a crystal of potassium manganate(VII) in abeaker of w ater. The colour spread s because p articlesleave the crystal and mix through the wa ter particles.The crystal dissolves .

2 Place an open gas jar of air upside d own on an op engas jar of red-brown brom ine vapou r. The colour spread sup ward s because par ticles of bromine mix throug h thepa rticles of air.

DiffusionIn all the examples above, particles bounce off all directions when theycollide. In this way, they get mixed. This mixing process is called diffusion .

So what are these particles?The smallest pa rticle that cannot be broken d own by chemical means is calledan atom .

In some substances, the particles are just single atoms. For example the gasargon, found in air, is mad e up of single argon atoms

In many substances, the particles consist of two or more atoms joinedtogeth er. These p articles are called molecules . Water, bromine, and thegases nitrogen and oxygen in air, are made u p of molecules.

In other substances the particles consist of atoms or group s of atoms thatcarry a cha rge. These par ticles are called ions . Potassium mang anate(VII)is mad e of ions.

You ll find out more a bou t all these p articles in later sections.

Seeing particlesWe are now able to see the particles in some solids, using very powerfulmicroscopes. For examp le the image on the right sh ows th e atoms in silicon,wh ich is used to m ake compu ter chips. In this image, the atoms app ear over

70 million times larger than they really are!

Questions1 The particles in liquids and gases show random motion .

What does that mean, and w hy does it occur?2 Why does the pu rple colour spread when a crystal of

potassium mangan ate(VII) is placed in w ater?

3 Bromine vap our is heavier than air. Even so, it spread sup ward s in the experiment above. Why?

4 a What is diffusion ? b Use the idea of diffusion toexplain how th e smell of perfume travels.

The atoms in silicon. The imag e wastaken u sing a tun nelling electronmicroscope. The colour wa s add ed tohelp you see them m ore clearly.


3/20

Solids, liquids, and gases1.2

8

Water: solid, liquid and gas

Water can be a solid (ice), a liquid (water), and a gas (water v apou r orsteam). Its state can be changed by h eating or cooling:

1 Ice slowly changes to water ,wh en it is put in a war m place.This chang e is called melting .The thermom eter shows 0C until allthe ice has melted , so 0C is called itsmelting point .

2 When the w ater is heated itstemperature rises, and some of itchanges to water vapour . This changeis called evaporation . The hotter th ewater gets, the more quickly itevaporates.

3 Soon bu bbles appear in the w aterIt is boiling . The water vap ourshows u p as steam. The thermom etestays at 100C while the water boilsoff. 100C is th e boiling point of water.

You can see th at:

condensing is the opp osite of evaporating

freezing is the opposite of melting

the freezing point of water is the same as the melting point of ice, 0 C.

Whats the difference?It is easy to tell the difference between a solid, a liquid and a gas:

A gas has no definite volume orshape. It spread s ou t to fill itscontainer. It is mu ch lighter th an th esame volum e of solid or liquid.

A liquid flows easily. It has a definitevolume bu t no d efinite shape. Ittakes the shap e of the container.

A solid has a d efinite shape and adefinite volum e.

And wh en steam is cooled, the op posite changes take place:


4/20

Heating curve for water

0 1 2 3 4 5 6 7 8 9 10Time (minutes)

150

125

100

75

50

25

0

-25

water turning to vapour

vapourgettinghotter

ice warming up

icemelting

T e m p e r a

t u r e

( C )

water warming up

Molten iron being pou red out at an ironwor ks. Hot over 1540 C!

Questions1 Write down two p roperties of a solid, two of a liquid, and

two of a gas.2 Which word m eans the opp osite of :

a boiling? b melting?3 Which has a lower freezing p oint, oxygen or ethanol?4 Which has a higher boiling point, oxygen or ethanol?

5 Look at the heating curve above.a About how long did it take for the ice to melt, once

melting started?b How long did boiling take to complete, once it started?c Try to th ink of a reason for th e difference in a an d b .

6 See if you can sketch a h eating curve for sodium .

Other things can change state tooIts not just water! Nearly all substances can exist as solid, liquid and gas.Even iron and d iamond can melt and boil! Some melting and boiling pointsare given below. Look how different they are.

Substance Melting point/C Boiling point/C

oxygen 219 183ethanol 15 78sod ium 98 890sulphur 119 445iron 1540 2900d iamond 3550 4832

Showing changes of state on a graph

Look at this graph. It shows how the temp erature changes as a block of ice issteadily heated. First the ice melts to water, then th e water gets w armer an dwarmer, and eventually turns to steam:

A graph like this is called a heating curve .

Look at the step wh ere the ice is melting. Once melting starts, the temp eraturestays at 0C until all the ice has melted. When th e water starts to boil, thetemp erature stays at 100C un til all the water has tu rned to steam. So themelting and boiling points are clear and sharp .


5/20

Particles in solids, liquids, and gases1.3

0

How the particles are arrangedWater can change from solid to liquid to gas. Its particles do n ot change.They are the same in each state. But their arrangement changes.The same is true for all substances.

State How the particles are arranged Diagram of particles

Solid

Liquid

Gas

The particles in a solid are packedtightly in a fixed pattern. There arestrong forces holding them together.So they cannot leave their p ositions.The only movem ents they make aretiny vibrations to and fro.

The particles in a liquid can moveabout an d slide p ast each other. Theyare still close together, but a re not ina fixed pattern. The forces that holdthem together are weaker than in asolid.

The particles in a gas are far ap art,and they move abou t very quickly.

There are almost no forces holdingthem together. They collide with eachother an d boun ce off in all directions.

Changing stateMelting When a solid is heated, its particles get more energy and vibratemore. This makes the solid expand . At the m elting p oint, the par ticles vibrateso mu ch that they break away from their positions. The solid tu rns liquid.


6/20

Questions1 Using the idea of particles, explain w hy:

a you can pour liquids b solids expand on heating2 Draw a diagram to show w hat hap pens to the particles,

when a liquid cools to a solid.

3 In wh ich su bstance is the force between p articles stronger?(Use the table above.)a in solid sodium, or in ice?b in liquid sodium, or in water?

Boiling When a liquid is heated, its particles get more energy and movefaster. They bum p into each other m ore often, and boun ce furth er apar t.This makes the liquid expan d. At the boiling point, the p articles get enoughenergy to overcome the forces between them. They break awa y to form a gas:

T e m p e r a t u r e

( C )

he a t a dded kJ

he a t formelting

he a t forboiling

melting

boiling

Even though you keep on add ing heat,the temperature stays steady w hile thesubstance melts, and a gain w hile it boils.

Evaporating Some p articles in a liquid have m ore energy than others. Evenwell below the boiling point, some hav e enough energy to escape and form agas. This is called evaporation . It is why pu dd les of rain d ry up in the sun.

Condensing and solidifying When a gas is cooled, the p articles lose energy.They move more and more slowly. When they bu mp into each other, they do nothave enou gh energy to bou nce away again. They stay close together, and aliquid forms. When the liquid is cooled, the p articles slow d own even more.Eventually they stop m oving, except for tiny v ibrations, and a solid forms.

How much heat is needed?The amou nt of heat n eeded to melt or boil a su bstance is different for everysubstance. Thats because the particles in each substance are different, withdifferent forces between them. The stronger the forces, the more heat energy

is needed to overcome them.This table shows the am ount of heat needed to melt and boil some substances.(1 mole of each substance has exactly the same number of particles.)

Substance Amount of heat (in kilojoules) needed to melt 1 mole of it boil 1 mole of it

ethanol 5.0 38.7water 6.0 41.2sod ium 2.6 97.0

Note that it always takes m ore heat to boil a substance than to m elt it.Thats because it takes a lot more energy to separate the particles completely(in boiling) than to free them from a lattice (in melting).


7/20

A closer look at gases1.4

2

When you blow air into a balloon,the gas par ticles exert pressure on theballoon and inflate it. The more youblow, the greater th e pressure.

The particles in this gas are m ovingfast. They hit the walls of thecontainer and exert pressure onthem. If you now heat the gas . . .

. . . the particles take in heat energyand move even faster. They hit thewalls more often, and w ith moreforce. So the gas p ressure in creases.

The same happ ens with all gases:When you heat a gas in a closed containe r, its pressure increases.That is why th e pressure gets very high inside a pressure cooker. In a pressure cooker, water vap our is

heated to w ell over 100C, so its pressure

is very high. You m ust let a pressurecooker cool before you open it!

The same thing is true for all gases:When a gas i s squeezed into a smaller space, its pressure i ncreases.

All gases can be squeezed into a sma ller space, or compressed . If enoughforce is app lied to the p lunger a bove, the gas pa rticles will get so close thatthe gas tu rns into a liquid. But liquids an d solids cannot be comp ressed.Their particles are already very close together.

What is gas pressure?When y ou blow up a balloon, you fill it with air p articles. They hit againstthe sides of the balloon and exert pressure on it. This pressure is whatkeeps the b alloon inflated.

In the same w ay, all gases exert a pressure. The pressure d epend s on thetemperature of the gas and the volume it takes u p, as yo ull see below.

When you heat a gas

There is a lot of space between th epa rticles in a gas. You can force them

closer by pushing in the plu nger

like this. Now the particles are in asmaller space so they h it the wa lls

more often. So the gas pressureincreases.

Cylinders of compressed a ir, that allowdivers to breathe under water.

When you squeeze a gas into a smaller space


8/20

Questions1 What causes the pressure in a gas?2 Why does a balloon burst if you keep on blowing?3 A gas is in a sealed container. How do you th ink the

pressure will change if the container is cooled?Explain you r answ er.

4 A gas flows from one container into a larger one.What do you th ink will happen to its pressure?Draw d iagrams to explain.

5 a Why does the scent of perfume spread?b Why d oes the scent of perfume wear off faster in

warm w eather than in cold?6 Of all gases, hydrogen diffuses fastest at any given

temperatu re. What can you tell from th is?7 Look at the glass tube above. Sup pose it was warm ed a

little in an oven , before the experiment. Do you think thatwould change the result? If so, how?

The diffusion of gasesOn pag e 7 you saw th at gases diffuse because the p articles collide with otherparticles, and bounce off in all directions. But gases do not all diffuse at thesame rate, every time. It depend s on th ese factors:

1 The mass of the particlesThe particles in hyd rogen chloride gas are tw ice as heavy as those inammonia gas. So which gas do you think will diffuse faster? Lets see:

Cotton wool soaked in ammonia solution is put into one end of a longtube. It gives off ammonia gas.

A t the same time , cotton w ool soaked in hyd rochloric acid is pu t into theother end of the tube. It gives off hydrogen chloride gas.

The gases diffuse along the tube. Smoke forms where they meet:

The white smoke forms closer to the h ydrochloric acid end of the tube.So the amm onia particles have travelled furth er than th e hydrogen chlorideparticles which mean s they h ave travelled faster .The lower the mass of its particles, the faster a gas will diffuse.

That makes sense when you think abou t it. When p articles collide and boun ce

away, the lighter particles will bounce further.The particles in the above two gases are molecules. We use the term relativemolecular mass for the mass of a molecule. So we can also say:The lower its relative molecular mass, the faster a gas will diffuse .

2 The temperatureWhen a gas is heated, its particles take in heat energy, and move faster. Sothey collide with m ore energy, and bou nce furth er away. And so the gasdiffuses faster.The higher the temperature, the faster a gas will diffuse.

The scent of flowers travels faster in awarm room. Can you explain why?

The faster a particle is moving wh en it hitsanother, the faster and further it willboun ce away. Just like snook er balls!


9/20

The sugar has dissolved in the w ater, giving a solution.Sugar is the solute , and water is the solvent :

solute + solvent = solution

You can t get the su gar ou t again b y filtering.

Not everything dissolves so easilyNow think abou t chalk. If you m ix chalk powd er with wa ter, most of the pow der even tually sinks to the bottom. You can get it ou t again byfiltering.

Why is it so different for sugar an d chalk? Because their particles are verydifferent! How easily a su bstance dissolves d epend s on th e particles in it.Look at the examp les in th is table:

Mixtures, solutions, and solvents1.5

4

A mixture of sugar and water. Thismixture is a solution.

A mixture of chalk powder and water.This is not a solution. The tiny chalk particles do not separate and spreadthrough the wat er particles. They stayin clusters big enough to see. In time,most sink to the bottom.

MixturesA mixture contains more than one substan ce. The substances are justmixed together, and not chemically combined. For examp le:

air is a mixture of nitrogen, oxygen, and small amounts of other gases shampoo is a mixture of several chemicals and w ater.

SolutionsWhen you mix sugar with w ater, the sugar seems to disapp ear. That isbecause its particles spread all through the w ater p articles, like this:

Compound Mass (g) dissolving in 100 g of water at 25C

silver nitrate 241.3calcium nitrate 102.1sug ar (glucose) 91.0potassium nitrate 37.9 d ecreasingpotassium su lphate 12.0 solubilitycalcium hydroxide 0.113calcium carbonate (chalk) 0.0013silver chloride 0.0002

So silver nitrate is much more soluble than sugar but p otassium sulph ateis a lot less soluble than su gar. It all dep end s on th e par ticles.

Look at calcium hydroxide. It is only very slightly or sparingly so lublecompared with th e compou nd s above it. Its solution is called lime water .

Now look at the last two substances in the table. They are u sually calledinsoluble since only a very tiny am oun t dissolves.

Whats so lub le, and w hats no t?

The solubility of every substanceis different.

But there are some overallpattern s. For example all sodiucompounds are soluble.

Find out more on page 126.


10/20

Questions1 a Are all solutions m ixtures? b Are all mixtures solutions?2 Explain each term in your own words. (Check the glossary?)

a insoluble b solubility c aqueou s solution3 Look at the table on p age 14.

a Which substance in it is the m ost soluble?b About how many times more soluble is this substance

than potassium sulp hate, at 25 C?c The substance gives a colourless solution. What w ill you

see if you ad d 300 g of it to 100 g of wa ter at 25 C?d What will you see if you heat u p the m ixture in c?

4 Now turn to the table at the top of page 126.a Name two m etals that have no insoluble salts.b Nam e one other group of salts that is always soluble.

5 See if you can give t hree exam ples of:a solids you dissolve in w ater, at hom eb insoluble solids you u se at home.

6 Name tw o solvents other than water that are used in thehome. What are they used for?

7 Many gases dissolve in water too. See how manyexamples you can think of. (Fish need one of them!)

Nail p olish is insoluble in w ater. So shewill remove it later by dissolving it inpropanone.

So sugar is more solu ble in hot water tha n in cold w ater.If a solid i s sol uble in water, it usually gets more soluble as thetemperature rises.

Water is not the only solventWater is the worlds most common solvent. A solution in water is called anaqueous solution (from aqua , the Latin w ord for w ater).

But ma ny other solvents are used in ind ustry and about the hou se, to dissolvesubstances that are insoluble in wa ter. Some examp les are:

Solvent It dissolves

white sp irit gloss pain t

propanone (acetone) grease, nail polishethanol glu es, p rinting inks, the scented su bstances that are

used in perfumes and aftershaves

All three solvents above evaporate easily at room temp erature they arevolatile . This means th at glues an d paints d ry easily. Aftershaves feel coolbecause ethanol cools the skin wh en it evaporates.

Helping a solute dissolve

water

stirring rodsugar

Sugar dissolves quite slowly in w aterat room tem peratu re. Stirring helps.But if you keep on ad ding su gar

eventually no m ore of it w illdissolve, no matter how hard youstir. The extra sinks to the bottom.The solution is now saturated .

But look what h app ens if you heatthe solution. The extra su gardissolves. Add more sugar an d it willdissolve too, as the temperatu re rises.

extra sugar sinksto b ottom

all the sugar

has dissolved

heat


11/20

Pure substances and impurities1.6

6

What is a pure substance?

This water has particles of apoisonous su bstance in it. It is notpu re and could be dead ly.

But this water has p articles of othersubstances mixed with it. So it isnot pure.

This is water. It has only waterparticles in it, and nothing else. So itis 100% pure .

wa terpa rtic le

wa terpa rtic le

wa terpa rtic l

A pure sub stance has no particles of any other substance mixed w ith it.

In real life, very few substances are 100% pure. Tap water contains smallamou nts of m any different p articles (for example calcium ions andchloride ions). The p articles in it are not usu ally harmful and some areeven good for you. Distilled w ater is mu ch pu rer, but still not 100% pu re.For example it has gas p articles from th e air d issolved in it.

Does purity matter?Often, it d oes not m atter if a substance is not pu re. Most of the time w euse tap w ater without th inking about w hats in it. But sometimes pu rity isvery importan t. If you are making a new med ical drug , or food flavouring,you must make sure it contains nothing that could harm people.

An u nw anted su bstance, mixed with the su bstance you wan t, is called animpurity.

Baby foods and milk powd er are tested in the factory,to make sure they contain no harmful impurities.

Vaccination against p olio, by mouth . Medicines must be safe,and free of harm ful impu rities. So they are tested heavily.


12/20

How can you tell if a substance is pure?Chemists use some comp lex methods to check pu rity. But th ere is one simplemethod you can use in the lab: you can check melting and boiling points .

A pu re substance has a definite, sharp , melting point and boiling point.You can look these up in d ata tables.

When the substance contains an impurity, its melting point falls and itsboiling p oint rises. And melting and boiling no longer take place sharp ly,but over a range of temperatu re.

The more impu rity present, the bigger the change in melting and boilingpoints, and the wider the temperature range over which the substancemelts and boils.

Compa re these examp les:

Questions1 What does a pure substance mean?2 You m ix instant coffee with w ater, to make a cup of coffee.

Is the coffee an impurity ? Explain.

3 Explain w hy melting and boiling points can be used as away to check purity.

4 Could th ere be impu rities in a gas? Explain.

This sulphu r sample melts sharply at119C an d boils at 445C. So it is p u re.

This water freezes aroun d 0.5C andboils aroun d 101C. So its not pu re.

So, melting an d boiling points w ill give you an idea of pu rity. But if you aremaking a n ew med ical drug , you will also do several other pu rity checks.

Separation: the first step in obtaining a pure substanceWhen you carry ou t a reaction to make something, you u sually end up with amixture of substances. Then you have to separate the one you wan t.

The table below shown some separation method s. These can give quite puresubstances. For examp le when you filter off a solid, and rinse it really w ellwith distilled water, you remove a lot of impurity. But it is just not possible toremove every tiny p article of impur ity, in the school lab.

Method of separation Used to separate

filter a solid from a liqu idcentrifuge a solid from a liqu idevaporate a solid from its solu tioncrystallise a solid from its solu tiond istil a solvent from a solu tionfractional distillation liquid s from each other

chromatograp hy a mixture of substances from a solution

There is more about th ese methods in the next three un its.

At the end of this reaction, the beakermay contain several products, plusreactants that have not reacted.Separating them can be a challenge!

ID check!

Every substance has a unique pairof melting and boiling points.

So you can use melt ing andboiling p oints to identify asubstance.

First, measure them. Then look updata tables to find out w hat thesubstance is.


13/20

Separation methods (I)1.7

8

c lea r liquid

pa rtic lesof solid

suspendedin liquid

solid flungto bottom

Before centrifuging , the solid ismixed all throug h the liquid.

After centrifuging, all the solid hascollected at the bottom.

The liquid is poured ou t of the test tubes, or removed with a sm all pipette.The solid is left behind.

3 By evaporating the solventIf the mixture is a solution , the solid cannot be separa ted by filtering orcentrifuging, because its tiny p articles are spread all through the solvent.

Instead, the solution is h eated to evaporate the solvent. So the solid is leftbehind. You could use this method to obtain salt from its aqueou s solution,for example:

Which method?As you saw in the last unit, there are several ways toseparate mixtures of substances. The method you choosedep ends on the ph ysical state of the substances.

Separating a solid from a liquid1 By filteringChalk can be separated from water by filtering, as shownhere. The chalk gets trapp ed in th e filter paper w hile thewater passes through.

The chalk is called the residue . The w ater is the filtrate .

Filtering is very w idely used to separate solids from liquids.For example it is used in cleaning u p river water, to make itfit to d rink. Can you th ink of a use in the kitchen?

2 By centrifugingA centrifuge is used to separ ate small amou nts of solid an d liquid.Inside the centrifuge, test tubes are sp un very fast, so the solid gets flungto the bottom:

Evaporating the w ater from a solutionof salt in water.

A centrifuge.


14/20

1 A solution of copper(II)sulpha te is heated, to get rid of somewater. As the w ater evap orates, thesolution becomes more concentrated.

2 The solution can be checked to seeif it is ready b y placing one d rop on amicroscope slide. Crystals shouldform qu ickly on th e cool glass.

Evaporating the etha nol from a solution

of sugar in etha nol, over a water bath .

This is a saturated solution of copper(II) sulph ate in water at 70 C.If it is cooled to 20 C . . .

. . . crystals begin to appear, becausethe compoun d is less soluble at 20 Cthan at 70 C.

Separating a mixture of two solidsYou can separate two solids by choosing a solvent that w ill dissolve just oneof them. Sup pose the solids a re salt and sand. Water dissolves salt but n otsand. So you can separate th em like this:

1 Add water to the m ixture, and stir. The salt d issolves.2 Filter the mixture. The sand is trapp ed in the filter pap er, but th e salt

solution passes through.3 Rinse the sand with w ater, and d ry it in an oven.4 Evaporate the salt solution u ntil dry salt is left.

Water could not be used to separate salt and sugar, because it dissolves both.But you could u se ethanol, which dissolves sugar bu t not salt. Ethanol isinflamm able, so should be evap orated over a w ater bath, as shown h ere.

Questions1 What does this term mean? Give an example.

a filtrate b residue2 You h ave a solution of sugar in w ater. You w ant to get t he

sugar from it.a Explain w hy filtering w ont w ork.b Which method will you use instead?

3 Describe how you w ould crystallize potassium nitratefrom its aqueous solution.

4 How w ould you separate salt and sugar? Mention anyspecial safety precaution you w ould tak e.

5 Now see if you can think of a w ay to get clean sand froma mixture of sand and little bits of iron w ire.

4 By crystallisingYou can obtain man y solids from their solutions by allowing them to formcrystals. Copper(II) sulphate is an example:

3 Then the solution is left to cooland crystallize. The crystals areremoved by filtering, rinsed withwater and dried w ith filter pap er.

The p rocess is called crystallisation . It is carried ou t like this:


15/20

Separation methods (II)1.8

0

Simple distillationThis is a way to obtain the solvent from a solution. The apparatu sis shown on the right. It could be used to obtain water from saltwa ter, for examp le. Like this:

1 The solution is heated in the flask. It boils, and steam rises intothe condenser. The salt is left behind.

2 The condenser is cold, so the steam conden ses to water in it.

3 The water drips into the beaker. It is called distilled water .It is almost pu re.

You could use this method to get drinking water from sea w ater.Many countries in the Middle East obtain d rinking water bydistilling sea water in giant d istillation p lants.

Fractional distillationThis is a way to separate a mixture of liquids from each other. It makesuse of their d ifferent boiling poin ts. You cou ld u se it to separat e amixture of ethanol and water, for example. The ap paratu s is shownon the right.

These are the steps:1 The mixture is heated. At about 78C, the ethanol begins to boil.

Some w ater evaporates too, at that temp erature. So a mixture of ethanol vapour and water vapour rises up the column.

2 The vapou rs condense on the glass beads in the column, makingthem hot.

3 When the bead s reach abou t 78C, ethanol vapour no longercondenses on them. Only the water vapou r does. So water dr ipsback into the flask, while the ethanol vapou r goes into thecondenser.

4 There it cond enses. Pure liquid ethanol d rips into the beaker.

5 Eventually, the thermometer reading rises above 78C. This is asign that all the ethanol has gon e. So heating can be stopp ed.

Fractional distillation in industry

Fractional distillation is very importan t in indu stry.

It is used in the oil industry to refine crude oil into grou ps of similarcompou nd s. The oil is heated and the vap ours rise to different heights,up a tall steel fractionating column. (See page 241 for more.)

It is also used in p rodu cing ethanol , in indu stry. Some ethan ol is madeby fermenting sugar cane and other plant m aterial. When fermentationis over, the fermented liquid is run off. Then it und ergoes fractionaldistillation, to separate th e ethanol from the oth er substances in it.The ethanol is used as a solvent, and as fuel for cars on its own or m ixedwith petr ol. (See page 250 for more.)

An oil refinery. Petrol is produ ced here,as well as man y other u seful chemicals.


16/20

1 A drop of black ink is put at thecentre of a piece of filter pa per, andallowed to d ry. Three or four moredrops are added on the same spot.

2 Water is then d ripped on to theink spot, one drop at a time. The ink slowly spreads out and separatesinto rings of different colours.

1 Concentr ated solution s of X, A, B,C, and D are made u p in propanone.A spot of each is placed on a line, ona sheet of filter paper, and labelled.

2 The pap er is stood in a littleprop anone, in a covered glass tank.The solvent rises up th e pap er; whenits near the top , the pap er is takenout again.

3 X has separated into three spots.Two are at the same height as A andB, so X mu st contain su bstan ces Aand B. Does it also contain C and D?

The substances in the ink tra vel across the p aper at d ifferent rates. Thats wh ythey separate into rings. The filter paper sh owing th e separatesubstances is called a chromatogram .

Paper chromotograp hy can also be used to identify substances. For example,mixtu re X is thoug ht to contain su bstan ces A, B, C, and D, which are allsoluble in propan one. The mixture could be checked like this:

The substances shown here are all coloured. You can also use pap erchromatography for colourless substances, and to identify a substance fromtables, by measuring how far it travels. See the next u nit for more.

Questions1 How w ould you obtain pure w ater from sea water?

Draw the app aratus, and explain how the method works.2 Why are condensers called th at? Whats the cold w ater for?3 You w ould n ot use exactly the same apparatus you

described in 1, to separate ethan ol and w ater. Why not?

4 Explain h ow fractional distillation w orks.5 In the last chromatogram above, how can you tell that X

does not contain substance C?6 Look at the first chromatogram above. Can you think of a

way to separa te the coloured su bstances from the p aper?

blue ring

red ringyellow ring

Paper chromatographyThis method can be u sed to separ ate a mixture of substances. For examp le,you could u se it to find out h ow m any d ifferent coloured substances there arein black ink:

3 Sup pose there are three rings:yellow, red and blue. This shows th atthe ink contains three su bstances,coloured yellow, red a nd blue.


17/20

More about chromatography1.9

2

Making use of chromatographyChromatograp hy is really useful. You can u se it to:

separate mixtures of substances

pu rify a substance, by separating it from its impurities

ident ify a subs tance .

Example: Identify substances in a colourless mixtureOn pag e 21, pap er chromatograph y was used to identify coloured substances. Now for a bigger challenge!

Test tu bes A E on th e right below contain five colourless solutions of compou nd s called am ino acids. A contains several amino acids. The otherscontain just on e each. Your task is to id entify all the amino acids in A E.

1 Place a sp ot of each solution along a line draw n on slotted filter paper,as show n h ere. (The slots are to keep the sam ples separate.) Label eachspot in p encil at the top of the pap er.

1 The dots represent the twodifferent su bstances in this m ixture.They are carried along in the m obile

ph ase, dissolved in the solvent.

2 The two su bstances travel overthe stationary p hase at d ifferentspeeds because they have

d ifferent levels of attraction to it.

3 Eventually they get comp letelyseparated from each other. Now youcan collect each substan ce, and / or

identify it.

How chromatography worksYou already met p aper chrom atograph y. There are other kinds too. All of them depend on the interaction between:

a n on -m ov in g or stationary ph ase (such as pap er) and a moving or mobile ph ase. This consists of the mixture you wan t to

separate, dissolved in a solvent.

The five mystery solutions. The solventin each is wa ter.

A scientist using gas chromatography toidentify the substances in a mixture.The sample goes into the machine on th eleft of the screen. From th e peaks on th egraph , she can tell what is in it.


18/20

2 Place a suitable solvent in the bottom of a beaker. (A mixture of water,ethanoic acid and butan ol is suitable.)

3 Roll the filter paper into a cylinder and place it in the beaker. Coverthe beaker.

4 The solvent rises up the p aper. When it has almost reached the top,remove the pap er.

5 Mark a line on it to show w here th e solvent reached . (You can t tell wherethe am ino acids have reached because they are colourless.)

6 Put the paper in an oven to dry out.

7 Next spray it with a locating agent to make the amino acids show up .Ninhydrin is a good choice. (Use it in a fume cupboard!) After spraying,heat the p aper in the oven for 10 minu tes. The spots turn p urp le. So nowyou have a proper chromatogram. ( Chroma means colour .)

8 Mark a p encil dot a t the centre of each sp ot. Measure from th e base line toeach dot, and to the line show ing the final solvent level.

Questions1 Say wha t each term means, in chromatograph y:

a stationary phaseb mobile phase

2 Explain in your own word s how chromatography w orks.3 a What do you think a locating agent is?

b Why is one needed in the experiment above?

4 For the chromatogram above:a Were any of the amino acids in B E also present

in A? How can you tell at a glance?b Using a ruler, work ou t the R f values for the amino

acids in AE.c Now use the R f table above to name them.

solventlevel

sta rtingpoint

A B C D E

9 Now work out the R f value for each amino acid. Like this:

distance moved by amino acid R

f value = distance moved by sol vent

10 Finally, look up R f tables to identify the am ino acids. Part of an R f tablefor the solvent you used is shown on the right.

The R f value of a compound is always the same for a given solvent, underthe same conditions.

Rf values for amino acids

(for water/ butanol/ ethanoicacid as solvent)

amino acid R f value

cysteine 0.08

lysine 0.14

glycine 0.26

serine 0.27

alanine 0.38

proline 0.43

valine 0.60

leucine 0.73


19/20

Questions on Chapter 1

4

1 A large crystal of potassium mangan ate(VII) wasplaced in the bottom of a beaker of cold w ater andleft for several hour s.

a Describe what w ould be seen:i after five minutes ii after several hou rs

b Explain your answers u sing the idea of particles.c Nam e the two processes that have taken p lace

du ring the experiment.

2 Use the idea of particles to explain why:a solids h ave a d efinite shapeb solids cannot be pou redc liquids fill the bottom of a contain erd you cant store gases in open containerse you cant squeeze a sealed plastic syringe that is

comp letely full of waterf a balloon expand s as you blow into it.

3 The graph below is a heating curv e for a pu resubstance. It shows how the temperature rises with

time, when th e solid is heated steadily until it melts,and then th e liquid is heated un til it boils.

a What is the melting point of the substance?b What is its boiling p oint?c What happ ens to the temperature while the

substance changes state?d The graph sh ows that th e substance takes longer

to boil than to melt. Give a reason for this.e How can you tell that the substance is not w ater?f Sketch a rough heating curve for pu re water.

4 A cooling curve is the opp osite of a heating curve.It show s how the temp erature of a substancechanges with time, as it is cooled from a gas to asolid. H ere is the cooling curve for one substance:

a What is the state of the su bstance at roomtemperatu re (20C)?

b Use the list of melting and boiling p oints onpage 9 to iden tify the su bstance.

5 Using the idea of particles explain why:a the smell of burnt food can travel all over

the houseb wh en two solids are p laced on top of each other,

they do not m ixc a liquid is used in a cars braking systemd pu mping u p you r bike tyres quite hard gives a

smooth ridee compressing a gas into half the volume will

dou ble its pressuref pollution from just one factory can affect a large

area of land.

6 Ammon ia is an alkaline gas that turn s litmus blue.It is lighter than a ir. A test tube of amm onia gas isplaced over a test tube of air, like this:

a After a short time the red litmus p aper in thelower test tube turns blue. Explain why.

b Would it make any difference if you reversedthe test tubes? Explain you r answ er.

c What w ill you see if the test tu be of air isreplaced by on e containing hyd rogen chloride?

T e m p e r a

t u r e

/ C

Time

115

17solid

liquid

ga s


20/20

7 a Which of these are examples of diffusion?i a helium balloon rising in airii a hy drogen-filled balloon d eflating, du e to

gas passing through the skinii i the smell of perfume from a person stand ing

on the other side of a roomiv sucking a dr ink from a bottle, using a straw

v an ice lolly turning liquid wh en left out of thefreezervi all the tea in the cup changing colour w hen

you add milk vii a heavy, coloured gas spreading d own

through a gas jarviii rice going soft when left in hot waterix a blue crystal forming a blue solution, when

it is left sitting in a glass of water.b For one of the examples of diffusion, draw a

diagram showing th e particles before and afterdiffusion h as taken place.

8 The rate of diffusion of a gas can be measured usingthis apparatus:

The glass tube is sealed at on e end with a p lug of plaster. This has tiny holes in it, just large en ough tolet gases pass through .Water w ill rise in the tu be if a gas escapes from thetube faster th an air enter s it. (Air is mainly nitrogenand oxygen.)a Explain why the water level in the tube rises,

when h ydrogen is the gas used.b What d oes this tell you about the rate of

diffusion of hyd rogen compared to air?c Explain your an swer to b using the idea of

particle mass.d The molecules in carbon dioxide are h eavier

than those in nitrogen and oxygen.So what d o you think will happ en to the levelof the water in the tu be, if the gas in the tube iscarbon dioxide? Explain you r an swer.

9 RelativeGas Formula molecular mass

methane CH 4 16helium He 4oxygen O 2 32nitrogen N 2 28chlorine Cl 2 71

Look at the table above.a Which two gases will mix fastest? Explain.b Which gas will take least time to escape from

a gas syringe?c Would you expect chlorine to d iffuse more

slowly than the gases in air? Explain.d An un know n gas diffuses faster than nitrogen,

but m ore slowly than m ethane. What you cansay abou t its relative m olecular m ass?

10 A mixture of salt and sugar h as to be separated,

using the solvent ethanol.a Which of the two substan ces is soluble inethanol?

b Draw a diagram to show how you wou ldseparate the salt.

c How could you obtain sugar crystals from thesugar solution, without losing th e ethanol?

d Draw a d iagram of the app aratus for c.

In a chromatograp hy experiment, eight colouredsubstances were spotted on to a piece of filterpap er. Three were the b asic colours red, blu e, andyellow. The other five were dyes, labelled AE.The resulting chromatogram is shown below.a Which dye contains only one basic colour?

b Which dy e contains all three basic colours?c Which basic colour is the most solu ble inpropanone?

12 You h ave th ree colourless solutions. Each containsan am ino acid you mu st identify.Explain how to do th is using chromatogra ph y.(Use the terms R f an d locating agent in your answ er,and show that you u nderstand wh at they mean.)

11

airH2plug ofporousplaster

hydrogengas (H2) in

water risingin tu b e

water

0

10

20

30

40

complete chemistry for igcse chapter 1

Documents