Calcium and magnesiumLook at samples of calcium and magnesium burning in oxygenand reacting with water and complete a table like this one.

... compound ... element ... Group II

ReactingmetalsReactingmetals

66

You are learning to:� Recognise that calcium reacts

quickly� Identify similarities

between calcium andmagnesium

� Construct equations fortheir reactions

BIG IDEASBIG IDEAS

Many of the hills andmountains in Europe are madefrom two rocks, limestone anddolomite. They are bothsedimentary rocks, oftencontaining marine fossils.Samples of the rocks fizz withdilute acids, releasing carbondioxide gas. Limestone ismostly calcium carbonate anddolomite is a combination ofcalcium and magnesiumcarbonates.1 Give three similarities

between dolomite andlimestone.

2 What evidence is therethat they formed beneaththe sea?

FIGURE 1: Mountainsfrom metals.

FIGURE 2:Groups I and II.

calcium � oxygen calcium oxide

Ca CaO O O++2 2

Calcium burns with an orange-red flame. Calcium oxide (CaO) isproduced, which is a white powder. The common name forcalcium oxide when it is made from limestone is quicklime.

Magnesium also reacts violently with oxygen, burning with abrilliant white flame to leave white magnesium oxide (MgO).

Both metals react with water giving off hydrogen gas butcalcium is more reactive.

Magnesium does react slowly with cold water, but it takes afew days to produce just a few bubbles of hydrogen gas.

FIGURE 3: Calcium burning in airwith an orange-red flame. What isthe solid product of this reaction?

calcium � water calcium hydroxide � hydrogenmagnesium � steam magnesium oxide � hydrogen

Group trendsMagnesium and calcium are in Group II in thePeriodic Table. Elements in the same group reactin similar ways – they show trends. Magnesium andcalcium burn vigorously in air. The word equationfor the reaction of calcium burning in air is:

Symbol With oxygen Metal with cold water

Mg

Ca

Why is it not surprising that magnesium and calcium reactin a similar way, when you study the Periodic Table?

What is the reactivity trend in Group II?

Symbolic changesAlthough the reactions of magnesium and calcium with air andwater are similar, they are not identical. With air, or with oxygenalone, the equations are:

With water or steam in the caseof magnesium:

... physical state ... quicklime ... state symbol 67

2Mg � O2 2MgO2Ca � O2 2CaO

Mg(s) � H2O(g) MgO(s) � H2(g)Ca(s) � 2H2O(l) Ca(OH)2(aq) � H2(g)

The letters in brackets tell us about the water in each case. Steam is a gasso H2O(g) is used and water is a liquid and is written H2O(l). These labels arecalled state symbols. They are used in equations to show what physicalstate a substance is in. The symbol for solid is (s) and for solution (aq).

a The element strontium (Sr) is below calcium in Group II. Predicthow fast strontium reacts with water compared to calcium.

b Write both word and symbol equations for the reaction of strontium(Sr) with water.

If the chemical formula for calcium sulphate is CaSO4, what is theformula for strontium sulphate?

All new elementRead the details of this unknown newelement and some of its properties.

What evidence is there that Nm is metallic?

Would you place it in Group I or II? Explainyour answer.

Write a symbol equation for the probablereaction of Nm with water.

What is the grey coating on the surface of Nm and what happensto a weighed sample of pure Nm as the coating forms?

Strontium compounds are used togive the red colour to somefireworks. It was once used as aspecial effect in the theatre inthe 19th Century and was called‘crimson fire’.Magnesium flares are also used togive a brilliant white light.

What metal compoundis in this firework?

Name Symbol

Newium Nm

How stored In dry air

Appearance Grey, shiny on a clean surface

With water Sinks, bubbles, flammable gas,solution has a high pH value

Mg O OH H

H HMg++ ++

Reaction speedsIf a metal reacts with water it is likely to react even faster withacids. Calcium fizzes more violently in acid than in water. Somemetals do not react at all. Gold stays shiny in water and in acidtoo. Gold is very unreactive.

Unreactive metals are used to make jewellery and coins and foruse outside in the weather.

Why does a gold bottle not corrode when it is filled withacid?

Gold never wears out so why is it not used for roofing?

Comparing reactivityMany metals react with acids and release bubbles of hydrogengas. We can use the rate of bubbling to compare the reactivityof metals. Reactivemetals bubble faster than unreactive metals.This allows a reactivity series to be drawn up of themetals that are tested.

Describe an investigation to find out which of twometals is the more reactive.

Describe an investigation to find out how much gasis released when a piece of metal reacts with anacid.

Developing a reactivity seriesWe can test metal samples with a variety of reagents such asoxygen, water, acids and alkalis. Comparison of the reactivities andthe products of the reactions allows us to construct a reactivityseries but there are problems. Aluminium appears unreactive, weuse it for window frames and tube trains. The metal is very reactiveindeed but it becomes coated in an unreactive layer of aluminiumoxide protects the metal underneath.

Iron goes rusty by reaction with oxygen and water. Whydoesn’t this rust coating stop any further corrosion?

... rate ... reactive ... reactivity

Metals and acidsMetals and acids

68

You are learning to:� Describe what you see

when metals react withacids

� Construct equations for thereactions

� Use evidence fromexperiments to makepredictions

BIG IDEASBIG IDEAS

FIGURE 1:Which metal, Aor B, is the more reactive?

A

B

Artists use acids and heattreatment to produce interestingcolour effects. Some metals suchas titanium can show a range ofcolours and surface patterns.Each piece made by the artist isunique. The technique allowsartists to be very creative.

Basketball competitionThere is always great competition to play for a sports team. Inbasketball, extra players wait to see if they get a chance to play. Thecoach can replace one player with another player. There are chemicalreactions where one metal is replaced by a different metal. The morereactivemetal displaces another metal. We can see changes whenone metal displaces another. These might be a change in the colour ora change in the temperature.

Which word describes what happens when one metal takes theplace of another?

Give one example of the kind of change there might be whenone metal replaces another.

Displacement reactionsWhen the reactivities of copper and iron are compared it is clear that iron is more reactive. Thecompound copper sulphate has a very distinctive blue colour. The two metals look very different too.Iron is silver-grey and copper is brown.

Reaction of an iron nail in copper sulphate solutionWhen an iron nail is placed in blue copper sulphate solution, changes are observed.

... displace ... displacement reaction

Displacement ofmetals

Displacement ofmetals

74

You are learning to:� Discuss how one metal can take

the place of another� Recognise that relevant

observations provideevidence

� Construct equations fordisplacements

BIG IDEASBIG IDEAS

FIGURE 1: The coach for thisbasketball team is replacingone player with another.

Material Start of reaction End of reaction

copper sulphate blue paler blue

iron metal silvery grey brown coating

Some of the iron in the nail has reacted with the coppersulphate solution.

iron � copper sulphate copper � iron sulphategrey blue brown very pale green

Some people have made moneyout of displacement reactions.Scrap iron is cheap but coppermetal is worth a lot of money.Old copper mines often becomeflooded and the water turnsblue. It is a solution of coppersulphate. Adding scrap iron tothe solution displaces coppermetal, a useful source of avaluable raw material.

So4 So4Cu

This displacement reaction takes place because iron is more reactive than copper. Amore reactivemetal displaces a less reactive metal from its solution. In this case, iron has displaced copper fromcopper sulphate.

Observations of similar reactions let us work out a reactivity series for metal displacements. Fromthis, we can predict which other displacements should be successful.

Why does the blue colour become paler in this investigation?

a What would be the final colour of this solution if all of the copper was displaced?

b Part of the reactivity series reads: Zn>Fe>Sn>Pb>Cu where > means more reactive than.How many of these metals could zinc displace? Explain your answer.

Displacement equationsZinc is more reactive than iron. This means that zinc can also displace copper from solutions such ascopper sulphate or copper nitrate.

The equations are very similar:

The reaction of zinc with copper sulphate works better if zinc powder is added to the blue solution.The powder provides a large surface area for reaction with the solution. Zinc powder is pale grey.There is a temperature rise as the displacement occurs. This is shown by placing a thermometer in thetest tube during the reaction.

Describe two changes you would expect when zinc powder is added tocopper sulphate solution.

Magnesium is more reactive than zinc. Write a symbol equation for thereaction between magnesium and copper sulphate solution.

Would anything happen if you added magnesium to a solution of zinc sulphate?Explain your answer.

Changing placesWhen metals displace each other from solutions, the particles rearrange themselves. The metalparticles that were originally in the soluble compounds are displaced as particles of the elementsthemselves. For example:

What can you conclude about the relative reactivities of magnesium and copper?

Write a symbol equation for the reaction of magnesium with zinc nitrate solution Zn(NO3)2where zinc is displaced.

Why is calcium metal of limited use in investigating displacement reactions of metals?

... reactive ... replace ... surface area 75

++++O O

O O

SO O

O O

SCu CuMg Mg

++++O O

O O

SO O

O O

SCu CuZn Zn

Predicting displacement reactionsIn chemical reactions we can often make predictions. If we have seen several reactions that aresimilar, we can predict other reactions we have not tried. If four different metals all fizz with acidswe might predict that another metal would fizz too.

By testing many metals to see how they react with oxygen, water or acids, we can develop areactivity series. The most reactive metals are written at the top with the metals becoming lessreactive down the list. More reactive metals displace less reactive ones. The reactivity series is usedto predict which displacement reactions will be successful.

... aluminium oxide ... displace

PredictingreactionsPredictingreactions

76

You are learning to:� Recognise that predictions can

be useful� Explain how to use data

to predict reactions� Construct equations to

match your predictions

BIG IDEASBIG IDEAS

Symbol Name Reacts with Reacts with Reacts withoxygen? water? dilute acid?

K potassium � �, fast in cold �

Na sodium � �, fast in cold �

Ca calcium � �, fast in cold �

Mg magnesium � �, slow in cold, �fast in steam

Al aluminium � �, needs steam �

Zn zinc � �, fast in steam �

Fe iron � �, fast in steam �

Pb lead � no �

Cu copper � no no

Au gold no no no

Does iron displace copper?The table above shows that iron is more reactive than copper.Therefore this displacement reaction can be predicted to besuccessful:

iron � copper sulphate copper � iron sulphate �

Does copper displace iron?The reaction between copper and iron sulphate can bepredicted not to be successful according to the data.

... predict ... reactivity series 77

copper � iron sulphate iron � copper sulphate �

a Which metal in the table do you predict could notdisplace any other metal?

b Name one metal that could displace magnesium frommagnesium sulphate solution but would be toodangerous to try. Explain why.

c Why should the surfaces of metals be cleaned beforetesting displacement reactions?

d Write both word and symbol equations for the reaction ofzinc with iron sulphate, FeSO4.

Describing displacement reactionsWith blue copper sulphate it is obvious if a displacement reaction has occurred because there is acolour change. Most solutions are colourless and there may not be a colour change. Often thereis a temperature change and this shows that a reaction has occurred.

Balanced symbol equations can be written– even for reactions that do not work.

Only one of the reactions shown by the equations above is predicted to work. It is the reactionbetween magnesium (Mg) and zinc sulphate (ZnSO4). From the data in the table on page 76magnesium is more reactive and displaces zinc.

Write a word equation for the reaction between zinc and lead nitrate solution.

Write a symbol equation for the reaction between magnesium and iron sulphate.

Predict if the reactions in Q2 and Q3 will be successful.

Particles and bondsThe number of metal particles that can combine with sulphate particles can vary. Metals canhave different numbers of bonds to combine with other groups or elements. For example:

Sodium sulphate Na2SO4

Zinc sulphate ZnSO4

Aluminium sulphate Al2(SO4 )3

We need to find out the numbers of bonds before writing more difficult chemical formulae.The sulphate particle in the examples has two bonds.

Zn � MgSO4 ZnSO4 � MgMg � ZnSO4 MgSO4 � Zn

Would you predict that reactivealuminium would be a badchoice for use as cooking foil ina hot oven? Aluminium is highin the reactivity series andshould react with food or drink.But it does not. Aluminiummetal is covered in a thin layerof aluminium oxide that stopsit reacting further. This is anexample of a prediction notbeing enough – a reaction mustbe carried out to test theprediction.

What is rust?Cars are made from steel. Steel is mostly made fromthe element iron. The problem with iron is that itrusts. If the paint chips off a car, the iron underneathstarts to go rusty. Rust can even spread under paint.Rusty iron is brown in colour. Iron loses its strength as itgoes rusty. This is why rusty cars become unsafe to driveand end up in a scrap yard.

Which element that is in steel goes rusty?

Describe two changes to iron caused by rusting.

The golden alternativeRusting is just one example ofcorrosion. When somemetals react with air andwater they start to corrode.They lose their shinyappearance and become dull.In some cases the corrosioncontinues until no metalremains, as with iron.

But some metals resistcorrosion. They are unreactivemetals that are not affected by air orwater, or metals that react extremely slowly. Examples of unreactivemetals are gold and platinum. A golden car would never corrode but itwould be too expensive and also very heavy! Copper and lead corrodeslowly but are still used for roofing.

Copper jewellery also corrodes slowly – it turns green. It can even stainyour skin green too.

Aluminium is the shiny metal we use for cooking foil. It does not seemto corrode, even at high temperatures inside an oven. Aluminium is aspecial case. The metal reacts rapidly with oxygen in the air to formaluminium oxide which protects the metal underneath fromfurther corrosion.

... alloyed ... corrosion ... element ... rust

Corrosion ofmetals

Corrosion ofmetals

78

You are learning to:� Recognise that iron goes rusty

in certain conditions� Explain why some metals do not

corrode� Make connections

between the availabilityand uses of metals

BIG IDEASBIG IDEAS

FIGURE 1: Rusty cars end up in scrap yards.Why are rusted cars unsafe to drive?

FIGURE 3: St Paul’s Cathedral inLondon. What is its dome made from?

FIGURE 2:Why does gold jewellerystay shiny whereas copper jewelleryeventually turns green?

Give two examples of metals that resist corrosion.

What colour change is seen when copper corrodes?

Suggest how acid rain affects the speed at which metalscorrode.

Making the right choiceMetals that resist corrosion have special uses. Gold, silver andplatinum are all used to make jewellery, coins and medals. It isimportant that metals chosen for these objects do not corrode,for example iron coins would be unsuitable. Fine gold wires areused in computers. If these wires corroded, data would be lost orthe computer might stop working.

Iron can be made to resist corrosion if it is alloyed (mixed) withchromium and nickel which gives stainless steel. This shiny alloyis used for cutlery and for the insides of washing machines and inhospitals where corroded metals might harbour germs.

Name two metals that are used in coinage.

Which elements are alloyed with iron in stainless steel?

a Why are the insides of dishwashersmade from stainless steel?

b Where in the reactivity series wouldyou find metals such as gold andplatinum?

c Why are you unlikely to readequations that show gold displacinganother metal?

d Write a word and symbol equationfor the reaction of sodium with silvernitrate solution, AgNO3 solution.What problems might you find intesting this reaction?

Why use steel?Both iron and steel go rusty and yet we still use steel to make millions of cars each year. There areseveral reasons for choosing steel. Steel is a low-cost construction material compared withalternatives such as aluminium or stainless steel. Steel is malleable and very strong. Both galvanising(zinc coating) and painting can slow down the rusting process.

If the nitrate particle has a single bond to attach to other things, how many bonds must beformed by aluminium and by silver in this equation?Al + 3AgNO3 Al(NO3)3 + 3Ag

... stainless steel ... steel ... unreactive 79

The metal titanium is used forsome replacement body parts.People whose knee joints need tobe replaced can have a titaniumhinge joint. Titanium does notcorrode inside the body.

FIGURE 4:Why are hospital surfacesoften made from stainless steel?

Keeping iron dryThe things that make iron rust are:

water air (oxygen).

... anodising ... corroding ... galvanised iron

How to stopcorrosionHow to stopcorrosion

80

You are learning to:� Decide how to stop iron from

rusting� Discuss the factors that affect

rusting� Compare the range of anti-

corrosion methods

BIG IDEASBIG IDEAS

Dry iron does not go rusty. In a desert there is very little waterand so iron hardly rusts at all.

In our climate we need ways of keeping iron dry to stop itfrom rusting.

Iron can be painted, for example cars and trucks.

Iron can be covered in plastic, for example a washingline or clothes-drying rack.

Iron can be covered in grease or oil, for example abicycle has oil or grease put on its iron chain.

All of these methods work by keeping water away from iron.

Why do cars not go rusty in deserts?

a How can you stop a bicycle chain from rusting?

b Water cannot soak through plastic or grease. Explainwhy this helps keep bicycle chains shiny andplastic-coated steel washing lines in new condition.

Zinc to the rescueIron and steel can be coated with zinc to stop the ironunderneath corroding. The process is called galvanisingand the product is galvanised iron.

In the 19th Century much of thecentre of Paris was rebuilt. Thearchitect chose zinc sheeting asthe roofing material. Zinccorrodes very slowly and producesan attractive grey surface, calleda patina. These zinc roofs canlast for up to 100 years and havebeen favourite subjects for artistsin Paris to paint.

The steel cables supporting theForth bridge are slowly corrodingand some thin wires have alreadysnapped.Engineers are pumping a currentof dry air through the cables tostop the corrosion. Without this,the bridge might collapse.

Forth road suspension bridge,near Edinburgh in Scotland.

FIGURE 1: These have not rustedbecause the climate is dry.

For iron to rust oxygen and water must bepresent. If either oxygen or water is excluded,the iron does not corrode. The zinc coatingexcludes oxygen and water.

An example of galvanising is in iron roofingsheets and nails. They are coated with zinc toprotect them from the weather. (Zinc is tooweak to make nails.)

Food cans are often called ‘tins’ because thesteel can is covered with a thin layer of tin.This prevents the steel from corroding andpossibly reacting with the food inside the can. Zinc cannot be used forcans because some foods dissolve the zinc coating.

What is the corrosion coating called that forms on some metals?

Tin follows iron in the reactivity series. Why is this a problemwhen a tin-plated steel can is scratched?

The sunken wreck of the ship The Titanic is surrounded bycold seawater. Why has the wreck not rusted away completely?

Stopping the rotAluminium does not corrode because it has a coating of aluminium oxide on itssurface that is unreactive. This coating can be improved by anodising the metal.During this process an aluminium object is placed in a special electrical cell thatproduces oxygen and this strengthens the surface coating. The coating can absorbbright colours. We see examples of this in Christmas decorations and in cookware.

Most cars are now partly galvanised to resist corrosion. Many layers of paint aresprayed on to further protect the steel underneath. Some modern cars are madecompletely from aluminium, just like aircraft and underground trains. Thealuminium does not need to be painted. Stainless steel planes would be too heavyeven to take off.

Give two ways in which cars can be protected from corrosion.

Why might you describe a steel plane as an ‘expensive caravan’?

Designer problemsAluminium cars do not go rusty, unlike steel. The reactive aluminium is protected byan impermeable surface layer of aluminium oxide that protects the metal. However,aluminium is much more expensive than steel to produce. We could also use theunreactive metal titanium to build cars but they would cost a fortune to buy.

Aluminium requires large amounts of expensive electricity to extract frombauxite ore. Why is this an argument in favour of recycling drinks cans?

... galvanising ... patina 81

FIGURE 2: Galvanised iron roofingsheets. Why is it necessary to galvaniseiron materials that are used outside?

FIGURE 3:Why isaluminium anodised? Whydo anodised aluminiumobjects look attractive?