Lesson video

Hello, my name's Dr.

Warren, and I'm so pleased you've decided to join me today for this lesson on Alloys and Their Properties.

It's part of the Structure and Bonding unit and it follows on from previous learning about the metallic structure of metals and alloys.

We're gonna work together, and I'm here to support you through all the tricky parts of today's lesson.

By the end of today's lesson, you should be able to compare pure metals with their alloys and give examples of alloys and their uses.

We've got some keywords for you: alloy, steel, carat, and brittle.

And now we're gonna show you some these keywords in some sentences.

You may wish to pause video and copy them down so you can refer to them later in the lesson.

An alloy is a mixture of two or more elements, where at least one element is a metal.

Steel is an alloy of iron that contains specific amounts of carbon and may contain other metals.

The purity of gold is measured in carats.

Pure gold is 24-carat gold.

A substance that has the ability to break up easily is brittle.

So, the lesson today has two learning cycles.

The first one is metal alloys and the second one, properties and uses of alloys.

So, let's get started with metal alloys.

Adding different elements changes the properties of pure metal, making it more useful, which is why alloys are really useful.

So, let's take a look at, for example, steel.

Steel is an alloy of iron and it's used in the construction industry to build bridges and many different buildings, high-rise buildings, factories, it's used to reinforce concrete.

Why is it used? Well, when we compare it to iron, steel is generally stronger, harder, more resistant to corrosion, and less malleable and less ductile, making it more suitable for building purposes.

Now, let's just remind ourselves about what the structure of a pure metal looks like.

There's a regular lattice structure in a pure metal of metal ions.

You'll see the rows and columns, and then you've got the sea of delocalized electrons in between, and it's that strong force of electrostatic attraction between the metal ions and the sea of delocalized electrons, which is the metallic bond that accounts for quite a few of the properties.

So, pure metals are often soft, weak, ductile, and malleable.

And the force needed to make the layers of metal ions slide over each other is relatively small compared to those in alloys.

Now, how is the structure of an alloy different? Well, once we introduce a different size atom or ion, whether that be a larger one or a smaller one, into the lattice structure, it's disrupted.

And you can see in the diagram that we have a regular lattice structure in an alloy.

This means that the properties of the alloy are different, they're often stronger and harder, why? Well, this is because a greater force is needed to make the layers of metal ions slide over each other in the lattice.

So, steel, we've already mentioned that, is an alloy of iron and carbon, and it contains other elements as well to enhance its properties.

In fact, there's several different types of steel.

You get mild steel, which is called low carbon steel, and that has a very small amount of carbon, which is iron and carbon and less than 0.

2%.

High carbon steel, which is iron and carbon, it also has other elements such as tungsten and chromium added to it.

And stainless steel, iron, carbon, and chromium.

So, you can just see from that example that we can even vary the properties of the alloy by changing the different elements that are present.

So, a quick check, which of the following statements are correct? Steel is an alloy of carbon, steel is an alloy of iron, all steels contain iron and carbon, steel is stronger than pure iron.

So, well done.

If you answered B, C, and D, they are correct.

Remember it's a metal, steel is an alloy of iron, it's stronger, and all steels contain carbon even though some steels contain other elements as well, so well done if you got that correct.

We're going to consider another example now.

Gold used in jewellery is usually an alloy with silver, copper, and zinc.

So, you can see some gold rings there.

They look like they may be pure, but in reality they're probably an alloy.

The purity of gold is measured in carats.

This is a special unit just for gold.

And pure gold is 24-carat gold.

That's a number that you need to remember, especially when we come to do some calculations in a moment.

But what it means is if we've got a pure gold bar that every single atom in that gold bar is gold.

If those rings that we can see are actually 18-carat gold, what that means is the rings are made of a mixture of 75% gold and 25% other metals.

So, when gold jewellery, for example, is manufactured, often it comes with a mark telling us about the purity.

So, if I was to buy a pair of 4-carat gold earrings and I found they had a mass of 2 grammes, I would be able to calculate how much gold they contained.

Now, to do this, what we'd need to do to start with, we'd have to find the fraction of gold in 4 carats.

So, remember I said that 24-carat gold was pure gold, 100%.

So, to find that fraction, it's 4 divided by 24, which gives 1 over 6, and that becomes our multiplier.

So, if our gold earrings have a mass of 2, to find the mass of gold in the earrings, we multiply 1 over 6 times 2, which gives us.

33 grammes, so.

33 grammes is the actual mass of gold in my earrings.

Let's have a quick check of understanding, which of the following statements about a 6-carat gold chain are correct? It's made from an alloy, it's made from pure gold, it contains 25% gold, it contains 75% gold.

So, well done if you managed to give the answer A, it's made, it's an alloy, yes, it is an alloy and it contains 25% gold, so that is C is correct as well.

Moving on, bronze is an alloy of copper, and that's a mixture of copper and tin.

So, the properties of bronze include it's got a brown, metallic colour, it's strong, and it's resistant to corrosion.

And one of the uses of bronze is bronze coins, and they're being used all the time in circulation so it's important that they are strong and resistant to corrosion.

It's also used for statues, and you may have seen those around the place that you actually live in.

Another alloy of copper, which is important, is brass.

Mustn't get confused these two, but brass is a mixture of copper and zinc and it's used for brass instruments.

So, if you're an instrumentalist, you may play the trumpet or the saxophone and they are made of brass.

Compared to copper or zinc, brass is more malleable and it has much more desirable acoustic properties, so it sounds better when it's blown into so that's why it makes brass more suitable for making musical instruments than copper or zinc.

Again, we want to make sure we make these alloys properly, we can actually calculate the amount of zinc in a brass alloy because it varies from 5% to about 45%.

And it's important that as we add more of the zinc, the properties will change slightly.

So, here we have a particle model of brass.

We have our zinc ions, we have our copper ions, which have been disrupted in the regular structure and we have our sea of electrons.

So, to calculate the percentage of zinc ions present in the sample, what we need to do first is we need to count up the number of zinc ions.

The zinc ions are represented by the purple blobs, and you can see we have 4.

If we count the number of copper iron, it comes to 16.

So, the total number of ions in this particular part of the lattice that we can see is 20.

So, to find the percentage of zinc ions in this particular structure, we have to take the zinc ions divided by the total and times by 100.

So, that's 4 divided by 20 times 100, which equals 20%.

So, in this particular structure that we can see of brass, there are 20% by mass of zinc atoms. Okay, quick check again, bronze is an alloy of copper and aluminium, zinc, tin, or iron? Well done if you picked tin, that is correct.

That's a bit of knowledge that you need to remember.

Well done.

So, we have another calculation here.

An aluminium alloy saucepan as a mass of.

5 kilogrammes and contains 89% aluminium by mass.

That means that out of every 100 atoms in the lattice, 89 of them are aluminium.

What mass of aluminium does it contain? And give it to 3 significant figures.

So, the first thing we want to do to work out the mass is to work out the multiplier, so it's 89 divided by 100 and then we multiply that by.

5 'cause that's the actual mass.

That gives us an answer of 0.

445 kilogrammes.

Okay, here's a question for you to have a go at.

A bronze 2 pence coin has a mass of 7.

12 grammes and contains 97% copper by mass.

What mass of copper does it contain? And again, give your answer to 3 significant figures.

You might want to pause the video while you do this calculation.

So, very well done if you remember to write out that 97 divided by 100 is the multiplier, then multiply that by 7.

12, which is the mass of the coin, and you will find that 6.

91 grammes of the bronze 2 pence is copper.

Very well done if you got that correct.

That brings us onto our first task.

So, we've got 2 questions for you to work through.

First of all, in question 1, I want you to write down what is steel and then draw a labelled diagram showing structure of steel.

Then the second question is about gold.

Alloys of gold often used to make jewellery.

Different alloys of gold have different carat values.

So, again, in part A, if you can write down what we mean by the term carat and then there are a couple of calculations for you to do.

Pause the video while you do these on your worksheet, and then when you're ready, press Play to restart and we'll go through the answers together.

Okay, so let's have a look at the answers.

What is steel? Well, steel is an alloy of iron that contains specific amounts of carbon and other metals.

So, if you've got that definition right, well done because that's something that you've had to recall.

For your labelled diagram, it should look something like this.

It's important that you have the sea of delocalized electrons, you show a disrupted or a regular lattice of iron ions and have those labelled, and also that you show some carbon atoms in between the iron ions that are basically disrupting it.

So, if you've got that diagram, absolutely fantastic, well done.

And if you didn't, just make a note of where you went wrong.

So, question 2, alloys of gold are used to make jewellery.

So, we've got a couple of calculations here, but first of all, the term carat.

Well, the purity of gold is measured in carats, so well done if you've got that and a special well done if you manage to say that pure gold is 24 carats.

For our calculation, don't forget when you're doing calculations to show your working so that if you have got the method right, but you just make a mistake, you'll still get some credit.

We have first of all to work out the fraction of gold in 9 carats, which is 9 divided by 24 'cause remember 24 is our pure 100% carat gold and this gives us a mass of gold in the ring, the fraction times the mass, which is 5.

63 grammes.

So, well done if you got that right.

Part C, the price of gold is 58 pounds 50 per gramme, I told you it was expensive.

What we need to do now is calculate the cost of the gold in the ring.

Well, we know how much gold is in the ring because we've just worked that out in part B.

It's 5.

63 grammes.

We're gonna multiply that by 58 pounds 50 and that gives us an answer of 329 pounds, 36 pence.

Okay, I'm gonna move on to question 3.

This time it's a chemical analysis of a Bronze Age spearhead and it shows that it has a mass of 1.

5 kilogrammes and contains 85% copper and 15% tin by mass.

I want you to calculate the mass of copper and tin in the spearhead and then kind of work out how much it would cost to make 10 spearheads if we're given the copper has a cost of 3 pounds 50 per kilogramme and tin costs 22 pounds per kilogramme.

Okay, so let's have a look at the answer.

First of all, part A of our calculation, calculate the masses of copper and tin in spearhead.

So, let's start with the calculation of the mass of copper.

We need to work out that multiplier.

We know 85% of it is copper, so it's 85 divided by 100 and then multiply that by the total mass, which is 1.

5, so we have 1.

28 kilogrammes of copper.

To work out the mass of tin, it's quite simple, we can just do a subtraction, 1.

5 minus 1.

28 equals 0.

22 kilogrammes of tin.

Always remember to show your working just in case you make a mistake on the final answer.

So, well done if you've got both of those correct.

Let's move on to the second part, this time we want to make 10 spearheads and work out how much it's gonna cost.

And we've been told that copper is relatively cheap at 3 pounds 50 per kilogramme and tin costs quite a lot more at 22 pounds per kilogramme.

So, how do we do this calculation? First of all, we want to work out the cost of copper.

We know that it has a mass of 1.

28 and we know that it is 3 pounds 50 per kilogramme, so that gives us 4 pounds, 48 pence for a bit of copper.

The cost of the zinc in the spearhead is.

22 times 22.

It's more expensive, but the cost comes out similar, 4 pounds 84.

Now, that is the cost to make one spearhead, we need to add those 2 together, which gives us a total of 9.

32 or 9 pounds 32 pence.

We've been asked to work out the cost to make 10 spearheads, so what we need to do is multiply it all by 10.

So, our overall cost for 10 spearheads is 93 pounds and 20 pence.

There's quite a lot in that calculation, so well done if you actually managed to get all the way through it.

And if you got lost in the way, you've got the instructions there to follow through.

But it's really good to make sure that you always lay out your work clearly and follow it through logically.

So, a very, very well done if you've got those answers right.

Okay, so that brings us to the end of our first learning cycle on metal alloys, and now we're going to move on to our second learning cycle and think about the properties and uses of alloys.

Scientists and engineers really like alloys and often trying to develop new ones.

And this is because they have more desirable properties over the individual elements that they're made from.

And an example of kind of quite a modern alloy is nitinol.

It's an alloy of nickel and titanium and has many uses.

And just a couple of uses here.

It's used to make glasses frames because it's super elastic.

This means that if you bend it or you stand on it, it won't break, it'll just bounce back into its normal shape.

It's also got shaped memory properties, which makes it useful for making orthodontic braces.

Another alloy that is really useful is called solder, and this is something that you might have used in design and technology because it's used to assemble electrical opponents and join wires together.

Now, the real reason it's used is because solder has a very, very low melting point.

What you can do is you can take the solder, which is often just given as a strip of metal.

You can put it onto the iron, you can melt it.

Then the liquid metal is spread over the electrical components that need to be joined together.

When it cools, it solidifies, making the joint permanent, so it's a very easy way to join electrical components and wires.

Now, you may have already realised that most metals have high melting points due to the strong electrostatic forces between the metal ions and the delocalized electrons, but this is where solder has its advantage.

You can see it's made from lead and tin, which mu both have much higher melting points than solder itself.

So, that's why it's much more useful for making electrical components and joining wires as it doesn't need to get quite as hot to melt.

Why is it then that solder has a much lower melting point? Well, it's because the different size ions in the alloy disrupt the regular lattice structure in the pure metal.

And what this actually does, it weakens the bonds between the metal ions and the delocalized electrons, so it doesn't need as much energy to break that bond, meaning that the melting point is much lower.

So, it's quite common for a lot of alloys to have lower melting points than the metals that they are actually made from.

Okay, let's have a quick check of understanding, true or false, solder, an alloy of lead and tin, has a higher melting point than both lead and tin? Is that true or false? Well done if you chose false.

Now, what's the reason why? Have a look at these sentences and make your choice.

Very well done if you chose A, metal alloys usually have a lower melting point than the metals that they are made from.

So, we're going to consider another property, and that's density.

We've not talked about density yet, but density is a measure of mass per unit volume of material.

So, if we had a square centimetre cubed of a material, that's a volume, it's how much mass that has gives us the density.

And you can see from the table that iron, copper, and zinc all have quite a high density of between 7 and 9 grammes for cubic centimetres.

But when we look at aluminium, it is much lower.

Now, this is really important because it gives aluminium some desirable properties which make it good for aircraft parts.

The low density, so when you got an aircraft, you want it to fly up in the air, it's really good that it's low density, but there's another property that's really good and that is its resistance to corrosion.

It has an oxide layer that forms on its surface, protecting the metal below.

But there's one problem with pure aluminium, and that its soft and so not really very good for aircrafts.

We don't want a soft metal that maybe will bend or anything when it's up in the air, so we use an alloy instead.

Now, there are several different alloys that have aluminium in.

So, for example, you can see we've got duralumin, which has copper, magnesium, and manganese, and magnalium, which has magnesium in as well.

And both of these alloys are used to make aircraft parts, and you can see they still maintain that low density.

In fact, magnalium is even lower than aluminium.

But what is really important is compared to pure aluminium, they have a much, much stronger and higher strength level.

So, pure aluminium has 80 compared to 420-450.

So, duralumin has similar density, but it's over 5 times stronger, and magnalium has a lower density and it's over 8 times stronger.

So, this shows you how we can really change those properties of the alloys and make them more suitable for an aircraft.

So, alloys are generally stronger than pure metals, just remind ourselves going back to that structure, we have our larger atom or ion inside the irregular disrupted lattice structure.

So, this means that a greater force is needed to make the layers of metal ions slide over each other, so the alloy is much, much stronger than the pure metal.

And that is exactly what's happened when we've introduced these other metal ions into the aluminium lattice, and that makes it much more suitable for aircrafts.

So, a quick check for understanding, aluminium alloys are stronger than pure aluminium, true or false? Well done if you picked true, that is correct.

Now, what is the reason? Have a read of A and B and make your choice.

Well done if you picked A, adding different sized metal ions into the structure makes it harder for the aluminium ions to slide over each other.

Fantastic.

Okay, so we're gonna have a look at a slightly different use of a alloy, and that's a medical use.

Magnesium alloys are used for bone implants to promote bone tissue regeneration.

That means basically if you've broken a bone, like you can see in this X-ray here, that we can implant some magnesium and basically it will help the bone to grow, the tissue to grow and heal.

But there is a problem if we use pure magnesium, and that is it's biodegradable and sometimes it can degrade far too quickly.

So, actually, the metal is actually gone before the tissue is regrown, and that means we might have to have more surgery to implant some more magnesium.

But what they found is if we use a magnesium alloy, it can degrade as the bone tissue heals, so it kind of slows down that degradation process, which means that the implant does not need to be removed once the tissue is regrown and it also reduces the need for surgery.

And that's absolutely fantastic medical use of a magnesium alloy.

We mentioned steel earlier in the lesson, so we're gonna return to that.

Pure iron is soft, it's brittle, and it's corrosive.

But when it's alloyed with carbon and other metals to make steel, it's much, much more useful.

And here are some uses of the different types of steel.

We mentioned previously that mild steel, which has a low percentage of carbon, it's used for machine parts and pipelines.

And this is because it's malleable and docile, we can really shape it quite easily and it is strong.

High carbon steel, that's used for cutting tools and for drilling because again, it's very, very hard working.

It's got a high melting point, that's quite important because when you're drilling, things get hot.

It's strong, but brittle.

And stainless steel is something that you may be familiar with because it's used in the home, in the kitchen for cooking utensils and things like dishwasher parts.

And one of the main properties that is important for that is its resistance to corrosion.

And again, it's very hard.

So, steel is a very, very versatile alloy.

Okay, let's have a quick check for understanding, which of the following objects is likely to be made from stainless steel? So, we have a knife, looks like we've got some engine parts, and we've got some nails.

Okay, well done if you picked A.

We have a knife, it's gonna be used in the kitchen for food so we want to make sure that it is resistant to corrosion.

That brings us to our second task, task B, about the properties and uses of alloys.

We've got three questions here.

Give two reasons why cutlery is made from alloys of stainless steel and not pure iron.

2, suggest why pipelines are made from mild or low carbon steel and not pure iron.

And then we've got a calculation for question 3, a 750 gramme sample of aluminium magnesium alloy contains 3% by mass of magnesium.

Calculate the mass of aluminium in the sample and also explain why alloy is stronger than pure aluminium.

So, if you want to answer those questions on the worksheet, pause the video and then when you're ready we'll have a look at the answers together.

Okay, so let's have a look at the answers.

The first answer, stainless steel is harder than pure iron, so it's more suitable for cutting food.

It's also resistant to corrosion, but iron rusts so steel makes it more hygienic, and that's really important when we're thinking about food so very well done if you got that right.

So, pipelines, why would they be made from mild low carbon steel, and not pure iron? Well, first of all, mild carbon steel is more malleable and more ductile than pure iron, and that's quite important when wanting to make a pipe, so it needs to be moulded into a pipeline.

It's also stronger than pure iron and it will be able to withstand more force.

And again, in a pipeline, if it's gonna have water or something going through, there's gonna be quite a high force or pressure going through it, so very, very well done if you've got that, those answers correct as well.

Now, what about our calculation in question 3? Well, first of all, to calculate the mass of aluminium in the sample, what we need to do is calculate the mass of magnesium first.

We're told it's 3% of magnesium, so that's 3 divided by 100, that gives us our multiplier.

We multiply by 750, and that gives us 22.

5 grammes of magnesium.

We were asked to calculate the mass of aluminium.

We know the total mass of 750, so 750 minus 22.

5 gives us a mass of aluminium of 727.

5 grammes.

So, well done there if you've got that right.

It's a 2 step calculation.

It's always important to work out what you can do first, get that starting point, so really good if you've got that right.

Okay, part B, explain why the alloy is stronger than pure aluminium.

Right, okay, where do we start here? Aluminium is a pure metal with a regular lattice.

Always go back to that lattice structure.

The alloy contains some larger magnesium ions, which disrupt the regular lattice structure.

The force needed to make the layers of metal irons slide over each other is greater in the alloy than the pure metal, making it stronger.

So, when you're answering that sort of question, there's always that logical thought process you need to go through, starting with the aluminium pure metal and then getting down to the property that you want.

So, really well done if you've got that answer correct, you're doing absolutely great.

Okay, we've got another question in this task.

Which of the materials in the table below would you recommend for building a helicopter, and why? So, you've got some information here.

You're told about the density, you're told about the melting point, and you're told about the strength.

Now, remember, helicopters go up in the air so that will be a bit of a hint.

Something's gonna be flying, so which material would be best for that? Okay, so pause the video while you answer the question and then when you're ready to start, when you've done it, press Play and we'll have a look at the answer together.

Okay, so let's have a look at the answer.

Remember, when you do a question like this, we can only use information from the table.

We've been asked about a helicopter, so the important bits of information are gonna be density and strength.

And really we can ignore for the purposes of this question, the information about the melting point.

So, your answer might be something like mine.

I said I would recommend using duralumin as it has a low density so the helicopter would be light enough to get off the ground.

And if we look in our table, it's 2.

69 compared to aluminium that's 2.

7.

But the other reason for choosing my alloy over the pure metal is because it's strong, and this is important so the helicopter will be able to withstand the forces experienced during operations, such as lift and thrust.

And if we compare the strength, the pure metal is 80, the alloy is 420 so it is much, much stronger, and that's why I would choose that one.

So, if you've got that answer, well done.

Remember there was two parts to it, the density and the strength.

That's great, we are doing really, really well.

Okay, so let's go over the key learning points of this lesson.

Alloys are used over pure metals as they have more desirable properties than elements that they are made from.

The properties of alloys are due to the disruption to the regular ladder structure caused by atoms of different sizes being added to the metal.

Iron is alloyed with carbon and other metals to produce different types of steel.

Gold, copper, aluminium, and magnesium all form useful alloys with other metals.

I hope that you've enjoyed learning with me today and look forward to you joining me on another lesson very soon.