Materials of the modern world

Materials of the Modern World: Study Notes

Curriculum note – In the 2025 curriculum, useful materials from crude oil (including plastics) and the environmental problems of using materials are Extension topics. Alloys were Core in the 2017 curriculum but are not listed in the 2025 curriculum.

Hello! Get ready to explore the amazing materials that build our world. From the metal in your phone to the plastic in your water bottle, we use special materials every single day. In these notes, we'll discover how we make metals stronger, what incredible things we can get from crude oil, and how we can use these materials wisely to protect our planet. It's all around you, so let's dive in!

Alloys: Better Than Pure Metal!

Have you ever wondered why a steel bridge is so much stronger than a simple iron nail? The secret is in mixing things up! Just like you mix flour, sugar, and eggs to bake a better cake, scientists mix metals with other elements to make them better. This special mixture is called an alloy.

What is an Alloy?

An alloy is a material made by mixing a metal with at least one other element. This other element can be another metal or a non-metal. The whole point of making an alloy is to give it better properties than the pure metal has.

These improved properties could be:

• Strength: Making the metal harder to bend or break.
• Hardness: Making it more resistant to scratching.
• Corrosion Resistance: Stopping it from rusting or tarnishing.
• Lighter Weight: Making it strong but not heavy.

How does this mixing work? (A simple look)

Imagine a box filled with perfectly stacked tennis balls. You can easily slide the layers of balls over each other. Pure metals are a bit like this, with their atoms arranged in neat, sliding layers, which can make them soft.

Now, imagine you throw some smaller golf balls into the box. The neat layers are all messed up! It's much harder to slide the layers now. This is what happens in an alloy. By adding atoms of a different size, we disrupt the neat layers, making the material much harder and stronger. It's a simple trick with amazing results!

Common Alloys and Their Uses

You use alloys all the time, maybe without even realising it!

• Steel: This is an alloy of iron and a tiny bit of carbon. Pure iron is relatively soft and rusts easily. Steel is incredibly strong, which is why we use it for everything from skyscrapers and bridges to cars and ships.


• Stainless Steel: This is a special type of steel with chromium and nickel added. The chromium stops it from rusting. That's why your knives and forks, kitchen sinks, and doctor's tools are made from it!


• Bronze: This is an alloy of copper and tin. It's much harder than pure copper and has a beautiful colour. It's famously used for statues, medals, and bells.


• Brass: This is an alloy of copper and zinc. It's strong, looks a bit like gold, and doesn't corrode. You can find it in musical instruments like trumpets, as well as taps and door handles.

Key Takeaway: Alloys

Quick Review:

• An alloy is a mixture containing at least one metal.
• We make alloys to improve their properties, like making them stronger or rust-proof.
• Real-world examples include steel (for buildings), stainless steel (for cutlery), and bronze (for medals).

Crude Oil: Black Gold and Its Secrets

Deep underground, there's a thick, black liquid called crude oil. We often think of it as just fuel for cars, but it's actually a treasure chest of ingredients for making some of the most important materials in our lives, especially plastics!

What is Crude Oil?

Crude oil is a complex mixture of thousands of different compounds called hydrocarbons. Hydrocarbons are molecules made of only hydrogen and carbon atoms, linked together in chains of different lengths.

Analogy: Think of crude oil like a big pot of alphabet soup. It's a messy mix! To use the letters, you first need to separate them. We need to do the same thing with the different hydrocarbon molecules in crude oil.

Size Matters: How Molecule Size Changes the Properties

The hydrocarbon molecules in crude oil come in many sizes, from tiny ones with just 1 to 4 carbon atoms to huge ones with more than 40. As the molecules get bigger (longer chains):
• Boiling point gets higher. The smallest ones (such as methane and butane) are gases at room temperature, middle-sized ones are liquids, and the biggest ones are very thick liquids or solids, such as bitumen (used to surface roads).
• They get more viscous – thicker and slower to flow. Compare runny petrol with thick, sticky lubricating oil.
• Their colour gets darker, from colourless to yellow and then dark brown or black.
• They are harder to set on fire, and they burn with a smokier, sootier flame.

Because boiling point depends on molecule size, we can use boiling points to separate the molecules. That is exactly what fractional distillation does!

Separating Crude Oil: Fractional Distillation

We separate the mixture of hydrocarbons using a process called fractional distillation. It works because different-sized hydrocarbon molecules have different boiling points.

Don't worry, it's simpler than it sounds! Here's the step-by-step process:

1. Heat it up: The crude oil is heated to a very high temperature (over 350°C), turning most of it into a gas (vapour).
2. Into the Tower: This hot gas mixture is pumped into the bottom of a tall tower called a fractionating column. This column is very hot at the bottom and gradually gets cooler as you go up.
3. Rise and Cool: The hot hydrocarbon vapours start to rise up the column. As they rise, they cool down.
4. Condense and Collect: When a hydrocarbon vapour cools to its boiling point, it turns back into a liquid (this is called condensing). It is then collected on trays at different levels.

The key idea is that bigger molecules have higher boiling points, so they condense and are collected lower down in the hot part of the tower. Smaller molecules have lower boiling points, so they travel further up to the cooler parts before condensing.

Each collected liquid is called a fraction, and each fraction has a specific use.

Uses of the Fractions

The separated fractions are incredibly useful!

• Fuels: Some fractions become petrol for cars, kerosene for jet engines, and diesel for lorries and buses.
• Solvents: Chemicals used to dissolve other substances.
• Raw Materials: One of the most important fractions (called naphtha) is the starting ingredient for making plastics, medicines, and many other chemicals.

Did you know?

The plastic that makes up your school chair, your TV remote, and even some of the fibres in your clothes all started their journey as crude oil buried miles beneath the Earth's surface!

Plastics: The Super Molecules

Plastics are everywhere! They are amazing materials that can be moulded into almost any shape. But what exactly are they?

What is a Plastic?

Analogy: Imagine you have a huge box of single paper clips. Each paper clip is a small molecule called a monomer. If you link thousands of these paper clips together, you create one very, very long chain. This giant chain is called a polymer.

That's exactly what a plastic is! Plastics are materials made of very large molecules called polymers. These polymers are formed by chemically joining thousands of smaller monomer molecules together. Because these polymer molecules are so huge, we also call them macromolecules.

Examples of Common Plastics

Different monomers create different polymers with different properties.

• Poly(ethene) / PE: You see this every day! It's used to make plastic bags, shampoo bottles, and kids' toys because it's flexible, lightweight, and cheap.


• Poly(vinyl chloride) / PVC: This plastic is tough, strong, and a poor conductor of electricity. It's perfect for window frames, water pipes, and covering electrical wires.

Always Inventing!

Our world is always changing, and we need new materials to solve new problems. Scientists are constantly designing new plastics with special properties, like plastics that are super-strong for bulletproof vests, or plastics that can self-heal when scratched!

Sorting Mixed Plastics by Type

Recycling works best when each type of plastic is collected on its own, because different plastics melt at different temperatures and a mixture makes a weak product. So how could you separate a pile of mixed plastic pieces?

Method 1: Read the recycling code

Many plastic items have a small triangle of arrows with a number inside. The number tells you the type of plastic:
1 = PET (drink bottles), 2 = HDPE (milk and shampoo bottles), 3 = PVC (pipes), 4 = LDPE (plastic bags and cling film), 5 = PP (food boxes and bottle caps), 6 = PS (cups and foam boxes), 7 = other plastics.
HDPE and LDPE are both kinds of PE.

Method 2: Float or sink (density)

Each plastic has its own density. A piece of plastic floats in a liquid that is denser than it, and sinks in a liquid that is less dense. Approximate densities:
PP about 0.90 g/mL; PE about 0.91–0.97 g/mL; water 1.00 g/mL; PS about 1.05 g/mL; concentrated salt solution about 1.2 g/mL; PET about 1.38 g/mL; PVC about 1.4 g/mL.

A plan you could use:
Step 1: Cut the plastics into small pieces of similar size. Wash and dry them. (Use solid pieces, not foam – foam floats because it is full of air.)
Step 2: Put the pieces in water and stir. Add a drop of detergent so that air bubbles do not stick to them. PE and PP float, so scoop them off.
Step 3: Move the pieces that sank into concentrated salt solution. PS floats, while PET and PVC sink.
Step 4: PET and PVC have similar densities, so use another clue to tell them apart, such as the recycling code on the original item (write the codes down before you cut the items up).

Safety first: never burn plastics to identify them. Burning PVC gives off poisonous, acidic gases.

Key Takeaway: Plastics

Quick Review:

• Plastics are made of giant molecules called polymers.
• Polymers are long chains made of repeating small units called monomers.
• Plastics are made from chemicals that come from crude oil.
• Different plastics, like PE and PVC, have different properties for different jobs.

Using Materials Responsibly

Alloys and plastics have changed our world for the better, but their success has created some big environmental challenges. It's important to understand these problems so we can find solutions.

The Problems with Our Materials

• Plastic Pollution: Most plastics are not biodegradable, which means they don't naturally rot away. A plastic bottle can stay in the environment for hundreds of years, polluting our land and oceans and harming animals that mistake it for food.


• Using Up Resources: Making plastics uses crude oil, and extracting metals from the ground (mining) uses up finite resources. Both processes require a lot of energy and can cause pollution.

More Problems to Know About

From plastics:
• Burning plastic waste gives off harmful gases, such as carbon monoxide, and adds carbon dioxide (a greenhouse gas) to the air. Burning PVC also gives off acidic hydrogen chloride gas.
• Microplastics: plastic waste slowly breaks into tiny pieces smaller than 5 mm. Sea animals eat them, and they can pass along food chains – even to us.
• Landfills fill up: plastic waste takes up landfill space for a very long time.

From metals (see also the chapter Metals):
• Mining destroys habitats and leaves huge piles of waste rock.
• Extracting metals from ores burns a lot of fuel, which releases carbon dioxide. Heating some ores also releases sulphur dioxide, which causes acid rain.
• Thrown-away batteries and electronic waste can leak toxic heavy metals, such as lead, mercury and cadmium, into soil and water.

Finding Solutions: Be a Planet Hero!

We all have a part to play in solving these problems. A great way to remember how to help is with the "3 Rs".

1. REDUCE: The best solution is to simply use less stuff. For example, carry a reusable water bottle and shopping bag instead of using disposable ones.


2. REUSE: Before you throw something away, think if you can use it again for another purpose. For example, an old jam jar can be used to store pens, or a food container can be used for leftovers.


3. RECYCLE: This is when we take old materials and turn them into new products. Recycling metals and plastics saves resources and uses much less energy than making them from scratch. For example, old plastic bottles can be recycled into clothing, park benches, or new bottles!

More Ways to Solve the Problems

• Sort your waste: put clean plastics, metals and paper into separate recycling bins, so each material can be recycled properly.
• Recycle metals: making a can from recycled aluminium uses about 95% less energy than extracting new aluminium from its ore (see the chapter Metals).
• Use biodegradable plastics for some single-use items. For example, PLA is made from plants such as corn, and microorganisms can break it down in the right conditions (for example, in a composting plant).
• Collect batteries and electronic waste separately, so that toxic metals are treated safely and valuable metals can be recovered.
• Design products that last and can be repaired, and choose long-lasting alternatives such as glass or stainless-steel bottles.
• Government rules also help. For example, Hong Kong charges for plastic shopping bags and has banned some disposable plastic tableware, such as plastic straws and foam food boxes.

Key Takeaway: Be a Planet Hero!

Quick Review:

• Using and disposing of plastics and metals can cause pollution and use up the Earth's valuable resources.
• We can all help make a difference by following the 3 Rs: Reduce, Reuse, and Recycle.