Physical properties: metals, non-metals and the main material groups

The Statue of Liberty is green from its crown to its toes. Was it always that colour, or did somebody paint it green? Paint is the usual guess, and it is wrong. When the statue was unveiled in eighteen eighty-six, it was a shiny reddish-brown, the colour of a brand-new penny. Within about twenty years it had turned green on its own. So what turned a metal statue green with no paint, and why is that green coat now protecting it?

Every product starts with what it is made of. The materials a designer works with fall into five main groups, and you already own something from each one. Papers and boards are the first group, like the card of a cereal box. Timbers are the second. Natural timber is wood cut straight from a tree, like pine. Manufactured timber is a board made from wood fibres or thin layers glued together, like MDF. Metals and alloys are the third group, like the aluminium of a drinks can. An alloy is a metal mixed with another element, the way steel is iron with a little carbon added. Polymers are the fourth group. Polymer is the proper word for a plastic, like the polypropylene of a lunch box. Textiles are the fifth, like the cotton of a T-shirt. A physical property is something you can measure or observe about a material without bending, hitting or stretching it. How heavy it is for its size, whether heat and electricity pass through it, whether it soaks up water, whether it melts. How a material copes when you do bend, hit or stretch it is a working property, like strength or hardness. Working properties have their own video, the one on strength, hardness and toughness. This video stays with the physical kind. Bending a cork tile until it snaps: is that testing a physical or a working property? A working one. Bending it to breaking point tests how it copes with a force. Its density, or whether it soaks up water, would be physical. Now the quickest physical sort of all: metal against non-metal. Most metals let heat and electricity pass through well. They are heavy for their size, and they shine where they are cut or scratched. Most metals can also be bent or hammered without snapping. And over time, most of them react with oxygen and water from the air. That reaction is the rust on an old bike chain, and the dull brown tarnish on an old copper pipe. The non-metals in products are timber, polymers, paper and board, and textiles. Most let heat and electricity through poorly, and they are lighter for their size. They also char, burn or soften at far lower temperatures than a metal. That is enough to classify a material you are handed. Picture an unlabelled bar from a workshop scrap bin. It feels heavy for its size, it is cold in your hand, and it gleams where a file has scratched it. Metal or non-metal, and which two clues tell you? A metal. A timber or a polymer of that bulk would weigh less, and it would stay dull under the tool. Any two clues pointing the same way settle it.

Thermal conductivity is how easily heat passes through a material. A good thermal conductor lets heat through fast, and most metals are good ones. Heat always flows from hot to cold, so picture it as arrows leaving the hotter thing. In a conductor, the arrows pass straight through. In an insulator, a material that lets heat through poorly, the arrows are slowed or stopped. Now a test of that. A metal railing and a wooden bench stand side by side outside all night, and on a frosty morning a thermometer reads the same on both. Which feels colder to touch, and why, if both are the same temperature? The railing feels far colder. Your hand is the hot thing, so heat flows out of your skin into whatever you touch. Steel conducts that heat away fast, so your skin cools quickly. Timber conducts it slowly, so your hand stays warm. The railing is no colder than the bench at all. It draws your heat away faster, and your skin reads that loss of heat as cold. In some products, conduction is the job. An aluminium pan base conducts, so heat from the hob passes through quickly and the food heats fast and evenly. Insulation is the opposite job. A wool hat insulates, so the warmth of your head stays in instead of escaping into the cold air. A card sleeve on a coffee cup, a copper pan base: which conducts, which insulates? The copper base conducts, so heat from the hob reaches the food fast. The card sleeve insulates, so heat from the coffee is slowed before it reaches your fingers. One examiner's report, on a question about a takeaway lid made of card lined with foil, records this slip. Some responses confused conduction with insulation, of the foil card composite, when discussing suitability of the lid to reflect heat and keep food warm. Keeping food warm is insulation, because the heat is held in. The fix is one question before you write: should the heat pass through, or be held back? Through is conduction, and held back is insulation. Electrical conductivity is the same idea for electricity: how readily it flows through a material. Copper is an excellent electrical conductor, which is why the wires inside a phone charger cable are copper. And the cable's outer coat: conductor or insulator? An insulator, so the current cannot reach your hand. The coat is polyvinyl chloride, PVC for short, a polymer that current cannot cross. Two materials share the job, then, each picked for one physical property. Now a different task: give a physical property that metals share, with a product that needs it. Here are two students' answers to that. The first writes: metals are strong. The second writes: metals conduct electricity well, so the wires in a charger cable are copper. Which of the two answers scores marks, and what is wrong with the other? The second scores. Strength is a working property, not a physical one, and strong on its own gives no reason. The second names a physical property and says what it does for a product.

Density is how much mass is packed into a given volume. Two parts of exactly the same size can weigh very different amounts, and the heavier one is the denser. Picture a block of pine and a block of mild steel, cut to the same size and set on a balance. The steel side drops. Same size, more mass, so steel has the higher density. Here's a different balance. A big block of polystyrene foam sits on one pan, a small steel washer sits on the other, and the foam side goes down. Does that make the polystyrene foam denser than the steel? No. Density compares mass for the same volume. A piece of foam cut to the washer's size would weigh almost nothing. Heavier overall is not the same as denser. Resistance to moisture is a material's ability to keep water out instead of soaking it up. Absorbency is the opposite end of the same scale: an absorbent material soaks water up. A cotton towel is absorbent, which is what a towel is for. A polypropylene lunch box resists moisture, so soup cannot soak into its walls, stain them or make them soft. Kitchen roll and a raincoat's outer fabric: absorbent, or resistant to moisture? Kitchen roll needs to be absorbent, so it takes up a spill. The raincoat fabric needs resistance to moisture, so rain stays on the outside and runs off. Fusibility is a material's ability to melt when it is heated, then set solid again as it cools. It is a useful ability, because a melted material can be poured, moulded or used to join parts. Solder shows it on a circuit board. It is melted into a joint, flows round a component's leg, then hardens to hold the part in place and carry the current. Polymer granules are fusible too. They are melted, pushed into a mould, and set as they cool into a shaped part, like a bottle cap. Now a weak answer about why a steel saucepan suits its job: it has a high melting point, so the pan will not melt. What is wrong with 'high melting point, so it will not melt' as a reason? A pan on a hob never gets anywhere near its melting point, so that reason says nothing about its job. Its job is to cook food. The property that does that is thermal conductivity, carrying heat through the base to the food. A separate examiner's report met that same weak chain on a question about a pan. Quite a few candidates referred to a high melting point but then incorrectly justified this by saying the pan would not melt. The fix is to ask what the product is for, then name the property that does that job. Keep melting for fusibility, where melting is useful. Now three of these properties on one product. A hiker's camping mug is made from aluminium, and here are two students' reasons for that choice. One writes: aluminium is light. The other writes: aluminium has a low density, so the mug adds little weight to a rucksack carried all day. Which reason scores, and what is the other one missing? The second. It names the property, low density, and says what that does for this mug. Light is only the everyday word for low density, and on its own it gives no reason. One examiner's report puts it plainly, on a question about a cardboard box. Other candidates stated generic, one word properties of cardboard such as strong, light, cheap et cetera that did not give a reason why this made them suitable for the box. The habit that fixes it is the linked answer: the property, what it means, then what it does for this product. Now, what is one more physical reason aluminium suits the mug, and what does it do? Two more work. Aluminium conducts heat well, so a camping stove boils the water fast and saves fuel. It resists moisture too, so it can be rinsed and used for years, unlike a paper cup that goes soggy. There is a trade-off. The conductivity that boils the water fast also makes the rim hot on your lips, which is why many camping mugs add a silicone sleeve to insulate it.

Three more physical properties complete the set: resistance to corrosion, resistance to chemicals, and flammability. Resistance to corrosion is a material's ability to keep its surface and its strength when air and water attack it. Plain steel resists poorly: it rusts, flakes and weakens. Aluminium resists corrosion well. A thin protective layer forms on its surface as soon as air touches it, and that layer keeps air and water off the metal underneath. Stainless steel, most polymers and glass resist corrosion too. Now back to the Statue of Liberty. Its skin is copper, and over the years copper reacts with the air and rain and turns green. Why would that green layer help the statue, rather than harm it? It works like the layer on aluminium. The green coat, called verdigris, covers the metal and shields what lies beneath, so the metal under it is protected from further corrosion. So nobody painted the statue. The copper made its own green coat, and that coat has guarded the copper beneath it ever since. Resistance to chemicals is a material's ability to stay unharmed when a chemical touches it, without dissolving, softening, staining or reacting. That is why bleach comes in a bottle of high density polyethylene, HDPE for short, which holds it for months without leaking or weakening. Flammability is how easily a material catches fire and keeps burning. Timber, paper and board, most polymers and many textiles are flammable, and most metals are not. So a child's nightdress or a tent needs a material of low flammability, or a flame-retardant finish, a treatment that slows burning. Here's a garden products firm with three jobs. A sprayer tank moulded in HDPE, a patio chair frame of aluminium left out all year, and a fire pit bowl of steel. Tank, chair frame, fire pit: which of the three properties does each one need? The tank resists chemicals, so weedkiller cannot soften or eat into its walls and it never leaks. The chair frame resists corrosion, so the weather cannot rust it. The fire pit has low flammability, so it will not catch fire while it holds burning logs. One catch with the fire pit. Plain steel does corrode, so it will rust unless it is coated, and that is why the corrosion answer went to the aluminium chair.

Four questions to finish, three of them on products this video has not shown you.

A home radiator is steel. Which physical property helps it warm the room? Thermal conductivity. Heat from the hot water inside passes through the steel quickly and out into the room. Now a lifejacket's foam: why does it need a low density? Low density means little mass for its size, so the foam floats and holds the wearer up. Next up, a glue gun melts a glue stick that sets as it cools. What property is at work? Fusibility. The glue melts when heated and turns solid again as it cools, and that joins the parts. One more, back to the green statue: what has the green coat given the copper? Resistance to corrosion. The verdigris keeps air and water off the copper beneath, so the statue's metal is protected from further corrosion.

If all four came back to you, you have fully understood this one, and a thumbs up will tell you so the next time you scroll past it. If some were slow, save the video, because the rest of this chapter keeps using these property words, so they come back round.

Next in the chapter: Working properties of materials: strength, hardness, toughness and more, the properties you feel when you bend, hit or stretch something. Hopefully that's next in your feed, but if not, search for ScholaFly D T oh three, oh two.

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