Coaching Centre
Physical and chemical properties
The difference between physical and chemical properties, including state, melting and boiling point, density, conductivity, malleability, ductility and viscosity, and flammability, reactivity, corrosion and toxicity, and how to test an unknown element.
The Statue of Liberty was a shiny copper-brown when it was unveiled in 1886. Within about 30 years it had turned completely green. Nobody painted it: the copper slowly reacted with the air and rain to form a thin green coat called a patina, which now protects the metal underneath.
Two kinds of property
Every substance can be described by its properties, and these decide what it is useful for. You wouldn't want clothes made of iron or a bridge made of cotton. Properties come in two kinds.
Physical properties can be observed or measured without turning the substance into something else. They include:
- state at room temperature (solid, liquid or gas), melting point and boiling point;
- appearance: colour and lustre (shininess);
- density, hardness and strength;
- electrical and heat conductivity;
- malleability (can be hammered or bent into shape) and ductility (can be stretched into wire);
- viscosity, how thick and slow-flowing a liquid is. Honey is more viscous than cooking oil, which is more viscous than water. Most liquids become less viscous when heated: warm honey pours much more easily.
Chemical properties describe how a substance reacts to form new substances. You can only find them out by trying a reaction. They include:
- flammability: how easily it catches fire. Petrol, ethanol and paper are highly flammable; water is non-flammable.
- reactivity: how readily it reacts with other substances such as water, acids or oxygen. Sodium fizzes violently in water; gold doesn't react with almost anything.
- corrosion: how easily it is worn away by reacting with its surroundings.
- toxicity: how much harm it can do to living things.
Corrosion
When iron reacts with oxygen and water it forms rust, a flaky, reddish-brown iron oxide. Rust crumbles away and exposes fresh metal underneath, so iron keeps corroding until it is eaten right through. Salt water speeds it up, which is why ships and seaside railings need constant painting.
Some metals corrode in a more helpful way. Aluminium, titanium, zinc and stainless steel quickly form a thin, tough layer of oxide that sticks tightly to the surface and seals the metal underneath from air and water. This is called a passive layer. Copper's green patina does a similar job. Gold and platinum hardly corrode at all, which is why gold jewellery thousands of years old still shines.
Toxicity
Toxicity depends on the dose. Even water can be dangerous if someone drinks several litres in an hour. Other substances are toxic in tiny amounts. The most toxic natural substance known is made by the bacterium Clostridium botulinum: a fraction of a microgram can paralyse the muscles used for breathing. Yet in carefully controlled tiny doses the same toxin (Botox) is used as a medicine to relax muscles.
Mercury and lead are toxic metals that build up in the body over time. Mercury released by industry accumulates in fish, which is why pregnant women are advised to limit large predatory fish such as shark (flake) and swordfish. Lead was once added to petrol and paint; it was removed from Australian petrol in 2002 after it was linked to health problems in children.
Testing an unknown element
Chemists identify substances by testing their properties. Looking, hammering, testing conductivity and heating can quickly tell a metal from a non-metal.
- Pick copper and run all five tests. Then do the same for sulfur. Which tests show the biggest differences?
- Test graphite. Which result surprises you?
- Test iron, copper and aluminium with the magnet. Is a magnet a good test for "metal"?
- Test silicon and decide which group it belongs to.
Test yourself
Which is a chemical property?
Burning makes new substances, so flammability is chemical.
Copper can be drawn out into thin wires. This property is called
Malleability is being hammered into sheets.
Honey pours more slowly than water because it has a higher
Viscosity is resistance to flowing.
Aluminium window frames don't rust away because
This passive layer protects the metal underneath.
Which test is best for telling most metals from non-metals?
Metals conduct; most non-metals don't (graphite is the exception).
Gold is used for jewellery mainly because it
It keeps its shine for thousands of years. Gold is actually quite soft.
Sort the properties of iron.
A substance that can be hammered into shape is . How easily a substance catches fire is its . When iron reacts with oxygen and water it forms . The shine of a metal is called its . A thin protective oxide coating is called a layer.
The Sydney Harbour Bridge is made of steel (mostly iron) and must be painted constantly. Explain why, and suggest why engineers still chose steel for the bridge.
The iron in steel reacts with oxygen and water to form rust, a type of corrosion. The salty air around the harbour speeds this up. Rust doesn't stick to the metal; it flakes away and exposes fresh steel, so without protection the bridge would slowly be weakened. Paint acts as a barrier that keeps oxygen and water away from the steel, but it wears and must be renewed.
Engineers still chose steel because it is very strong, it can be made into huge beams and rivets, and iron is common and cheap compared with corrosion-resistant metals such as aluminium or titanium. Painting is cheaper than building the bridge from a more expensive metal.