☰ Year plan · Science

Mixtures, compounds and properties

Week 5 · Lesson 3 Matter and Its Properties: mixtures and pure substances, elements, compounds and molecules, physical and chemical properties, uses of materials · 3 lessons
3

Properties decide uses

Science Prep 1 Term 1, Unit 1: pp. 18–25
GoalTell physical from chemical properties and explain how properties decide the uses of materials.
New words
physical property · a property we can observe or measure without changing the substancechemical property · a property that shows only when the substance reactsalloy · a material made by mixing a metal with other elements
Explanation

Physical properties can be seen or measured without changing the substance: colour, state, density, viscosity, melting point, solubility and the ability to conduct electricity. Chemical properties show only when a substance reacts, for example burning, rusting or reacting with an acid. Melting ice is a physical change: it is still water, and ice floats because it is less dense than liquid water. Rusting is a chemical change: iron turns into a new substance. We choose materials by their properties. Helium does not burn and is lighter than air, so it fills balloons. Nitrogen is little affected by heat and does not react with rubber, so it is used in tyres. Silicon, a metalloid, conducts electricity better than nonmetals but worse than metals. Stainless steel resists rust. An aluminium-titanium alloy is light and keeps its strength when hot. Aerogel is one of the lightest solids and a very good insulator.

MaterialPropertyUse
heliumlighter than air, does not burnballoons
nitrogendoes not react with rubbertyres
stainless steelresists rustkitchen tools
aerogelvery light, insulatesinsulation
Worked examples
ice floatsiron sinks
A cork floats on water, an iron nail sinks and an ice cube floats. These are physical properties linked to density. When ice melts you still have water, so no new substance forms.
An iron nail left in damp air slowly turns brown and flaky. That is rusting, a chemical property of iron: it can react with air and water. A stainless steel spoon does not rust easily, so it is a good choice for a kitchen.
Why is the base of a cooking pot made of metal and its handle of plastic or wood? Metal conducts heat well, which cooks the food. Plastic and wood conduct heat badly, so the handle stays cooler. The use follows the property.
Class activity

Float or sink. With an adult, fill a bowl with water and collect a cork, a paper clip, a coin and a piece of ice. Write your prediction for each one, then test. Next, pour a little cooking oil on water in a clear glass and watch what happens. Say which property you observed in each test, and whether it is physical or chemical.

Practice
1
Give two physical properties of a substance.
2
Is rusting a physical or a chemical change? Why?
3
Why is helium used to fill balloons?
4
Why does ice float on water?