Properties decide uses
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.
| Material | Property | Use |
|---|---|---|
| helium | lighter than air, does not burn | balloons |
| nitrogen | does not react with rubber | tyres |
| stainless steel | resists rust | kitchen tools |
| aerogel | very light, insulates | insulation |
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.