10.5 Materials: Properties, Uses & Reversible and Irreversible Changes
Key Takeaways
Materials are chosen for a job by their properties, such as transparency, absorbency, strength, flexibility, magnetism and how well they conduct heat.
Magnets attract iron, steel, nickel and cobalt, but not aluminium, copper, plastic, wood or glass.
Reversible changes such as melting, freezing, evaporating, condensing and dissolving can be undone, while irreversible changes such as burning, cooking and rusting make new materials.
Household chemicals must be stored in labelled original containers out of children's reach, never mixed (bleach with ammonia makes toxic gas), and never poured into gullies or drains.
A fair test changes only one variable, such as the material, while keeping the amount of water, size of sample and time the same.
10.5 Materials: Properties, Uses & Reversible and Irreversible Changes
Materials: Properties and Uses is a Grade 6 Science unit in the National Standards Curriculum (Term 2, Unit 1). Students examine materials to test their transparency, absorbency, strength, magnetic properties and heat conductivity; explain why particular materials are chosen for particular objects; use and dispose of household materials safely; and classify changes as reversible or irreversible.
The Properties of Materials
| Property | What it means | Good example | Poor example |
|---|---|---|---|
| Transparency | Lets light through so you can see through it | Glass, clear plastic | Wood, metal |
| Absorbency | Soaks up liquid | Cotton towel, sponge, paper towel | Plastic, glass, rubber |
| Strength | Holds weight or force without breaking | Steel, concrete, nylon rope | Paper, thin plastic |
| Flexibility | Bends without breaking | Rubber, cloth, thin plastic | Glass, brick |
| Hardness | Resists scratching and denting | Diamond, steel, stone | Clay, wax, soft wood |
| Magnetism | Is attracted by a magnet | Iron, steel, nickel, cobalt | Aluminium, copper, plastic, wood, glass |
| Heat conductivity | Lets heat pass through easily (a conductor) or not (an insulator) | Conductors: aluminium, copper, iron | Insulators: wood, plastic, cloth, air |
| Waterproofing | Does not let water through | Plastic, rubber, glass, galvanised zinc sheets | Cotton cloth, paper |
Note
Not all metals are magnetic. A magnet will pick up a steel nail but not an aluminium drink can or a copper wire. This makes magnets useful for sorting steel cans from aluminium cans for recycling.
Choosing the Right Material for the Job
Ask which properties the object needs, then choose a material that has them.
| Object | Properties needed | Suitable material |
|---|---|---|
| Cooking pot (dutchie) | Conducts heat well, does not melt on the stove | Aluminium or cast iron |
| Pot handle | Does not conduct heat (insulator) | Wood or heat-resistant plastic |
| Roof sheeting | Waterproof, strong, light | Galvanised (zinc-coated) steel sheets |
| Window | Transparent, waterproof | Glass |
| Towel | Absorbent, soft, flexible | Cotton |
| Raincoat | Waterproof, flexible | Plastic or rubber-coated fabric |
| Fishing line | Strong, flexible, waterproof | Nylon |
In a test item, an option is wrong if the material lacks a property the job needs, even if it has other good properties (a glass handle on a pot would not conduct heat much, but it would break easily).
Testing Materials Fairly
To find the most absorbent paper towel:
- Predict which towel will absorb most, based on what you have observed before.
- Cut squares of the same size from each towel.
- Dip each square into the same amount of water for the same time.
- Measure how much water is left in each container, or how much each towel holds.
- Change only the type of towel (the independent variable); keep everything else the same.
- Compare results with your prediction and be willing to change your mind if the evidence disagrees.
Using, Storing and Disposing of Household Materials Safely
- Keep bleach, kerosene, pesticides and medicines in their original labelled containers, never in drink bottles.
- Store them out of reach of children, away from food and away from heat or flames.
- Never mix cleaning products. Mixing bleach with ammonia-based cleaners releases a poisonous gas.
- Read labels and follow instructions; wear gloves when the label says to.
- Dispose of materials responsibly: do not pour oil, paint or chemicals into gullies, drains or rivers; take used batteries and electronics to collection points where they exist; and reduce, reuse and recycle plastics and glass.
- Think about the environmental impact of materials. Plastic bags and polystyrene food boxes last for hundreds of years, clog drains and harm marine animals, which is why Jamaica has banned certain single-use plastic bags, plastic drinking straws and polystyrene foam food containers.
Reversible and Irreversible Changes
| Reversible changes (can be undone; no new material) | Irreversible changes (cannot easily be undone; new materials form) |
|---|---|
| Melting ice into water, then freezing it again | Burning paper or wood (ash and smoke form) |
| Melting chocolate or candle wax, then letting it set | Cooking an egg or baking a cake |
| Evaporating water and condensing the steam back to water | Rusting of an iron gate |
| Dissolving salt or sugar in water (evaporate the water to get it back) | Fruit and food rotting |
| Bending a wire or folding paper | Cement or plaster of Paris setting hard |
Signs that an irreversible change has happened include a new smell, a colour change that does not go back, gas bubbles, light or heat given off, and the formation of a new substance.
Changes of State
Changes of state are reversible changes caused by heating or cooling:
- Melting: solid to liquid when heated (ice melts at ).
- Freezing: liquid to solid when cooled.
- Evaporating: liquid to gas (water boils at at sea level, but puddles also evaporate slowly in the sun).
- Condensing: gas to liquid when cooled, like water droplets forming on a cold glass of juice.
The Effect of Heat on Different Materials
Heat affects materials differently. Butter, wax and ice melt; paper and wood burn; clay hardens into pottery in a kiln; an egg sets; and metals expand slightly when hot, which is why gaps are left between some bridge sections. You can predict whether a change will be reversible by asking: "Is a new material made?" If yes, the change is irreversible.
Which item would a magnet attract?
An aluminium drink can
A copper electrical wire
A plastic ruler
A steel nail
Which of these is an irreversible change?
Melting a block of ice in the sun
Dissolving sugar in a glass of water
Frying an egg in a pan
Condensing steam on a cold window
Why are cooking-pot handles usually made of wood or heat-resistant plastic?
They conduct heat quickly so the cook can tell when food is ready
They conduct heat poorly, so the handle stays cool enough to hold
They are magnetic, so the pot stays firmly in place on the stove
They are transparent, letting the cook see the food inside the pot
Sections you finish are checked off in the contents.