Science notes, unit 7

Rusting

Textbook Science Year 5 (DLP), pages 163–176

What this chapter covers

You can describe the characteristics of rusty objects: reddish-brown, rough surface and brittle (7.1.1)
You can state that objects made of iron rust and that plastic and wood do not (7.1.2)
You can explain that water and air together cause iron to rust, using a fair test (7.1.3)
You can describe ways to prevent rusting: oil or grease, a rustproof coating, a rustproof metal plating, and paint (7.1.4)
You can explain why preventing rusting is important (7.1.5)
You can communicate what you learned about rusting, for example in a mind map (7.1.6)

Look at an old bicycle, a gate or a window hinge. Some have reddish-brown patches on them. That is rust. In this unit you will find out what rusty objects are like, which things rust, what makes iron rust, how to stop it, and why this matters.

7.1Rusting

Characteristics of rusty objects

Rusting is what happens when an object made of iron slowly changes and a reddish-brown layer called rust forms on it.

Mr Zaki shows his pupils a rusty window handle and a rusty hinge. They can tell it is rusty by looking and touching.

A rusty window handle that is reddish-brown — A rusty window handle and a rusty hinge.

A rusty door hinge with reddish-brown surface and rusted screws — A rusty window handle and a rusty hinge.

Characteristic	What you notice
Colour	It is reddish-brown.
Surface	The surface is rough.
Strength	It is brittle. It breaks or crumbles easily, for example the screws of the hinge.

Did you know? A rusty hinge can also be stiff and make a loud "kreek" sound when you open the door.

State two characteristics of a rusty nail.

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Any two: reddish-brown colour, rough surface, brittle.

Objects that rust

Not every material rusts. Ravi and Chan compare two benches and two mailboxes.

A wooden bench and a rusty iron bench in a park, two pupils talking — The iron bench is rusty. The wooden bench is not.

Two pupils walking past a rusty mailbox; one mailbox is plastic — The iron mailbox rusts. The plastic mailbox does not.

Material	Does it rust?	Examples
Iron	Yes	Nail, needle, bench, hinge, axe
Plastic	No	Plastic mailbox, drinking straw
Wood	No	Wooden bench, wooden skewer
Glass	No	Glass rod

In the textbook activity, a needle, a glass rod, a drinking straw, a wooden skewer and a nail are wrapped in wet cotton (with vinegar) for five days. You expect only the iron objects (the needle and the nail) to rust.

Not every object rusts. In this unit, remember: objects made of iron rust. Plastic, wood and glass do not.

Factors that cause rusting

What makes iron rust? The textbook investigates this with iron wool in four test tubes (A, B, C and D). After four days the pupils look at the iron wool.

Part of the investigation	What the textbook says
Problem statement	Do air and water cause rusting?
Hypothesis	Water and air will cause iron wool to rust.
Manipulated variable	Presence of water and air
Responding variable	Condition of the iron wool
Constant variable	Quantity of the iron wool (20 g in each tube)

Four test tubes with iron wool. A has only iron wool, B has anhydrous calcium chloride, C has boiled tap water under a layer of oil, D has tap water — The set-up: iron wool only, dry (calcium chloride), no air (boiled water with oil) and water and air (tap water).

Why these materials? Anhydrous calcium chloride absorbs moisture, so the inside of the tube is dry. Boiled water does not contain air. A layer of oil stops air entering the water.

Tube	Condition	Water?	Air?	Iron wool
A	Iron wool only, nothing added	No	Yes	Does not rust
B	Anhydrous calcium chloride	No	Yes	Does not rust
C	Boiled water and oil	Yes	No	Does not rust
D	Tap water	Yes	Yes	Rusts

The textbook leaves the results column blank for you to fill in. The results above are what you expect from the conclusion.

Conclusion: the hypothesis is accepted. The presence of water and air causes iron wool to rust. Both are needed.

Water alone does not make iron rust, and air alone does not either. Iron needs both water and air.

Why is oil poured on the boiled water in tube C?

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Boiled water has no air in it. The oil stops air from entering the water again, so the iron has water but no air.

Ways to prevent rusting

Iron objects left in water and air will rust. We can stop this by keeping water and air away from the iron.

Apply oil or grease
For moving parts such as a bicycle chain.

Coat with a rustproof material
Plastic is coated onto the surface of the iron, for example on a coat hanger.

Plate with a rustproof metal
Tin or aluminium is plated onto the iron, for example on food cans.

Paint
Paint covers the surface of a fence or gate.

Think about it: every method makes a cover over the iron, so water and air cannot touch it.

Which way would you use for the chain of a bicycle? Why?

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Apply oil or grease. The chain moves, so paint would peel off, and oil keeps water and air away.

Why preventing rusting is important

Rusting causes problems. The textbook shows four situations.

A girl and her mother in a shop looking at a dented sardine can — Situation 1: a dented can may have rusted. Food exposed to rust is unsafe to eat.

A rusty iron fence being discussed by two people — Situation 2: painting a rusty fence makes it look better and last longer.

Two pupils on a rusty staircase with a brittle handrail — Situation 3: a rusty handrail is brittle and not safe to use.

In situation 4 a man finds that his axe is rusty and has to buy a new one. Preventing rusting is important because objects made of iron are:

safe to use;
durable and not easily damaged;
cheaper to look after, so we save on maintenance cost;
pleasant to look at.

Think about it: why do the spoons at home not rust although they touch water and air every day?

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The textbook's answer: the spoons are plated with stainless steel, a rustproof metal. So water and air cannot make them rust.

Fun science: rust removal solution

To make a rust removal solution, mix 100 mL of vinegar, three tablespoons of salt and three tablespoons of soda bicarbonate. Stir, pour into a spray bottle and spray on a rusty object. Leave it for five minutes, clean with a wire brush, then rinse with clean water.

Wash your hands with soap and water after preparing the solution and after removing rust.

Show what you learned

Draw a mind map with Rusting in the middle. Add one branch each for the characteristics of rusty objects, objects that rust, what causes rusting, ways to prevent it, and why preventing it is important. Use the summary table below to check your branches.

Pictures: Science Year 5 (DLP) textbook, Unit 7, pp. 164 (handle, hinge), 166 (benches, mailboxes), 169 (test tubes), 170 (oil, hanger, cans, painting), 171 (sardine can, fence), 172 (staircase).

Summary

Topic	Remember
Rusty object	Reddish-brown, rough, brittle
Materials that rust	Iron rusts. Plastic, wood and glass do not.
Cause of rusting	Water and air together
Ways to prevent rusting	Oil or grease, rustproof coating, rustproof metal plating, paint
Why prevent it	Safe to use, lasts longer, saves cost, looks pleasant