RUSTING

Rusting is a chemical change that occurs on the surface of iron materials and forms

reddish–brown coating. The reddish – brown coating is called rust.

Conditions for rusting

There are three conditions necessary for rusting of iron to occur which are:

(i) Iron

(ii) Oxygen

(iii) Water

Therefore, iron will not rust on exposure to dry air or air-free water (water that has

been boiled to expel all dissolved air). However, iron will easily and readily rust in

water that has dissolved air in it.

Experimental setup for investigating the conditions necessary for iron to

undergo rusting

Observation

Nails in test tube A and D will rust. Nails in tubes B and C will not rust.

Reasons

•

In test tube A nails are in contact with both water and air (oxygen), hence rusting

will occur.

In test tube B, iron nails will not rust because water has been boiled to remove

oxygen and oil prevent oxygen from dissolving in it

In test tube C, iron nails will not rust because they are in contact with air only.

Anhydrous calcium chloride absorbs any water in the test tube.

In test tube D, iron nails will rust but the process will take long time to occur

because they are in contact with air but the amount of water in air is less.

FACTORS THAT SPEED UP THE RATE OF RUSTING

There are various factors which make the process of rusting faster; some of them are

described below:

(a)

Soluble salts. Iron tends to rust faster in the sea water than it does in fresh

water, due to the presence of dissolved salts. Salt water contains many ions

that speed up the rusting via electrochemical reactions.

This makes ships, cars and other iron materials in coastal towns like Dar es

Salaam, Tanga and Zanzibar to rust more quickly than in other cities in

Tanzania like Songea, Mwanza or Arusha due to the presence of salty water in

the oceans.

(b)

(c)

(d)

Presence of acidic substances. Acidic substances corrodes the surface of

iron by breaking down the oxide layer that forms on the surface of iron, this

expose iron to oxygen and water hence accelerating rusting.

High temperature. As temperature increases, molecules move faster and the

rate of reaction increases due to more collisions. So, rusting as a chemical

reaction its rate increases as temperature increases.

Carbon dioxide. Carbon dioxide dissolves in moisture or water to form

carbonic acid which corrodes iron, so high concentration of carbon dioxide

makes the process of rusting to be more faster.

(e)

(f)

Impurity; pure iron tends to rust more slowly when compared to iron

containing a mixture of metals (alloys).

Amount of moisture; moisture content is one among the significant factor

that affects the rate of rusting. Iron tend to rust much faster in highly moist

environment than in dry areas.

METHODS USED TO PREVENT RUSTING

In order to stop rusting iron should be protected from coming in contact with either

water (moisture) or oxygen (air) or both. The following are some of the methods

used to prevent iron from rusting:

(i) Painting

(ii) Oiling and greasing

(iii) Galvanization

(iv) Tin plating

(v) Use of silica gel

(vi) Sacrificial protection

(vii) Alloying

1. Painting. This is a process of coating iron items with a special pigment or paint.

The paint coating prevent oxygen and water from coming into contact with the

iron material.

2. Oiling and greasing. This is the coating of iron with oil or grease, some

machine parts cannot be protected by painting so oil is used.

3. Galvanization. This is the process of coating iron or steel with zinc. This is

possible because zinc is more reactive than iron so will be reacting with water and

oxygen in preference to iron to form an impermeable coat of zinc oxide over the

surface of iron. This process is used for dustbins, car bodies, barbed wires,

motorway crash barriers and iron sheets.

4. Tin plating. This is a process of coating iron/steel with tin. It is used to protect

food cans.

5. Use of silica gel. Silica gel is a substance in the form of grains which absorbs

moisture. Silica gel prevent water from reaching iron/steel. A small bag of silica

gel is put inside bags or boxes used for storing or carrying iron items to absorb

any moisture that may cause rusting.

6. Sacrificial protection. In this technique the corrosion of a metal surface is

controlled by making it the cathode of an electrochemical cell. The sacrificial metal

then corrodes instead of the protected iron.

7. Alloying. Alloys are mixtures of two or more elements of which one is a base

metal. For example, iron can be mixed with small quantities of much less reactive

metals to form an alloy called stainless steel. Stainless steel contains iron mixed

with chromium, nickel and manganese. Stainless steel does not rust. It is used to

make cutlery and kitchen equipments.

NB: An alloy is a mixture of one metal with another metal or non-metal

8. Electroplating is the process of coating one metal with a layer of another metal

by means of electrolysis, where the metal to be coated is made the cathode and

the coating metal the anode. An iron or steel object can be electroplated with a

layer of chromium or tin to protect against rusting. If protective layer is broken,

then the steel beneath will begin to rust.

9. Use plastic. Parts of some machines or instruments and household items made

up of steel are coated with plastic in order to prevent them from rusting by

preventing iron material from becoming into contact with oxygen and moisture.

Plastic is used because is cheap and can be made to look attractive.

DISADVANTAGES OF RUSTING

(a) Lose of metal (iron) from the surface

(b) Lose efficiency of some machines due to friction.

(c) Decrease life span of iron objects

(d) Rusting can cause diseases like cancer and tetanus.

(e) Change in physical properties of iron objects. Example; thermal and electrical

conductivity

(f) Rusting is the source of land pollution. The rusted iron objects are damped on

land.

(g) Rusting causes wearing and tearing of machine parts. Because weakens metals.

Question

An engineer observed a reddish–brown coating on the surface of iron rods which had

to be used for a bridge construction project. The phenomenon occurred when the

iron rods were left on open environment containing water and air for a month.

Explain six (6) harmful effects of the phenomenon explained above to the

environment and daily activities.

Answer

Rusting is a chemical process that occurs on the surface of iron or steel material and

forms a reddish-brown coating known as rust. It occurs when an iron or steel material

is subjected to an open environment containing water and air for some days.

The following are the harmful effects of the phenomenon explained above to the

environment and daily activities.

Structural damage: Rust weakens metal structures such as bridges, buildings, and

vehicles, leading to potential collapses and safety hazards. Rusty surfaces on

furniture, tools, and vehicles reduce their visual appeal and market value.

Increased maintenance costs: Regular repairs and replacements due to rust

increase maintenance expenses in homes, vehicles, and industries. This tend to

decline different projects.

Reduced lifespan of equipment: Rust shortens the functional life of tools,

machines, and appliances, requiring early replacements. This is due to the fact that

rusting cause wearing and tearing of machine parts as a result of increased friction

between iron parts.

Water and air contamination: Rusting pipes can contaminate water supplies with

iron particles and lead to health risks. Fine rust particles in the air can cause

respiratory problems if inhaled, especially in industrial settings

Agricultural impact: Rusty equipment in farming can lead to inefficient operations

and contamination of soil and crops. Rust equipment cannot operate well and the

contamination to the soil affects soil fertility for better agriculture yields.

Transportation hazards: Rust compromises the integrity of vehicles, ships, and

aircraft, making them unsafe for travel and transport. Several accidents may be

reported due to malfunctioning of some engine parts or transmission system.

Environmental waste: Discarded rusted items contribute to environmental pollution

and increased landfill use. The amount of useful land decrease as a large part of the

land is damped with rust iron materials.

Electrical failures: Rusting of electrical components can cause short circuits,

equipment failure, and fire hazards. Electrical conductivity is totally affected and

contribute to decline of projects and activities depending on electricity.

Generally, rusting is destructive. To prevent iron and steel material from rusting, we

should ensure that both air and water do not come into contact with them. Different

methods may be employed to prevent rusting, like; painting, oiling, alloying,

galvanization, tin plating, electroplating, use of silica gel (desiccant) and cathodic

protection. Therefore, the prevention of rusting has become the basis of many

researchers because of the wide range usage of iron and steel in vehicles, industries

and other technologies.

REVISION EXERCISE FOUR

1. Classify the following as either physical or chemical changes

(a) Aluminium foil is cut into half (b) Evaporation of water from the ocean

(c) Milk turns sour

(f) Freezing of juice

(i) Amatch is lit

(d) Wood is burnt

(g) Drying of clothes (h) Food is cooked

(j) Melting of ice (k) Dissolving sugar in water

(e) Nail or iron sheets rust

(l) Grinding a piece of paper

2. Explain the importance of change in states of matter in daily life. Use any five (05)

points.

3. (a) When iron sheets are exposed to wet air for a long time, they turn brown in

colour. Explain

(b) Explain the effects of rusting in your environment.

(c) Why are commodities like hand bags and camera bags for sale are packed with

silica gel? Explain

4. Why anodizing method is useless in preventing rusting of iron materials? Explain

5. Giving reason, state whether rust will form or not in each of the following

situation.

(a) Iron bar is dipped into boiling water

(b) Painted iron is dipped into un-boiled water

(c) Iron bar is dipped in un-boiled water

(d) Oiled bar is left outside the room over night

(e) Aluminium wire is dipped in un-boiled water

(f) A dry iron bar is wrapped with cotton wool

6. Briefly explain four (4) methods of preventing rusting of iron/stell material.

7. Cars and corrugated iron sheets in coastal towns like Dar es Salaam, Tanga and

Zanzibar are found to rust more quickly than in other cities in Tanzania like

Songea, Mwanza or Arusha. Why is this so?

8. Why it is not advisable to wash vehicles by using sea water? Explain

9. Rusting of iron and burning of papers are both chemical changes. But, when iron

rusts its mass increases and when a bundle of papers burns its mass decreases.

Give reason(s) for this phenomenon.

10. A form one student has a balloon filled with a certain gas. If the student puts the

balloon in the freezer overnight, what do you think will happen to the balloon?

Explain.

11. When a container of coffee is opened in a room, people in different parts of the

room may notice its smell. Explain

12. Describe the role of changes in states of matter in water purification.

13. Explain how the changes in state of matter are linked to the formation of rain

cloud in areas of low pressure and moist air.

14. How dose temperature affect the state of matter

15. Give four real-life examples that illustrate the kinetic nature of matter.

16. Explain how physical and chemical changes are involved in cooking process.

17. When a metal such a copper is heated, it expands. Explain what happens to the

metal particles during expansion.

18. Explain how physical and chemical changes are involved in steel production and

food processing industries.