The study of chemical equilibrium is a fundamental concept in Chemistry Topics, as it examines the balance between forward and reverse reactions.
Factors that Affect the Rusting of Iron and What is Crystallisation ?
When an iron object is left in damp air (or water) for a considerable time, it gets covered with a red-brown flaky substance called rust. This is called rusting of iron. During the rusting of iron, iron metal combines with the oxygen (of air) in the presence of water (moisture) to form a compound ‘iron oxide’. This iron oxide is rust. The process of rusting can be represented by the following word equation :
Rust is iron oxide (Fe2O3). Thus, rust and iron are not the same substance. Rust is different from iron on which it gets deposited. Rusting of iron is a chemical change (because a new substance ‘iron oxide’ is formed in this process).
Almost every iron (or steel) object kept in the open gets rusted slowly. We can usually see the rusted iron gates of parks and iron benches kept in the gardens which always remain in the open (see Figure).
The agricultural tools such as spades and shovels also get rusted when exposed to the atmosphere for a considerable time. In the kitchen, the iron pan (tawa) often gets rusted if kept in wet state for some time. We can also usually see the formation of some rust on iron grills, iron railings, iron pipes, iron nails, old cars, bicycles and steel bridges, etc.
Conditions Necessary for Rusting
Both, oxygen and water (or moisture) must be present for the rusting of iron to occur. So, two conditions are necessary for the rusting of iron to take place :
- presence of oxygen (of air), and
- presence of water or water vapour (called moisture).
Iron rusts when placed in damp air (or moist air), or when placed in water. Now, damp air (or moist air), also contains water vapour. Thus, damp air alone provides both the things, oxygen and water, required for the rusting of iron to occur. Again, ordinary water always has some dissolved oxygen in it. So, ordinary water also supplies both the things, oxygen and water, needed for the rusting of iron.
(i) If the air at a place has a high moisture content (more water vapour), that is, if the air at a place is more humid, then the rusting of iron becomes faster. The rusting of iron is faster in coastal areas (sea-side areas) because the air at those places contains more water vapour (or more moisture).
(ii) The presence of salt in water makes the process of rusting of iron faster. Thus, an iron object will rust much faster when kept in sea-water (which is salty water) than when kept in fresh water (having no salts dissolved in it).
Rusting Damages iron Objects
Rust is soft and porous, and it gradually falls off from the surface of a rusted iron object, and then the iron below starts rusting. Thus, rusting of iron is a continuous process which slowly eats up the iron objects and makes them useless.
Since iron is used in making a large number of objects or articles such as bridges, grills, railings, gates, and bodies of cars, buses, trucks and ships, etc., rusting of iron causes a great loss over a period of time. It is obvious that we should have some ways of preventing the rusting of iron.
How Do We Prevent Rusting of Iron
Rusting takes place when an iron object comes in contact with air (containing oxygen) and water. So, if air and water are prevented from coming in contact with iron objects, then rusting will not take place.
Thus, most of the methods of preventing rusting of iron involve coating the iron object with ‘something’ to keep out air and water (which cause rusting). Some of the methods of preventing rusting of iron are given below :
(i) Rusting of iron can be prevented by painting. When a coat of paint is applied to the surface of an iron object, then air and moisture cannot come in contact with the iron object and hence no rusting takes place. The window grills, railings, steel furniture, iron bridges, railway coaches, and bodies of cars, buses and trucks, etc., are all painted regularly to protect them from rusting.
(ii) Rusting of iron can be prevented by applying grease or oil.
When some grease or oil is applied to the surface of an iron object, then air and moisture cannot come in contact with it and hence rusting is prevented. For example, the tools and machine parts made of iron and steel are smeared with grease or oil to prevent their rusting.
(iii) Rusting of iron can be prevented by galvanisation.
The process of depositing a thin layer (or coating) of zinc metal on iron objects is called galvanisation. Galvanisation is done by dipping an iron object in molten zinc metal. A thin layer of zinc metal formed on the surface of an iron object protects it from rusting (because zinc metal remains unaffected by air and moisture). The iron sheets used for making buckets, drums, dust-bins and sheds (roofs) are galvanised to prevent their rusting (see Figure).
The iron pipes used in our homes to carry water are also galvanised to prevent rusting.
(iv) Iron is coated with chromium to prevent rusting.
This is called chrome-plating. Chromium metal is resistant to the action of air and moisture. So, when a layer of chromium is deposited on an iron object, then the iron object is protected from rusting. Chromium-plating is done on steel furniture, taps, bicycle handle bars and car bumpers, etc., made of iron and steel to prevent them from rusting.
(v) Rusting of iron can be prevented by alloying it to make stainless steel.
When iron is mixed (or alloyed) with carbon, chromium and nickel, then stainless steel is obtained. Stainless steel is an alloy of iron. Stainless steel does not rust at all. Cooking utensils, knives, scissors and surgical instruments are made of stainless steel and do not rust at all.
The Case of Ships
Ships are made of iron (and steel) and a part of the ship always remains under sea-water. Even on the part of ships which remain above sea-water, water drops keep clinging to their outer surface. Moreover, the sea-water contains many salts.
The presence of salts in sea-water makes the process of rust formation on ships faster. So, inspite of being painted regularly, the ships suffer a lot of damage from rusting. In fact, the damage caused by rusting is so much that a fraction of ship’s iron has to be replaced every year. This causes a lot of monetary loss (money loss) to the world.
The Iron Pillar at Delhi
There is an iron pillar near the Qutub Minar in Delhi which is more than 7 metres high and weighs more than 6000 kg (see Figure). It was built more than 1600 years ago.
Even after such a long period, the iron pillar has not rusted at all. This shows that Indian scientists had made great advances in metal making technology as back as 1600 years which enabled them to make this iron pillar having the quality of great rust resistance.
Sea-water contains salts dissolved in it (which make it salty). We have learnt in Class VI that salt can be separated from sea-water by the process of evaporation. The process of evaporation (to dryness) is not a good technique of separation because :
- The soluble impurities do not get removed in the process of evaporation of a salt solution. So, the salt obtained by evaporation is not pure.
- The crystals of salts obtained by the process of evaporation are small. And the shape of crystals cannot be seen clearly.
Large crystals of pure substances can, however, be obtained from their solutions by the process of crystallisation. So, the solid substances are usually purified by the process of crystallisation. Crystallisation is an example of a physical change. This is described below.
The solid particles having flat surfaces, straight edges and regular shapes are called crystals. Many substances form crystals. The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. The process of crystallisation is used to obtain large crystals of a pure solid substance from the impure solid substance.
An impure solid substance usually contains two types of impurities : insoluble impurities and soluble impurities. The insoluble impurities are removed by filtering its solution whereas soluble impurities get removed during crystallisation. As an example, we will now describe how large crystals of pure copper sulphate can be obtained from an impure sample of copper sulphate powder by the process of crystallisation.
To Obtain Pure Copper Sulphate Crystals from an Impure Sample
Impure copper sulphate powder can be purified by the process of crystallisation to obtain large crystals of pure copper sulphate. This is done as follows: Take about 100 mL of water in a beaker and add a few drops of dilute sulphuric acid to it. Heat the water over a burner till it boils. Add copper sulphate powder slowly to the hot water with constant stirring [see Figure (a)].
Continue to add copper sulphate till no more copper sulphate can be dissolved. This will give us a saturated solution of copper sulphate. Filter the hot saturated solution of copper sulphate to remove insoluble impurities. Allow the hot and concentrated solution of copper sulphate to cool slowly. Do not disturb the solution when it is cooling.
After some time, we will see large copper sulphate crystals at the bottom of the beaker [see Figure (b)]. Separate the copper sulphate crystals from solution by filtration and dry. The soluble impurities present in copper sulphate do not crystallise and hence remain behind in the solution.
The crystals of other substances like alum (phitkari) can also be prepared in a similar way. Impure common salt obtained by the evaporation of sea-water is also purified by the process of crystallisation.