Rusting Of Iron And Crystallization

Whether it is a change taking place within our body or in our surroundings, we know that changes can be broadly categorized as physical and chemical changes

Physical change

In a physical change, a substance undergoes a change in its physical properties.During this change, the formation of new substances does not take place and most of the physical changes are reversible. Evaporation, melting of butter, and cutting of paper are some examples of physical changes.

Chemical change

In a chemical change, the formation of one or more new substances takes place. The new substance formed has different chemical properties from that of the substance that formed it. Burning of fuels, cooking of food, fermentation, and souring of milk are some examples of chemical changes.

We will now discuss two important changes:

Rusting (a chemical change) and crystallization (a physical change)

You must have observed that when articles of iron are kept out in the open for some time, they get covered by a brownish substance. This brownish substance is called rust. Rust is the new substance obtained from the combination of iron and atmospheric oxygen. The characteristic properties of rust are different from that of iron as well as oxygen. Hence, rusting of iron is a chemical change, which affects articles of iron and slowly destroys them.

The two main factors that cause rusting are oxygen and water. As soon as the article comes in contact with air (oxygen) and moisture (water), it starts rusting. The chemical process that occurs during rusting can be represented as follows:

Iron (Fe) + Oxygen (O2) + Water (H2O) → Rust (Fe2O3)

Rusting becomes faster with an increase in the content of moisture in air. That is why water pipes made of iron tend to get rusted easily.

Salt makes the process of rusting faster. Rusting is a complex process involving many steps. The presence of salt speeds up the first step, which in turn speeds up the whole process.

Did you know?

Ships are made of iron and a part of them remain under water. Seawater contains many salts dissolved in it and salt water makes the process of rusting faster. Therefore, ships suffer a lot of damage. As a result, a fraction of the ship’s iron has to be replaced every year. Hence, a large amount of money is spent to replace the damaged iron and steel.

Every year, our world suffers huge monetary losses owing to the process of rusting, which causes harm to the articles made of iron. Therefore, attempts are being made to prevent rusting. Here are some ways that can prevent rusting.

1.1. Painting or coating iron articles with grease: This does not allow iron to come in contact with oxygen and water. Hence, it prevents rusting. In fact, paints and grease should be applied regularly to prevent rusting.

2. Rusting can also be prevented by depositing a layer of metal such as chromium or zinc on the surface of the iron article. The process of depositing zinc on iron is called galvanization.

Alloying is another method used for the prevention of rusting. Stainless steel is an alloy. It is a homogeneous mixture of iron, carbon, chromium, nickel, and manganese. It does not rust.

Now, let us discuss crystallization in detail, which is a physical change.

We know that evaporation is a physical process in which a substance changes from liquid to gaseous state. By evaporation, salt is obtained from seawater and all water changes to water vapour. As a result, only the non-volatile salt remains. The salt obtained by this method is not pure and has to be purified. Hence, purified crystals of salt can be obtained by the method of crystallization.

In the crystallization method, a saturated solution of a substance (to be purified) is made. Then, this solution is boiled to make it a supersaturated solution. When this supersaturated solution is cooled, crystals of pure substances are obtained. Thus, large crystals of pure substances can be formed from their solutions by the process of crystallization.

Since no new product has been formed, this is an example of physical change. Let us watch the following animation to understand the process of crystallization.their solutions by the process of crystallization.

Since no new product has been formed, this is an example of physical change. Let us watch the following animation to understand the process of crystallization.

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