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Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirty one (Physical Properties of metals)

Lesson thirty one
ProfVietanhNgày: Thứ tư, 2010-05-19, 9:33 AM | Tin nhắn # 1
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Physical Properties of metals

Metals are widely used in our daily life. For example, you must have used vessels made of aluminum, copper, brass and stainless steel. Metals are used for the construction of bridges, automobiles, aero planes, ships, trains, etc. Some of the metals, like gold, silver, copper, lead, mercury, tin, etc., have been in use since ancient times. During the Bronze age, people produced copper by heating a rock containing copper with carbon. In fact, metals are very important for the national economy of any country. Some metals, such as titanium, chromium, manganese, zirconium, etc., are classified as strategic metals. That is, they are essential for the country’s economy or its defence. These metals and their alloys are used in atomic energy, space science projects, jet engines, highgrade steels, etc.

A metal is an element which can form positive ions by the loss of electrons. The number of electrons lost by an atom is known as the valency of the metal.
Thus, sodium, magnesium, copper and aluminium are metals. They are metals because they can form positive ions by the loss of electrons. Let us now study some of the characteristic properties of metals.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-05-19, 9:34 AM | Tin nhắn # 2
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1. Metallic Lustre

In pure state, metals have a shining surface. This property is called metallic lustre. Metals like aluminium and magnesium appear white. Gold is yellow in colour and copper is reddish brown.

Take a piece of magnesium ribbon, aluminium wire and a sheet of copper. Note their appearance. Rub the surface of each metal with a sand paper. Note their appearance again.

You must have observed that metals had a dull appearance, but on rubbing with a sand paper, they appear bright. How do you explain this? When metals are exposed to air for a long time, they lose their brightness. This is due to the formation of a thin layer of oxide, carbonate or sulphide on their surface. This layer is removed by rubbing. For example, aluminium, on exposure to air, is coated with a layer of aluminium oxide, which prevents the metal underneath from further corrosion; the surface of copper is coated with a green layer in moist air due to the formation of basic copper carbonate; silver articles acquire a blackish tinge due to the formation of silver sulphide.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-05-19, 9:34 AM | Tin nhắn # 3
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2. Hardness

Metals are generally hard. But all metals are not equally hard. The hardness varies from metal to metal. Metals like ir on, copper, aluminium, etc., are quite hard. They cannot be easily cut with a knife. But sodium and potassium are so soft that they can be easily cut with a knife.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-05-19, 9:35 AM | Tin nhắn # 4
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3. Malleability and Ductility

Some metals can be beaten into thin sheets with a hammer. This property is called malleability. Gold and silver are the most malleable metals. They can be hammered into very thin foils.

Another important physical property shown by metals is ductility. That is, metals can be drawn into very thin wires. But all metals are not equally ductile. Gold and silver are the best ductile metals. One can draw a wire of about 2 kilometre from only one gram of gold.

Metals are malleable and ductile. It is because of these properties that metals can be given different shapes. For example, silver foils are used for decorative purposes on sweets; aluminium foils are used for wrapping chocolates, foods stuff, etc., and to seal bottles and containers.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
ProfVietanhNgày: Thứ tư, 2010-05-19, 9:35 AM | Tin nhắn # 5
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4. Thermal and Electrical Conductivity

Thermal means heat. Metals are generally good conductors of heat. Silver is the best conductor of heat, followed by copper. Aluminium is also a good conductor of heat. That is why cooking vessels and water boilers are generally made of copper and aluminium. Lead and mercury are the poorest conductors of heat. How does a metal conduct heat? When a metal is heated, its atoms gain energy and vibrate more vigorously. This energy is transferred to the electrons, which can move through the metal. They transfer their energy to other electrons and atoms, and thus, heat is conducted.

Metals also conduct electricity because they have electrons which are free to move. They offer little resistance to the flow of current. Silver and copper are the best conductors of electricity, followed by gold, aluminium and tungsten. Mercury offers a very high resistance to the passage of electric current.


Phạm Bá Việt Anh

Department of Analytical Chemistry
Faculty of Chemistry
Hanoi National University of Education
Mobile - Tel: (84) 943 919 789
 
Diễn đàn » Hóa học đại học và sau đại học » Tiếng Anh chuyên ngành hóa học (English for Special Purposes) » Lesson thirty one (Physical Properties of metals)
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