Chapter 17: Metal and Non-metals

We use many types of matter in daily life. Matter is classified into pure and impure substances. Elements and compounds are pure substances, while mixtures are impure. About 118 elements have been discovered so far. On the basis of their properties, these elements are classified as metals, non-metals, and metalloids. Most elements are metals, some are non-metals, and only a few are metalloids.

SEE Focus: In this chapter, we study where and how metals are found in nature, and how pure metals are extracted from their ores (metallurgy).

Almost all metals (except a few) are found in the form of compounds in the earth's crust. The elements and compounds that occur naturally are called minerals. Minerals are pure, solid, inorganic, and crystalline substances found in nature, and they have a fixed chemical formula.

Picture 17.2: Potassium, sodium, and calcium rock samples

The ores obtained from mines contain a lot of impurities. The mineral from which a metal can be extracted in large amounts at low cost is called the ore of that metal. Minerals from which a metal cannot be extracted easily and economically are not ores.

Very Important for SEE: All ores are minerals, but all minerals are not ores.

Picture 17.3: Ores of Iron

Among these, hematite is the chief ore of iron. It contains the maximum amount of iron (about 75%).

Picture 17.4: An ore of aluminium (Bauxite)

Among these, Bauxite is the chief ore of aluminium, which contains about 40–60% aluminium.

Picture 17.5: Ores of copper

Copper pyrite (chalcopyrite) is the main ore of copper, containing about 34.5% copper.

Picture 17.6: Argentite

Among these, the chief ore of silver is argentite, which contains about 87% silver.

Gold is a non-reactive metal, so it is found in a pure (free) state in nature — mainly in rocks and in alluvial soil formed from rocks.

Metallurgy is the science that deals with the properties, production, and purification of metals. It includes the stepwise process of extracting metals from their ores.

Mining is the process of extracting useful substances and minerals from the earth's geological surface by digging. It is done to obtain coal, petroleum, gold, and ores of different metals.

Grinding is the first step of metal extraction. It is the process of crushing the ores into small particles using rollers of machines.

The crushed ore contains impurities like mud, sand, and rocks, called gangue. Concentration is the process of removing these impurities from the ore, thereby increasing the percentage of metal in it.

It is easier to obtain metals from their oxides, so the concentrated ore is converted into a metal oxide. There are two methods of oxidation.

Roasting is the process of strongly heating the ore to convert it into its oxide by passing air or in the presence of oxygen. It is mainly used to convert sulphide ores into oxide ores. Example: Zinc sulphide (ZnS) is converted to zinc oxide (ZnO) by roasting.

Calcination is the process of strongly heating the ore to convert it into its oxide in the absence of air (without passing oxygen). It is mainly used to convert carbonate ores into oxides. Example: Calcium carbonate is converted into calcium oxide by calcination.

Reduction is the process of removing oxygen from a metal oxide. Metal oxides such as copper oxide, lead oxide, and iron oxide are treated with reducing agents like carbon, carbon monoxide, or hydrogen, which remove oxygen from them.

After oxidation, metal oxides are reacted with reducing agents like carbon, coke, or hydrogen above the melting point of the metal. Metals separate out in molten form while remaining impurities separate as gases or slag. This process is called smelting.

Metals obtained after reduction may still contain some impurities. Refining is done to obtain metals in a pure state. It can be done through distillation, electro-refining, and other methods.

In this method, impurities present in a metal are removed by boiling. It is used when the metal (or impurity) changes into vapour on heating. It is used to purify metals with a low boiling point, such as mercury.

Picture 17.7: Electro-refining setup

Electro-refining uses the electrolysis method to refine metals obtained from reduction. It is used to purify metals like iron, silver, copper, and gold, giving about 99% pure metal. In a voltameter, the impure metal is connected to the anode (positive terminal), and a strip of pure metal is connected to the cathode (negative terminal). A salt of the metal to be refined is used as the electrolyte.

When electricity is passed, the impure metal at the anode dissolves/erodes and pure metal gets deposited on the cathode. Insoluble impurities settle at the bottom of the anode as 'anode mud'.