4.1 Varieties of Minerals
Minerals are naturally occurring solid elements or compounds found in the Earth's crust with definite crystalline structures and chemical compositions.
Types of Minerals
- Elemental Minerals: Unreactive minerals found in their free native elemental state in nature. Examples: Gold, Silver, Platinum, Diamond.
- Compound Minerals: Reactive elements chemically combined with other elements (such as oxygen, sulfur, or carbon). Examples: Hematite ($Fe_2O_3$), Galena ($PbS$), Bauxite ($Al_2O_3$), Calcite ($CaCO_3$).
Common Natural Metal Compounds
| Mineral Name | Chemical Name | Combination Elements |
|---|---|---|
|
Bauxite | Aluminium oxide | Aluminium + Oxygen |
|
Hematite | Iron(III) oxide | Iron + Oxygen |
|
Galena | Lead(II) sulfide | Lead + Sulfur |
|
Cinnabar | Mercury(II) sulfide | Mercury + Sulfur |
|
Cassiterite | Tin(IV) oxide | Tin + Oxygen |
|
Calcite / Limestone | Calcium carbonate | Calcium + Carbon + Oxygen |
|
Malachite | Copper(II) carbonate | Copper + Carbon + Oxygen |
Calcium Carbonate ($CaCO_3$) Properties
Natural forms include marble, limestone, chalk, seashells, and coral reefs.
- Effect of Heat: Decomposes when heated strongly to produce calcium oxide (quicklime) and carbon dioxide gas:
$$\text{Calcium carbonate } (CaCO_3) \xrightarrow{\text{heat}} \text{Calcium oxide } (CaO) + \text{Carbon dioxide } (CO_2)$$
- Reaction with Acids: Reacts with dilute acids to produce a salt, water, and carbon dioxide gas:
$$\text{Calcium carbonate} + \text{Dilute hydrochloric acid} \rightarrow \text{Calcium chloride} + \text{Water} + \text{Carbon dioxide}$$
- Test for Carbon Dioxide Gas: Bubbled into limewater ($Ca(OH)_2$); turns the clear limewater cloudy / chalky due to insoluble calcium carbonate precipitate.
4.2 Reactivity Series of Metals
The Reactivity Series of Metals is an arrangement of metals according to their reactivity with oxygen, from the most reactive to the least reactive.
Reaction of Metals with Oxygen
Metal + Oxygen → Metal Oxide
- Potassium (K) & Sodium (Na): Burn rapidly with a bright brilliant flame.
- Magnesium (Mg) & Aluminium (Al): Burn rapidly with a bright white flame.
- Zinc (Zn) & Iron (Fe): Burn moderately with glowing sparks.
- Lead (Pb) & Copper (Cu): Glow dimly without a flame.
- Gold (Au) & Platinum (Pt): Do not react with oxygen even when heated strongly.
Position of Non-Metals (Carbon and Hydrogen) in the Reactivity Series
Although Carbon ($C$) and Hydrogen ($H_2$) are non-metals, their relative positions in the series are crucial for metal extraction processes.
Position of Carbon: Determined by heating metal oxides with carbon powder.
- If Carbon can remove oxygen from a metal oxide (reduces it), Carbon is more reactive than that metal.
$$\text{Carbon} + \text{Metal Oxide} \rightarrow \text{Metal} + \text{Carbon Dioxide}$$
- Result: Carbon lies between Aluminium and Zinc.
Position of Hydrogen: Determined by passing dry hydrogen gas over heated metal oxides.
- If Hydrogen reduces the metal oxide to pure metal, Hydrogen is more reactive than that metal.
$$\text{Hydrogen} + \text{Metal Oxide} \rightarrow \text{Metal} + \text{Water}$$
- Result: Hydrogen lies between Zinc and Iron.
Summary of the Reactivity Series of Metals
K > Na > Ca > Mg > Al > [Carbon] > Zn > [Hydrogen] > Fe > Sn > Pb > Cu > Hg > Ag > Au
(Mnemonic aid: "Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold")
4.3 Mining and Extraction of Metals
Extraction of metal is the industrial process of obtaining pure metals from their natural ores.
Methods of Extraction Based on Position in Reactivity Series
| Position in Series | Metals | Method of Extraction | Reason |
|---|---|---|---|
|
High Reactivity | Potassium, Sodium, Calcium, Magnesium, Aluminium |
Electrolysis of molten metal ores | Metals form extremely stable compounds that cannot be reduced by carbon. |
|
Medium Reactivity | Zinc, Iron, Tin, Lead |
Reduction by Carbon (Smelting in Blast Furnace) | Carbon is more reactive than these metals and can displace oxygen from their oxides. |
|
Low Reactivity | Copper, Mercury |
Direct heating of sulfide ores in air | Metal compounds are unstable when heated. |
|
Very Low / Native | Silver, Gold |
Physical mining (panning / physical separation) | Exist naturally in elemental uncombined state. |
Extraction of Iron in a Blast Furnace
Iron is extracted from hematite ($Fe_2O_3$) using a blast furnace:
- Raw Materials: Iron ore (Hematite), Coke (Carbon), and Limestone ($CaCO_3$).
- Production of Carbon Monoxide:
$$C + O_2 \rightarrow CO_2$$
$$CO_2 + C \rightarrow 2CO \quad (\text{Carbon Monoxide is the main reducing agent})$$
- Reduction of Iron Oxide:
$$Fe_2O_3 + 3CO \rightarrow 2Fe + 3CO_2$$
$$Fe_2O_3 + 3C \rightarrow 2Fe + 3CO$$
Molten iron sinks to the bottom of the blast furnace.
- Removal of Impurities (Slag formation):
Limestone decomposes: $CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2$
Quicklime reacts with silica/sand impurity ($SiO_2$):
$$CaO + SiO_2 \rightarrow CaSiO_3 \quad (\text{Molten Slag})$$
Slag floats on top of molten iron, protecting it from oxidation, and is tapped off separately.
Environmental Issues in Mining Activities
- Soil Erosion & Deforestation: Destruction of habitats and biodiversity loss.
- Air Pollution: Acid rain caused by gases ($SO_2$, $NO_2$) released from sulfide ore roasting, and dust particulates.
- Water Pollution: Toxic heavy metal leaching into water supplies and river siltation.
- Noise & Ground Vibration: Blasting operations disrupt local ecosystems and nearby human settlements.