CBSE Class 10 Science: Occurrence of Metals

Let's explore the fascinating world of metals and how they occur in nature!

1. How Do Metals Occur in Nature?

Metals are rarely found in a pure form (also called the native state) in nature.
Most metals are chemically reactive. They easily combine with other elements such as oxygen, sulphur, carbon, or chlorine.
This combination forms compounds which are present in the Earth's crust.

Key points to remember:

Examples:

Gold is often found as shiny yellow lumps (nuggets) in river beds.
Iron is mostly found as iron oxide, not as metallic iron.
Aluminium is always found as a compound, like bauxite (Al₂O₃·2H₂O), never free.

Minerals: Naturally occurring substances which contain metals or their compounds, blended with earthy impurities like sand or soil.

Example: Clay is a mineral that contains aluminium compound.
Ores: Minerals from which metals can be extracted easily and profitably.

Important: Every ore is a mineral, but every mineral is not an ore.

Example: Bauxite (Al₂O₃·2H₂O) is the main ore of aluminium.
Gangue: The unwanted impurities (like sand or soil) found mixed with the ores.

Example: When mining iron ore, mud and sand present are the gangue.

Most metals are reactive. They occur as compounds combined with other elements.

These combinations include oxides, sulphides, carbonates, sulphates, chlorides, silicates, etc.

Examples:

Table of Common Ores:

Metal	Common Ore(s)	Formula	Nature of Compound
Iron	Hematite, Magnetite	Fe₂O₃, Fe₃O₄	Oxide
Copper	Copper glance, Malachite	Cu₂S, CuCO₃·Cu(OH)₂	Sulphide, Carbonate
Aluminium	Bauxite	Al₂O₃·2H₂O	Oxide
Zinc	Zinc blende, Calamine	ZnS, ZnCO₃	Sulphide, Carbonate
Lead	Galena	PbS	Sulphide
Mercury	Cinnabar	HgS	Sulphide
Silver	Argentite	Ag₂S	Sulphide

Mining: Ores are dug out (extracted) from the Earth, often by big machines or explosives.
Processing: The ore is purified (dressing or concentration) to remove gangue (earthy impurities).

Step-by-step Processing (Example):

Ore is crushed and ground into powder.
Concentration methods are used to remove gangue (e.g., washing, froth flotation, magnetic separation).
Metallurgical processes are then used to extract pure metal.
- Roasting: Heating a sulphide ore in presence of air to convert it to oxide (e.g., ZnS is roasted to ZnO).
- Calcination: Heating a carbonate ore strongly in absence of air to convert it into oxide (e.g., CaCO₃ to CaO).
- Reduction: Converting the oxide into free metal (e.g., Fe₂O₃ + 3CO → 2Fe + 3CO₂).
- Electrolysis: Passing electricity through a molten ore to obtain metal (e.g., extraction of Aluminium from bauxite).

Examples:

Reactivity matters!
Least reactive metals (like gold, silver, and platinum) do not react with air, water, or acids easily, so they occur in the native state.
More reactive metals (like iron, aluminium, sodium) react quickly and are always found combined with other elements.

Examples:

Collect samples of gold, silver, iron ore (like hematite), and copper ore (like malachite).
Observe appearance – metallic luster/shine is seen in native metals.
Try to dissolve each sample in dilute hydrochloric acid.
Native metals (like gold, silver) will not react with acid.
Combined state samples (iron ore, malachite) will react, showing bubbles or color change.

Gold and silver samples stay exactly the same – no bubbles or change.
Hematite and malachite start fizzing – gas releases or solution turns colored.

Scenario: You want to explain why gold is found in riverbeds in a pure form.

Question: What property of gold allows this?
Answer: Gold is very unreactive (least reactive in the reactivity series), so it does not combine with other elements and exists as pure metal in nature.

Scenario: Your friend finds a shiny mineral and thinks it is pure aluminium.

Question: What would you tell them?
Answer: Aluminium is highly reactive and never found freely in nature. That shiny mineral is likely an aluminium compound, not pure aluminium.

Question: Which form of copper should they look for and why?
Answer: They should look for copper ores like copper glance (Cu₂S) or malachite (CuCO₃·Cu(OH)₂) because it's profitable and practical to extract metal from these ores.

Question: What does this tell about iron's occurrence in nature?
Answer: Iron is reactive and combines with oxygen and water, forming rust. This is why iron is usually found in nature as compounds (like iron oxide), not freely.

Question: Is sodium found as metal in this mine?
Answer: No, sodium is highly reactive and always exists as a compound, such as sodium chloride (table salt), in nature.

Fun learning tip: Next time you look at jewelry or coins, think: which metals are native and which had to be extracted from ores?