CBSE Class 10 Science – Occurrence of Metals: Long Answer Questions
Medium Level (Application & Explanation)
Q1. Differentiate between minerals, ores, and gangue with suitable examples. Explain the statement “Every ore is a mineral, but every mineral is not an ore.”
Answer:
A mineral is a naturally occurring substance containing a metal or its compounds mixed with earthy materials. For example, clay is a mineral that contains compounds of aluminium.
An ore is a mineral from which a metal can be extracted easily and profitably. For instance, bauxite (Al₂O₃·2H₂O) is the chief ore of aluminium.
Gangue refers to the unwanted impurities like sand, soil, and mud that are found mixed with ores during mining; for example, mud and sand mixed with iron ore.
The statement “every ore is a mineral, but every mineral is not an ore” means that while all ores are minerals, only those minerals that allow economical extraction of metal are called ores. Thus, although many minerals contain metals, only those with high metal content and low extraction cost qualify as ores.
Q2. Explain with examples how metals occur in the native and combined states. Relate this to their position in the reactivity series.
Answer:
Metals occur in two main forms: native (free) state and combined state.
Least reactive metals do not react easily with air, water, or acids, so they are found in the native state. Examples include gold (Au) as shiny nuggets in river sands, platinum (Pt), and sometimes copper (Cu) as metallic red lumps.
Reactive metals combine with other elements and occur in nature as compounds such as oxides, sulphides, carbonates, chlorides, and sulphates. For example, iron occurs as hematite (Fe₂O₃) and magnetite (Fe₃O₄); zinc occurs as zinc blende (ZnS) and calamine (ZnCO₃); lead as galena (PbS).
Thus, a metal’s reactivity decides its occurrence: least reactive (like Au, Pt) appear native, while more reactive metals (like Fe, Al, Na) occur in the combined state.
Q3. Describe the general steps from mining to the extraction of a pure metal. Illustrate your answer with examples from zinc, iron, and aluminium.
Answer:
The process begins with mining, where ores are dug out using machines or explosives.
The ore is then crushed and ground to a fine powder to expose more surface area.
Concentration (dressing) removes gangue using methods such as:
Washing (hydraulic washing) to remove lighter impurities,
Froth flotation for sulphide ores like zinc blende (ZnS),
Magnetic separation for magnetic ores.
The concentrated ore is then converted to oxide:
Roasting: sulphide ores (e.g., ZnS → ZnO in air),
Calcination: carbonate ores (e.g., ZnCO₃ → ZnO on heating without air).
Reduction converts metal oxides to free metal (e.g., Fe₂O₃ + 3CO → 2Fe + 3CO₂).
Some metals like aluminium are extracted by electrolysis of molten ores (e.g., from bauxite, Al₂O₃·2H₂O), as chemical reduction is not feasible.
Q4. List the common chemical forms in which metals occur in nature. Give suitable examples of oxides, sulphides, and carbonates from the ores of iron, zinc, copper, lead, mercury, and silver.
Answer:
Metals in the combined state occur mainly as oxides, sulphides, carbonates, sulphates, chlorides, and silicates. Important examples include:
Oxides:
Iron as hematite (Fe₂O₃) and magnetite (Fe₃O₄),
Aluminium as bauxite (Al₂O₃·2H₂O).
Sulphides:
Zinc as zinc blende (ZnS),
Copper as copper glance (Cu₂S),
Lead as galena (PbS),
Mercury as cinnabar (HgS),
Silver as argentite (Ag₂S).
Carbonates:
Zinc as calamine (ZnCO₃),
Iron as siderite (FeCO₃),
Copper as malachite (CuCO₃·Cu(OH)₂).
These forms reflect the reactivity of metals and their tendency to bond with oxygen, sulphur, or carbon dioxide in nature.
Q5. Contrast roasting and calcination. When is each process used? Support your explanation with chemical examples.
Answer:
Roasting is the process of heating a sulphide ore in the presence of air to convert it into an oxide and remove volatile impurities as gases. For example, zinc blende (ZnS) is roasted to form zinc oxide (ZnO). This step makes the ore suitable for reduction to the metal.
Calcination is the process of heating a carbonate ore strongly in the absence or limited supply of air to form the oxide, releasing carbon dioxide. For example, calamine (ZnCO₃) on calcination gives ZnO, and limestone (CaCO₃) gives CaO.
Thus, use roasting for sulphide ores and calcination for carbonate ores. Both processes aim to produce the oxide form, which is easier to reduce to the metal in the next metallurgical step.
High Complexity (Analytical & Scenario-Based)
Q6. A sample from a mine contains ZnS, Fe₂O₃, and sand. Design a logical sequence to concentrate and extract zinc efficiently from this mixture.
Answer:
First, remove gangue (sand) by washing; lighter particles are washed away due to difference in density.
The remaining ore mixture contains zinc blende (ZnS) and hematite (Fe₂O₃). Since ZnS is a sulphide ore, use froth flotation to preferentially concentrate the sulphide; Fe₂O₃ will sink as tailings.
Subject the concentrated ZnS to roasting in air: 2ZnS + 3O₂ → 2ZnO + 2SO₂ (conceptually converting sulphide to zinc oxide).
Convert ZnO to zinc metal by reduction in the subsequent step, as per standard metallurgical practice.
Iron oxide (Fe₂O₃) may be processed separately by reduction to obtain iron, but it is not needed for zinc extraction. This sequence uses the nature of the ore to choose the right concentration and conversion methods.
Q7. Aluminium is highly abundant but never found in the native state. Analyse the reasons and explain why electrolysis is used for its extraction.
Answer:
Aluminium (Al) is a highly reactive metal. It readily combines with oxygen to form stable oxides, mainly present as bauxite (Al₂O₃·2H₂O). Because of this high reactivity, Al is never found free in nature.
The oxide of aluminium is very stable, making chemical reduction by carbon or carbon monoxide ineffective under ordinary conditions.
Therefore, the extraction of aluminium uses electrolysis, where electricity is passed through the molten oxide to break it down into aluminium metal and oxygen.
This method overcomes the strong Al–O bond by supplying electrical energy directly.
In short, aluminium’s reactivity determines both its combined occurrence and the choice of extraction method, which is electrolytic rather than simple chemical reduction.
Q8. You are given three unknown samples: a golden-yellow nugget, a black powdery ore, and a greenish rock. Propose tests and reasoning to identify which are native metals and which are combined forms.
Answer:
Observe appearance: the golden-yellow nugget with metallic lustre likely suggests a native metal such as gold (Au). The black powder and greenish rock suggest combined forms (ores).
Perform a gentle dilute HCl test:
Native metals like gold and silver will show no reaction (no bubbles, no change).
Combined forms may fizz or cause a colour change due to reactions of carbonates or hydroxides.
The greenish rock could be malachite (CuCO₃·Cu(OH)₂), a copper carbonate hydroxide, likely to release CO₂ (effervescence) in acid and impart a bluish-green solution tint.
The black powder could be a sulphide (e.g., Cu₂S, ZnS) or oxide (e.g., Fe₂O₃). Limited reaction with HCl suggests oxide; further tests would involve roasting behaviour.
Q9. A company has two aluminium-bearing minerals: clay and bauxite. Only one can be processed this year. Apply the concept of ore vs mineral to justify which should be chosen and why.
Answer:
Both clay and bauxite (Al₂O₃·2H₂O) are minerals containing aluminium compounds. However, only bauxite is the chief ore of aluminium because metal extraction from it is easier and economical.
Clay, although abundant, typically contains aluminium in forms that are hard to process and not profitable with standard metallurgical routes.
The rule “every ore is a mineral, but every mineral is not an ore” guides the decision: choose the mineral that allows efficient concentration and feasible extraction.
Since bauxite is directly suitable for conversion to aluminium oxide and then to aluminium via electrolysis, it ensures better yield, lower cost, and proven technology.
Therefore, the company should process bauxite and defer clay, aligning with economic viability and technical ease.
Q10. Given an ore sample that may be zinc blende (ZnS) or calamine (ZnCO₃), design an identification and extraction plan using suitable metallurgical steps.
Answer:
First, identify the ore using acid reaction:
If the sample is ZnCO₃ (calamine), it will effervesce with dilute acid due to CO₂ release.
ZnS (zinc blende) typically shows no effervescence with dilute acid.
Based on identification, select the conversion step:
For ZnS, use roasting in air to form ZnO.
For ZnCO₃, use calcination (strong heating without air) to form ZnO.
After conversion to ZnO in both cases, proceed with reduction to obtain zinc metal.
