Metals and Non-metals: Occurrence of Metals – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Why do most metals occur in the combined (compound) state and not in the native (free) state in nature? Give examples.
Answer:
Most metals are chemically reactive; they do not remain unchanged when exposed to air, water, or other substances in the Earth’s crust.
As a result, they react with other elements like oxygen, sulphur, carbon, and chlorine, forming compounds.
These compounds are more stable than the metals themselves, which is why we mostly find metallic oxides, sulphides, carbonates, and chlorides in nature.
For instance, iron is found as hematite (Fe₂O₃) or magnetite (Fe₃O₄), and aluminium as bauxite (Al₂O₃·2H₂O) instead of the pure metals.
Only least reactive metals like gold, platinum, and silver occur in a free state because they do not combine easily.
Thus, reactivity decides whether a metal is found native or combined.
Q2. Differentiate between minerals, ores, and gangue. Why is every ore a mineral but not every mineral an ore?
Answer:
Minerals are naturally occurring substances in the earth’s crust that contain metals or their compounds, mixed with other impurities.
Ores are those minerals from which metals can be extracted easily and profitably.
Gangue refers to the unwanted earthy impurities like sand, clay, etc., present in the ore.
Every ore is a mineral because it is naturally formed and contains metal, but not all minerals are ores as some do not have enough metal or are too costly to extract.
For example, bauxite is an ore as well as a mineral of aluminium, but cryolite (another mineral of aluminium) is not used as ore because extraction isn’t profitable.
This economic difference is why the terms are not interchangeable.
Q3. Why are gold, silver, and platinum often found in the native state, while metals like sodium, magnesium, and aluminium are never found free in nature?
Answer:
Gold, silver, and platinum are at the bottom of the reactivity series, meaning they are least reactive.
They do not react easily with air, water, acids, or other elements.
Therefore, they can exist in the native (free/uncombined) state as metals in nature.
In contrast, sodium, magnesium, and aluminium are highly reactive metals.
They quickly react with oxygen, water, and other substances, forming stable compounds like oxides, chlorides, or carbonates.
Thus, you never find these reactive metals as pure elements in nature; only as compounds.
Q4. Explain, with the help of examples, how different metals are found in different compound forms in nature.
Answer:
Metals combine with various elements, creating different types of compounds in the earth’s crust.
For example, iron is mostly present as oxides (hematite, Fe₂O₃; magnetite, Fe₃O₄) and also as carbonates and sulphides.
Aluminium is mainly found as bauxite (Al₂O₃·2H₂O, oxide) and cryolite (Na₃AlF₆, fluoride).
Zinc exists as sulphide (ZnS, zinc blende) and carbonate (ZnCO₃, calamine).
Copper is found as sulphide (Cu₂S, copper glance), oxide (Cu₂O, cuprite), and carbonate (CuCO₃·Cu(OH)₂, malachite).
Mercury is found as sulphide (HgS, cinnabar).
Thus, metals occur in different chemical forms depending on their reactivity and local environmental conditions.
Q5. Describe the main steps involved in the extraction of metals from their ores.
Answer:
The main steps are:
Mining: Extracting the ore from the earth’s crust.
Crushing and grinding: Breaking ore into smaller pieces for easier processing.
Concentration or dressing: Removing gangue (earthy impurities) using methods like froth flotation, magnetic separation, etc.
Conversion to oxide: For many ores, the next step is converting the ore into its oxide form by roasting (sulphide ores) or calcination (carbonate ores).
Reduction: Oxide ore is reduced to get the pure metal, either by chemical methods (using carbon, aluminum, etc.) or by electrolysis.
Purification: The crude metal obtained is purified to remove any remaining impurities.
Each step ensures the metal is separated from unwanted materials and becomes pure for use.
High Complexity (Analysis & Scenario-Based)
Q6. Iron and aluminium are both abundant in the earth’s crust but are extracted from different ores using different processes. Analyse why the extraction of aluminium is more expensive than iron, despite its abundance.
Answer:
Iron is extracted from its ores (like hematite) mostly by reduction with carbon in a blast furnace, which is comparatively simple and cost-effective.
Aluminium extraction from bauxite involves electrolysis of molten aluminium oxide (Hall–Heroult process).
The process needs high temperature and a lot of electricity, making it energy-intensive and expensive.
Also, aluminium oxide needs to be dissolved in molten cryolite to lower its melting point, adding further cost.
Iron extraction doesn’t need such high energy input, so the process is cheaper.
Thus, method of extraction and energy requirement make aluminium more expensive to obtain, despite it being abundant.
Q7. A newly found mineral contains 5% sodium and 25% gangue. Discuss whether this can be considered an ore of sodium and why pure sodium is not extracted directly from this mineral.
Answer:
For a mineral to be called an ore, it must allow easy and economical extraction of its metal.
This mineral contains only 5% sodium, which is a low percentage; hence, extraction would not be profitable or efficient.
The gangue content is high (25%), making purification difficult and costly.
Also, sodium is highly reactive and never found free; its extraction needs costly electrolysis of fused sodium chloride.
So, unless better minerals of sodium exist, this mineral would not be considered an ore.
Pure sodium is not extracted directly because of its high reactivity and is only obtained after expensive processing of concentrated, suitable ores like rock salt (NaCl).
Q8. Predict and justify the occurrence state (native or combined) of copper in two different mining regions: one with high sulphur content and another with rich river beds.
Answer:
In a region with high sulphur content, copper is likely found in the combined state, mainly as copper sulphide ores (like copper glance, Cu₂S).
Sulphur combines with copper due to its moderate reactivity, leading to stable copper sulphide compounds.
In rich river beds, copper may be found in the native (free) state as metals or nuggets.
This occurs because water action and low reactivity allow copper to remain uncombined or get separated from compounds.
Thus, environmental conditions and reactivity of copper result in its different forms in various regions.
Where reactive elements (like sulphur) are available, copper occurs as compounds; in pure, stable locations (like river beds), it is native.
Q9. Suppose scientists discover a new metal X that is always found in native state and does not react with strong acids or oxygen. What can you say about the possible position of metal X in the activity series and its commercial use?
Answer:
Since metal X is always found in native state and doesn’t react with strong acids or oxygen, it must be very low in the activity series.
Like gold, platinum, or silver, it is least reactive.
Such metals are often precious due to their stability and rarity.
Its lack of chemical reactivity means it will not corrode, rust, or tarnish easily.
This makes it suitable for jewellery, coins, decorative items, and electrical components (because it won’t degrade).
Like other noble metals, if rare, it will be commercially valuable.
Q10. Compare and contrast roasting and calcination as ore processing techniques. Discuss a scenario where choosing the wrong process could result in failure to obtain the metal.
Answer:
Roasting involves heating sulphide ores in the presence of air to convert them into oxides (and release SO₂).
Calcination means heating carbonate ores in the absence or limited supply of air to convert them to oxides (and release CO₂).
These steps are critical before reduction during extraction.
If roasting is used for a carbonate ore, it will not decompose as needed; it requires calcination to release CO₂ and make suitable oxide.
Similarly, if calcination is used for sulphide ores, sulphur will not be removed as SO₂, leading to inefficient extraction.
For example: Using roasting on calamine (ZnCO₃) will not yield zinc oxide efficiently; using calcination on zinc blende (ZnS) will also fail.
Thus, knowledge of ore type is vital; the wrong process can cause extraction failure and economic loss.
