Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
9 questions
Unit 17 · Metals and Non-metals
2. Give reasons
All ores are minerals, but all minerals are not ores.
Answer
Every ore is a naturally occurring compound or element in the Earth’s crust, so every ore is a mineral. But an ore is a mineral from which a metal can be extracted easily and profitably. Many minerals contain a metal in such small amounts, or so tightly bound, that extracting it costs too much; these are minerals but not ores. For example, clay contains aluminium but is not its ore; bauxite is.
Ores are oxidized after concentration.
Answer
A metal is most easily obtained from its oxide, by reducing the oxide with carbon, carbon monoxide or hydrogen. So sulphide and carbonate ores are first changed into oxides (by roasting or calcination). This is done after concentration so that only the ore, not the useless gangue, has to be heated, which saves fuel; the oxidation also burns off or drives out impurities such as sulphur, carbon dioxide and water.
3. Write the differences between
Mineral and ore
Answer
| Mineral | Ore |
|---|---|
| a natural, inorganic, crystalline element or compound in the Earth’s crust | a mineral from which a metal can be extracted easily and profitably |
| may contain little metal | contains a high percentage of metal |
| extracting metal from it may not be economical | extracting metal from it is economical |
| all minerals are not ores | all ores are minerals |
| e.g. clay (for aluminium) | e.g. bauxite (for aluminium) |
Roasting and calcination
Answer
| Roasting | Calcination |
|---|---|
| the ore is heated strongly in a supply of air | the ore is heated strongly without air (or with very little) |
| used for sulphide ores | used for carbonate (and hydrated) ores |
| sulphur dioxide is given off | carbon dioxide (and water vapor) is given off |
| e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂ | e.g. ZnCO₃ → ZnO + CO₂ |
Oxidation and reduction
Answer
| Oxidation | Reduction |
|---|---|
| addition of oxygen (or removal of hydrogen) | removal of oxygen (or addition of hydrogen) |
| in metallurgy, it changes the ore into an oxide | in metallurgy, it changes the oxide into the metal |
| done by roasting or calcination | done with a reducing agent (C, CO, H₂) or by electrolysis |
| e.g. 2ZnS + 3O₂ → 2ZnO + 2SO₂ | e.g. ZnO + C → Zn + CO |
4. Answer these questions
What are the sources of metals?
Answer
Metals come from the Earth’s crust, in the form of minerals. Unreactive metals such as gold and platinum (and sometimes silver and copper) are found in the free, native state; reactive metals are found as compounds (oxides, sulphides, carbonates, chlorides and others). The minerals from which metals can be extracted profitably are their ores. Sea water is a source of some metals too (magnesium is extracted from it), and recycled scrap is an ever more important source.
Make a list of the ores of iron, copper, aluminium and silver.
Answer
- Iron: hematite (Fe₂O₃), magnetite (Fe₃O₄), siderite (FeCO₃), limonite (2Fe₂O₃·3H₂O), iron pyrite (FeS₂).
- Copper: chalcocite or copper glance (Cu₂S), chalcopyrite or copper pyrite (CuFeS₂), cuprite (Cu₂O), malachite (Cu₂(OH)₂CO₃).
- Aluminium: bauxite (Al₂O₃·2H₂O), cryolite (Na₃AlF₆), corundum (Al₂O₃).
- Silver: argentite (Ag₂S), horn silver (AgCl), ruby silver (Ag₃SbS₃), silver copper glance ((Ag,Cu)₂S).
How is metallurgy a stepwise process? Explain.
Answer
A metal can’t be taken out of its ore in one go; each step prepares the ore for the next:
- Grinding: the ore is crushed into powder, so its surface area is large.
- Concentration: the gangue (sand, mud, rock) is removed by gravity separation, magnetic separation or froth flotation, so the percentage of metal rises.
- Oxidation: the concentrated ore is changed into an oxide, by roasting (sulphide ores, in air) or calcination (carbonate ores, without air).
- Reduction: oxygen is removed from the oxide by heating it with a reducing agent such as carbon, carbon monoxide or hydrogen (often as smelting), or, for reactive metals, by electrolysis.
- Refining: the impure metal is purified by distillation or electro-refining.
Example: zinc blende (ZnS) → froth flotation → roasting to ZnO → reduction with carbon to zinc → distillation to pure zinc.
How is pure metal obtained by electro-refining? Explain with an activity.
Answer
In electro-refining, a block of impure metal is made the anode and a thin sheet of pure metal the cathode, in a solution of a salt of the same metal, and an electric current is passed.
Activity (copper): fill a voltameter with copper sulphate solution. Connect impure copper to the positive terminal of a battery (anode) and a thin pure copper plate to the negative terminal (cathode), and pass a current.
At the anode, copper atoms lose electrons and dissolve as ions: Cu → Cu²⁺ + 2e⁻. At the cathode, copper ions gain electrons and are deposited as pure copper: Cu²⁺ + 2e⁻ → Cu. The anode wears away and the cathode grows thicker with pure copper, while the impurities fall below the anode as anode mud or stay in the solution. The copper obtained is about 99.99% pure.