Planet Earth: Rocks and Minerals
FoundationThe crust is dominated by oxygen, silicon, aluminium and iron. Rocks are made of minerals, and metals are extracted from ores. Limestone, chalk and marble are all calcium carbonate, which decomposes on heating into calcium oxide and carbon dioxide.
- Crustal elements (by mass): oxygen 46%, silicon 28%, aluminium 8%, iron 5%
- Rocks come as igneous, sedimentary and metamorphic, made of minerals; rocks with metal compounds are ores
- Three extraction routes: physical separation, heating alone (silver from silver oxide), heating with carbon (iron)
文體小知識：科學知識說明文——說明地殼的元素組成、金屬的提取與碳酸鈣的變化，學習用文字反應式描述化學變化。
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1The crust is the solid outer layer of the Earth, built from elements: by mass, oxygen is about 46%, silicon 28%, aluminium 8% and iron 5%, with calcium, sodium, potassium and others making up the rest. Oxygen and silicon combine into silicon dioxide and many silicates, forming sand, clay and most minerals — oxygen and silicon are the leading actors of crustal chemistry.
2Rocks are made of one or more minerals and fall into three families by origin: igneous, sedimentary and metamorphic. A mineral is a naturally occurring solid of fairly fixed composition; rocks containing metal compounds are metal ores, such as haematite with its iron oxide. Metals can be extracted from ores by physical separation, by heating alone, or by heating with carbon; the more reactive the metal, the harder the extraction, and the most reactive await electrolysis.
3In nature calcium carbonate takes three common forms — limestone, chalk and marble — chemically identical but different in appearance. When limestone is roasted strongly in a lime kiln, calcium carbonate decomposes into calcium oxide and carbon dioxide; this is the thermal decomposition of calcium carbonate, neatly expressed with a word equation. The calcium oxide, called quicklime, turns into slaked lime with water and is widely used in mortar and for neutralising acidic soil.
4Heat, water and acid all act on calcium carbonate, causing erosion: rainwater dissolves carbon dioxide to become a weak acid, which over the years dissolves limestone, forming caves and stalactites. To test a carbonate, add dilute acid — the gas given off turns limewater milky, proving it is carbon dioxide; calcium gives a brick-red flame test. Limestone is also the raw material for cement and glass, with a very wide range of uses.
1Summary 重點摘要
The crust is dominated by oxygen, silicon, aluminium and iron. Rocks are made of minerals, and metals are extracted from ores. Limestone, chalk and marble are all calcium carbonate, which decomposes on heating into calcium oxide and carbon dioxide.
2Vocabulary cards Vocabulary
The solid outermost layer of the Earth, made of oxygen, silicon, aluminium, iron and other elements.
A naturally occurring solid of fairly definite composition — the building block of rocks.
Rock containing minerals from which metals can be extracted profitably, such as haematite.
Formula CaCO₃ — the main substance of limestone, chalk and marble.
A common natural form of calcium carbonate and the raw material for cement and glass.
A reaction in which heat breaks a substance into two or more new substances, such as the thermal decomposition of calcium carbonate.
An equation written in words, e.g. calcium carbonate → calcium oxide + carbon dioxide.
Calcium hydroxide solution; it turns milky with carbon dioxide and is used to test for CO₂.
The gradual wearing away and changing of rock by heat, water and acid.
Obtaining a metal from its ore by physical means, by heating alone or by heating with carbon.
3Step by step Steps
- Step 1: What the crust is made ofBy mass, the crust is about 46% oxygen, 28% silicon, 8% aluminium and 5% iron, followed by calcium, sodium, potassium and magnesium. For example, sand is mostly silicon dioxide (SiO₂) — one silicon atom with two oxygen atoms — which explains why oxygen and silicon top the list. Remember the chant 'oxygen, silicon, aluminium, iron, calcium, sodium, potassium'; when answering, list the percentages in order and note that they are by mass. The total need not be forced to 100%.
- Step 2: Rocks, minerals and metal oresRocks are made of minerals and fall into igneous, sedimentary and metamorphic families by origin. A mineral is a naturally occurring solid of fairly fixed composition, and rocks bearing metal compounds are metal ores. For example, haematite is mainly iron oxide and feeds the iron industry, while galena holds lead sulfide from which lead is extracted. Colour, hardness and crystal shape help identify minerals. The Earth's resources are finite: mining must weigh cost and environmental impact — a value the curriculum stresses.
- Step 3: Three ways to extract a metalThe method depends on how reactive the metal is: an unreactive metal such as gold can be separated physically; a less reactive one is obtained by heating its ore alone; a more reactive one is extracted by heating the ore with carbon, which takes away the oxygen. For example, heating silver oxide alone yields silver; heating iron oxide with coke yields iron — the principle of the blast furnace. The most reactive metals, like sodium and aluminium, must wait for electrolysis, learnt later.
- Step 4: Three common forms of calcium carbonateLimestone, chalk and marble are all calcium carbonate — chemically identical, different only in how they formed and hence in appearance: limestone is common in hills, chalk is soft and white, and marble, transformed by heat and pressure, shows clear grains and is a favourite building material. For example, many columns are marble and blackboard chalk resembles chalk, yet both are CaCO₃ with identical chemistry — dilute acid fizzes on both with carbon dioxide. Stating 'different forms, same substance' is exactly what earns marks.
- Step 5: Thermal decomposition and the word equationWhen limestone is roasted strongly in a kiln, calcium carbonate decomposes into calcium oxide and carbon dioxide. The change is written as a word equation: calcium carbonate → (high temperature) calcium oxide + carbon dioxide. For example, cement works run lime kilns day and night at over 800°C; the calcium oxide leaving the kiln is quicklime, which reacts with water, giving out heat, to form slaked lime (calcium hydroxide) for mortar and for neutralising acidic soil — one word equation linking the whole lime industry.
- Step 6: The tests for carbon dioxide and calciumTo test whether the gas produced is carbon dioxide, bubble it through limewater: the limewater turns milky because carbon dioxide reacts with calcium hydroxide to form insoluble calcium carbonate. To test for calcium, use the flame test — calcium colours the flame brick red. For example, drip dilute acid on marble chips: the gas turns limewater milky, and with a brick-red flame result the sample is shown to be a calcium carbonate. Using both tests together is the complete answer.
- Step 7: Erosion and the uses of limestoneHeat, water and acid all change calcium carbonate: day-night temperature swings crack rock by repeated expansion and contraction, and rainwater with dissolved carbon dioxide slowly dissolves limestone, carving caves, stalactites and stalagmites. For example, the caves of Guilin's hills are the work of acidified rainwater attacking calcium carbonate over hundreds of thousands of years. Limestone is hugely useful: roasted into quicklime and slaked lime, made into cement and glass, spread to neutralise acidic soil, and used as a flux in ironmaking — from hillsides to every household.
4Diagrams Diagrams
5Worked example Example
Industrial context: a cement works roasts limestone strongly in a lime kiln. Write the word equation for the change, explain how to test the gas given off, and give two industrial uses of the solid product.
- Read the question: limestone is mainly calcium carbonate; 'strong roasting' signals thermal decomposition, giving calcium oxide and carbon dioxide
- Write the word equation: calcium carbonate → (high temperature) calcium oxide + carbon dioxide
- Test the gas: bubble it through limewater; the limewater turns milky, proving carbon dioxide
- Uses of the solid: calcium oxide (quicklime) becomes slaked lime with water — used to neutralise acidic soil, to make mortar, and as a raw material for glass and ironmaking
- Answer: Limestone is mainly calcium carbonate. On strong heating it undergoes thermal decomposition, written as the word equation: calcium carbonate → (high temperature) calcium oxide + carbon dioxide. To test the gas, bubble it through limewater; the limewater turns milky, proving the gas is carbon dioxide. The solid product, calcium oxide (quicklime), turns into slaked lime (calcium hydroxide) with water. It is used to neutralise acidic soil, to prepare building mortar, and as an important raw material for making glass and for extracting iron.
6Common mistakes Mistakes
7Quick check Quick check
Q1What are the four most abundant elements in the crust and their rough percentages?
Q2Write the word equation for the thermal decomposition of calcium carbonate.
Q3How do you test for carbon dioxide?
Q4What do limestone, chalk and marble have in common?
Q5What method extracts silver from silver oxide?
8Recap Recap
- Crustal elements (by mass): oxygen 46%, silicon 28%, aluminium 8%, iron 5%
- Rocks come as igneous, sedimentary and metamorphic, made of minerals; rocks with metal compounds are ores
- Three extraction routes: physical separation, heating alone (silver from silver oxide), heating with carbon (iron)
- Limestone, chalk and marble are all calcium carbonate; heating gives calcium oxide and carbon dioxide
- CO₂ test: limewater turns milky; calcium flames brick red; limestone makes cement, glass and neutralises soil