Ch4 rocks and minerals
4.1 Rocks as a source of minerals
Rock: a solid mass of a mineral or a mixture of minerals.
Minerals: naturally occurring chemicals. They may be elements or compounds.
Ore: the type of rocks that contains minerals, from which a high proportion of useful elements (usually contains
metal) can be extracted. Most of the metals used in daily life are extracted from their ores.
4.2 Extraction of metals from their ores
Physical change: a change in which no new substances are produced
e.g. sublimation - change directly from the solid state to the gaseous state on heating
Chemical change: a change in which one or more new substances are produced.
4.3 Limestone, chalk, and marble
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Limestone, chalk, and marble are common rocks which contain the mineral calcite with different appearance and
hardness.
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Uses of limestone: Powdered limestone to neutralize acidic soil, make glass, make cement, as the
construction materials for some footpath, build statue
4.4 Weathering and erosion of rocks
Weathering of rocks: It is the slow process (over thousands of years) in which exposed rocks are broken down into
smaller pieces (∵ atmospheric conditions)
Erosion: a process in which weathered rock pieces are transported away by gravity, wind and water.
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Weathering by temperature changes
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Weathering by frost action
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Weathering by acidic substances
Carbon dioxide in air dissolves slightly in rainwater, forming carbonic acid.
The carbonic acid formed reacts with calcium carbonate in rocks such as chalk and limestone.
Calcium hydrogencarbonate is soluble in water and thus the rocks are slowly worn away.
e.g. formation of sinkhole in limestone area, damage to limestone statue
4.5 Chemical changes involving calcium carbonate
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Thermal decomposition of calcium carbonate
Calcium oxide (quicklime) dissolve in water to form calcium hydroxide (slaked lime)
Observation: 1. give a large amount of heat
2. a saturated solution with white suspension forms.
- Limewater (filtrate of the solution): a saturated solution of calcium hydroxide in water.
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Test for carbon dioxide-using limewater
Observation: limewater turns milky
Lime water reacts with excess carbon dioxide
Observation: milky limewater becomes colourless
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Test for calcium ion in calcium carbonate: adding dilute acid
Test for carbonate ion in calcium carbonate: flame test
Observation: Brick red flame is observed