Rocks and Minerals: Treasures from the Earth's Crust
Hello, young scientists! The gold in a ring and the marble floor of a shopping mall both came from rocks. You will learn how we get metals from rocks, how acids attack metals and rocks, how flame colours reveal metals, and how rain and acid rain change our planet. The Extension topic part is an optional challenge.
1. Rocks, Minerals and Ores
A mineral is a natural solid with a fixed chemical make-up, such as quartz or calcite (calcium carbonate). A rock is made of one or more minerals, like a fruit cake with nuts and raisins inside. Granite, common in Hong Kong, is a mix of quartz, feldspar and mica.
Ores: rocks worth mining
An ore is a rock with enough metal (usually as a compound) to be worth digging up and extracting, e.g. haematite (iron oxide), bauxite (aluminium oxide) and green malachite (copper carbonate).
Metals as elements or as compounds
A few very unreactive metals, such as gold, silver and sometimes copper, are found as the pure element in Nature, often as shiny nuggets. They hardly react with air or water.
Most metals, such as iron, aluminium and zinc, are more reactive. They have already joined with oxygen, sulphur or other elements, so they exist as compounds in ores. We need chemical reactions to get them out. (For metal properties and alloys, see the chapter "Metals".)
Key takeaway: Rocks are made of minerals. Gold and silver can be found as elements, but most metals are locked up as compounds in ores.
2. Extracting Metals from Ores
Extraction means getting a metal out of its ore. The method depends on how reactive the metal is.
Method 1: Physical methods – panning for gold
Gold is already an element, so we only need to separate it from sand. In panning, river gravel and water are swirled in a shallow pan. Dense gold sinks to the bottom while lighter sand washes over the edge.
Did you know? In the 1850s many Chinese people sailed to California to find gold, so San Francisco is still called "Old Gold Mountain" in Chinese!
Method 2: Heating alone
Oxides of some unreactive metals break down when simply heated. The oxygen given off relights a glowing splint.
\(\text{silver oxide} \rightarrow \text{silver} + \text{oxygen}\)
\(2\text{Ag}_2\text{O} \rightarrow 4\text{Ag} + \text{O}_2\)
\(\text{mercury(II) oxide} \rightarrow \text{mercury} + \text{oxygen}\)
\(2\text{HgO} \rightarrow 2\text{Hg} + \text{O}_2\)
Mercury vapour is poisonous, so schools usually show this one on video.
Method 3: Heating with carbon
Carbon can "steal" oxygen from the oxides of metals of medium reactivity. Heat black copper(II) oxide with carbon powder and reddish-brown copper forms.
\(\text{copper(II) oxide} + \text{carbon} \rightarrow \text{copper} + \text{carbon dioxide}\)
\(2\text{CuO} + \text{C} \rightarrow 2\text{Cu} + \text{CO}_2\)
\(\text{lead(II) oxide} + \text{carbon} \rightarrow \text{lead} + \text{carbon dioxide}\)
\(2\text{PbO} + \text{C} \rightarrow 2\text{Pb} + \text{CO}_2\)
Iron is also made by heating iron ore with carbon in a blast furnace.
Writing a balanced equation
Write the word equation, change names into formulae, then count atoms. Put numbers in front of formulae until both sides match. In the copper equation above, each side has 2 Cu, 2 O and 1 C.
Why reactive metals are harder to extract
The more reactive a metal, the more tightly it holds on to oxygen, so the harder it is to extract:
Most reactive (potassium, sodium, calcium, magnesium, aluminium) – electrolysis, using lots of electricity
Middle (zinc, iron, lead, copper) – heating with carbon
Less reactive (mercury, silver) – heating alone
Least reactive (gold) – found as an element; physical methods
Memory aid: "Every Clever Hiker Pans" = Electrolysis, Carbon, Heating alone, Panning, from most reactive to least.
Key takeaway: The more reactive the metal, the harder the method needed to extract it.
3. Calcium Carbonate in Nature
Calcium carbonate (\(\text{CaCO}_3\)) comes in several natural forms:
Limestone – a rock formed over millions of years from shells and skeletons of sea creatures
Chalk – a soft, white, crumbly limestone made from tiny shells of sea plankton
Marble – a hard rock formed when limestone is heated and squeezed deep underground; it is polished for floors and statues
Living things make it too: shells, corals, pearls and eggshells are mostly calcium carbonate.
Key takeaway: Limestone, chalk and marble are different forms of the same compound, calcium carbonate.
4. Dilute Acids Attack Metals and Rocks
Acids with metals
Many metals react with dilute acids, giving off bubbles of hydrogen:
\(\text{metal} + \text{dilute acid} \rightarrow \text{salt} + \text{hydrogen}\)
\(\text{magnesium} + \text{hydrochloric acid} \rightarrow \text{magnesium chloride} + \text{hydrogen}\)
Hydrogen burns with a "pop" (see the chapter "The carbon cycle and gases from air"). Magnesium fizzes fast, zinc and iron slowly; copper and gold do not react.
Acids with limestone and marble
Dilute acids react with calcium carbonate, giving off carbon dioxide:
\(\text{calcium carbonate} + \text{hydrochloric acid} \rightarrow \text{calcium chloride} + \text{water} + \text{carbon dioxide}\)
So acids slowly eat away limestone and marble buildings and statues, and corrode metal railings and bridges.
Key takeaway: Metal + dilute acid gives hydrogen; calcium carbonate + dilute acid gives carbon dioxide.
5. Speeding Up the Reaction (Extension topic)
This section is an Extension topic in the 2025 curriculum. It is a key practical task.
Surface area: powder versus chips
Acid can only react with the surface of a solid. The same mass of marble crushed into powder has a much larger surface area, so more acid particles hit it at the same time. Powder fizzes faster than chips, just as crushed ice melts faster than one big ice cube.
Other factors
Concentration – more acid particles in the same volume: faster reaction.
Temperature – warmer particles move faster and collide more often: faster reaction.
Planning a fair test
Independent variable – size of the marble pieces
Dependent variable – e.g. volume of carbon dioxide made in one minute
Control variables – mass of marble; volume, concentration and temperature of acid
Key takeaway: Larger surface area, more concentrated acid and higher temperature all speed up the reaction.
6. Flame Tests: Colour Clues to Metals
Some metals give a special colour in a flame. We use this as evidence to infer the metal in a mineral sample.
How to do a flame test
Dip a nichrome or platinum wire loop into dilute hydrochloric acid and heat it in a hot, non-luminous (blue) Bunsen flame until it gives no colour, so it is clean. Dip it into the powdered sample, hold it in the flame again and observe the colour. Wear safety goggles.
Colours to remember
Sodium – golden yellow
Potassium – lilac (pale purple)
Calcium – brick red
Copper – bluish green
Memory aid: Sodium = Sunshine yellow; Potassium = Pale purple; Calcium = Clay-brick red; Copper = Cool blue-green.
Using flame tests as evidence
A rock giving a brick-red flame probably contains calcium (e.g. limestone). Rock salt gives golden yellow: sodium. Malachite gives bluish green: copper.
Did you know? The colours of the fireworks over Victoria Harbour come from metal compounds: the same chemistry as a flame test!
Key takeaway: Flame colour is a clue to the metal present.
7. Erosion: How Rocks Wear Away
Erosion is the slow wearing away and carrying away of rock.
Rain, rivers, wind and waves
Rain and rivers wash away loose pieces; tumbling pebbles become smooth and round.
Wind blows sand against rock, scraping it like sandpaper.
Sea waves carve sea caves and sea arches, as seen in the Hong Kong UNESCO Global Geopark.
Rainwater dissolving limestone
Rainwater is slightly acidic because carbon dioxide dissolves in it. Over thousands of years it seeps into cracks in limestone and dissolves the rock, forming caves. Dripping water leaves calcium carbonate behind as stalactites and stalagmites. Guilin's limestone hills formed this way.
Memory aid: stalactites hang from the ceiling; stalagmites rise from the ground.
Key takeaway: Rain, rivers, wind and waves wear rocks away; slightly acidic rainwater dissolves limestone to form caves.
8. Acid Rain and Ocean Acidification
Causes of acid rain
Burning fossil fuels in power stations, factories and vehicles releases sulphur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)). These gases dissolve in rainwater to form acids, making acid rain. (These pollutants also appear in the chapter "Air quality".)
Effects of acid rain
It dissolves limestone and marble buildings and statues, corrodes metal bridges, makes lakes too acidic for fish, and damages trees and soil. Power stations now remove sulphur dioxide from waste gases.
Ocean acidification
As more carbon dioxide enters the air, more dissolves in the sea, forming a weak acid. The sea is still slightly alkaline, but its pH is slowly falling: ocean acidification.
Corals, oysters and clams build shells or skeletons from calcium carbonate. In more acidic water this is harder, and shells may even dissolve. Weaker corals mean fewer homes and less food for sea life, and corals in Hong Kong, e.g. Hoi Ha Wan Marine Park, could be affected too.
Key takeaway: Sulphur dioxide and nitrogen oxides cause acid rain; extra carbon dioxide causes ocean acidification, which harms corals and shellfish.
Common Mistakes
Common mistake 1: "All metals are found pure." Only a few unreactive ones, like gold, are.
Common mistake 2: "Heating with carbon works for every metal." Very reactive metals like aluminium need electrolysis.
Common mistake 3: Changing the small numbers inside formulae to balance an equation. Only add numbers in front.
Common mistake 4: "Ocean acidification means the sea has turned into acid." The sea is just becoming less alkaline.
Quick Review
1. Ores are rocks worth mining for metals.
2. Gold and silver occur as elements; most metals occur as compounds.
3. Extraction: physical methods, heating alone, heating with carbon.
4. Limestone, chalk and marble are calcium carbonate.
5. Acid + metal gives hydrogen; acid + calcium carbonate gives carbon dioxide.
6. (Extension) Surface area, concentration and temperature affect the rate.
7. Flames: sodium golden yellow, potassium lilac, calcium brick red, copper bluish green.
8. Erosion wears rocks away; rain forms limestone caves.
9. \(\text{SO}_2\) and \(\text{NO}_x\) cause acid rain; extra \(\text{CO}_2\) causes ocean acidification.
Great work, rock detective! Try the flash cards next.