The Earth's Structure, Atmosphere and Water: Getting to Know Our Home Planet
Hello, young scientists! What is deep under your feet? What is the air made of? Why can't we just drink sea water? In this chapter you will learn the four layers inside the Earth, the main gases in air, the forms of fresh water, how the Earth's water is shared out, and safe ways to tell fresh water from salt water.
How water moves around the Earth is covered in The water cycle; how we make water safe to drink is covered in Water purification.
1. Inside the Earth: Four Layers
The Earth is a huge ball of rock and metal with a radius of about 6,400 km. Nobody has travelled to the centre, but scientists study how earthquake waves travel through the Earth to work out what is inside. Think of a peach: the thin skin is like the crust, the thick flesh is like the mantle, and the stone is like the core.
The four layers from the outside in
1. Crust – solid rock, the thinnest layer: about 5–10 km thick under the oceans and 30–70 km under the continents. Mountains, oceans and Lion Rock all sit on it.
2. Mantle – the thickest layer, about 2,900 km thick. Very hot rock that is mostly solid, but over millions of years it can flow extremely slowly, like stiff plasticine.
3. Outer core – about 2,200 km thick, made mainly of iron and nickel. It is so hot (about 4,000–5,000 °C) that the metals are liquid.
4. Inner core – a ball of iron and nickel with a radius of about 1,200 km. It is the hottest part (about 5,000–6,000 °C) but it is solid, because the huge pressure from the layers above stops the metal melting.
Memory aid: Crust, Mantle, Outer core, Inner core – "Cool Monkeys Order Ice-cream".
Hotter as you go deeper
The temperature increases with depth. That is why very deep mines need cooling systems for miners. Pressure also increases with depth, because each layer is squeezed by the weight of everything above it.
Did you know? The inner core is about as hot as the surface of the Sun! The moving liquid iron in the outer core produces the Earth's magnetic field, which makes a compass point north.
Skill: drawing the Earth's structure
Draw a large circle for the Earth. Inside it, draw a small circle for the inner core and a bigger one for the outer core. The space up to the surface is the mantle, and the crust is a very thin outer band. Label each layer with its state and thickness, and add an arrow: "temperature increases with depth".
Scale tip: If 1 cm stands for 1,000 km, the inner core has a radius of about 1.2 cm, the outer core reaches 3.4 cm and the mantle reaches about 6.3 cm. The crust would be thinner than your pencil line, so if you draw it thicker, write "not to scale".
Common mistake: The mantle is NOT a sea of liquid lava – it is mostly solid rock. Melted rock (magma) forms only in some places near the top. And only the outer core is liquid; the inner core is solid.
Key takeaway: Thin solid crust, thick mostly solid mantle, liquid outer core, solid inner core. Temperature increases with depth.
2. The Atmosphere: A Blanket of Gases
What is the atmosphere?
The atmosphere is a layer of gases surrounding the Earth, held in place by gravity. We call this mixture of gases air. Air is thickest near the ground and gets thinner higher up, slowly fading into space. Near the top of Mount Everest the air is so thin that many climbers carry oxygen tanks.
The atmosphere gives us oxygen to breathe and gives plants carbon dioxide for photosynthesis. Its ozone layer blocks most of the Sun's harmful ultraviolet (UV) rays, and it keeps the Earth warm like a blanket.
What is air made of?
Air is a mixture. Approximate percentages by volume in dry air:
Nitrogen – about 78%
Oxygen – about 21%
Argon – about 0.9%
Carbon dioxide – about 0.04%
Other gases (e.g. neon, helium) – tiny amounts
Air also contains a changing amount of water vapour – a lot on a sticky Hong Kong summer day!
Memory aid: 78 – 21 – 1. Nitrogen 78, oxygen 21, and the last 1% is mostly argon. Carbon dioxide is only about 4 particles in every 10,000, but it matters: plants need it, and it helps trap heat. Gas tests are covered in Air.
Common mistake: Air is NOT mostly oxygen. Nitrogen is the main gas (almost four-fifths); oxygen is only about one-fifth.
Key takeaway: The atmosphere is a layer of gases around the Earth. Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
3. Water on Earth: Salt Water and Fresh Water
Salt water and fresh water
Salt water (sea water) is found in the oceans and seas. It has a lot of dissolved salts, mainly common salt (sodium chloride) – about 35 g in every 1 kg, or about 3.5%. Fresh water has very little dissolved salt. Rain is fresh water.
Forms of fresh water on Earth
1. Ice caps and glaciers – huge ice sheets on Antarctica and Greenland, and slow "rivers of ice" (glaciers) in high mountains.
2. Groundwater – rainwater stored in tiny spaces in soil and rock underground. We reach it with wells; it flows out at springs.
3. Lakes and reservoirs – still fresh water. A reservoir is a human-made lake, such as Plover Cove Reservoir.
4. Rivers and streams – flowing fresh water, such as the streams in our Country Parks.
5. Wetlands – land soaked with water for all or part of the year, such as marshes and swamps. They are home to many birds, fish and insects.
Small amounts are also found as water vapour and clouds, in soil, and inside living things.
How is the Earth's water shared out?
About 97% of the Earth's water is salt water. Only about 3% is fresh water – and of that fresh water, about 69% is frozen as ice, about 30% is groundwater and only about 1% is in lakes, rivers, wetlands, air and living things.
Skill: showing the distribution in a table or diagram
Where | Fresh or salt? | Share of ALL water
Oceans and seas | Salt | about 97%
Ice caps and glaciers (polar regions, high mountains) | Fresh | about 2%
Groundwater | Fresh | less than 1%
Lakes, rivers, wetlands, air, living things | Fresh | much less than 0.1%
A pie chart also works. The fresh water slice is tiny, so draw a second pie chart just for fresh water: ice 69%, groundwater 30%, surface water 1%.
Picture it: If all the Earth's water filled a 1-litre bottle, about 970 mL would be salt water and 30 mL fresh water. Of that 30 mL, about 20 mL is ice and 9 mL is underground. All the lakes, rivers and wetlands would be just a few drops!
Did you know? Most Hong Kong homes flush toilets with sea water, saving precious fresh water. Most of our fresh water is piped in from the Dongjiang (East River) in Guangdong, and we also collect rainwater in reservoirs.
Key takeaway: About 97% of the Earth's water is salt water and only about 3% is fresh. Most fresh water is locked up as ice, so the fresh water we can easily use is very limited.
4. Telling Fresh Water from Salt Water
Both can look clear and colourless. But sea water has a lot of dissolved salt, which changes some properties we can measure.
Safety first: never taste!
In the laboratory, never taste any sample, even "just water". Samples and apparatus may carry harmful chemicals or germs. Use safe tests instead.
Ways to tell them apart
1. Evaporation
Heat the same small volume of each sample in an evaporating dish over a water bath until the water has gone. Sea water leaves white salt crystals; distilled water leaves nothing. Wear goggles – do not heat strongly to dryness, as hot salt can spit.
2. Density
Sea water is denser. Using \(\text{density} = \dfrac{\text{mass}}{\text{volume}}\): 100 cm³ of fresh water has a mass of about 100 g, but 100 cm³ of sea water is about 102–103 g. Weigh equal volumes on an electronic balance. Objects also float higher in sea water – try a straw with plasticine on one end as a simple hydrometer.
3. Electrical conductivity
Dip two carbon rods connected to a battery and a bulb, LED or ammeter into each sample. Sea water conducts electricity much better, because dissolved salts let a current flow. Distilled water hardly conducts. Use batteries only – never mains electricity.
4. Boiling point
Pure water boils at 100 °C at normal pressure; sea water boils slightly higher (about half a degree) and freezes lower (about −2 °C). The differences are small, so you need an accurate thermometer.
Be creative! Other safe ideas work too – for example, sea water forms much less lather when shaken with soap solution.
Designing a fair experiment (Key Practical Task)
Problem: Bottles A and B are unlabelled. One holds fresh water, one holds sea water. Which is which?
Hypothesis: The sample that leaves more solid after evaporation is sea water.
Independent variable: the type of sample (A or B).
Dependent variable: the mass of solid left.
Control variables: volume of sample, type of dish, heating method.
Control set-up: distilled water, tested the same way.
Steps: 1. Weigh three labelled, dry evaporating dishes. 2. Add 10 cm³ of each sample. 3. Heat gently over a water bath until dry. 4. Cool and weigh again. 5. Repeat and average.
Example: A leaves about 0.35 g of white solid; B and distilled water leave almost nothing. Conclusion: A is sea water, because sea water contains much more dissolved salt, which stays behind when water evaporates. Check with a second method (e.g. conductivity) – if two methods agree, your result is more reliable.
Common mistake: "The water is clear, so it is fresh." Wrong – dissolved salt is invisible. Also, "fresh" means "not salty", not "clean": river water can still be unsafe to drink.
Key takeaway: Never taste. Sea water leaves salt after evaporation, is denser, conducts electricity better and boils at a slightly higher temperature.
Quick Review
1. Layers from outside in: crust (thinnest), mantle (thickest), outer core (liquid), inner core (solid, hottest).
2. Temperature increases with depth.
3. The atmosphere is a layer of gases surrounding the Earth.
4. Dry air: nitrogen 78%, oxygen 21%, argon 0.9%, carbon dioxide 0.04%.
5. Fresh water: ice caps and glaciers, groundwater, lakes, rivers and streams, wetlands.
6. 97% salt water, 3% fresh water – mostly ice.
7. Tell them apart by evaporation, density, conductivity or boiling point – never taste.
Great work! Every drop of fresh water is precious – now you know why.