Week 1 · Unit 1 Aquatic Ecosystems, opener and introduction; 1-1 The Hydrosphere on Planet Earth: four spheres, the share of salt, fresh and frozen water, the water cycle, transpiration and excretion · 3 lessons
1
Four spheres and where Earth's water is
Integrated Science Sec 1 Term 1, Unit 1: pp. 3–7
GoalName the four spheres, tell living (biotic) from non-living (abiotic) parts of an environment, and give the shares of salt water, ice and usable fresh water on Earth.
New words
ecosystem · all the living things of one place, together with the non-living parts they depend on, all acting on one anotherbiotic and abiotic factors · living things in an environment (such as plants, animals, people and microbes) and its non-living parts (such as light, air, water and soil)hydrosphere · all the water on and near Earth: oceans, seas, lakes, rivers, groundwater and ice
Explanation
The word environment comes from a French word meaning what surrounds us. An environment has biotic factors, the living things in it (for example plants, animals, people and microbes), and abiotic factors, its non-living parts (for example light, air, water and soil). An ecosystem is all the living things of one place together with the non-living parts they depend on, all acting on one another. Earth’s natural environment is made of four connected spheres: the hydrosphere (water), the atmosphere (air), the lithosphere (rock and soil) and the biosphere (life). They change all the time and affect one another. About 70% of Earth’s surface is covered by water, and this figure is about area, not volume. Of all the water on Earth, about 97% is salty (oceans, seas and salt lakes), about 2% is frozen (polar ice and glaciers) and only about 1% is fresh water that people can use (rivers, lakes, streams, reservoirs and water underground). Salt water cannot be drunk or used to irrigate fields directly, because it holds too much salt.
Worked examples
Kind of water
Cups out of 100
Litres out of 1000
salt water
97
970
ice
2
20
usable fresh water
1
10
Picture 100 equal cups for all of Earth’s water. 97 cups hold salt water, 2 hold ice and only 1 holds usable fresh water. Now use 1000 litres instead: 97% of 1000 is 970 litres, 2% is 20 litres and 1% is 10 litres. The three parts add up to 100% and to 1000 litres.
Sort four things into spheres. The water of a river belongs to the hydrosphere. A fish swimming in it is part of the biosphere. The air above the river is the atmosphere. The rock on its bed is the lithosphere. The four spheres meet in one place: the fish needs water, air and rock-made minerals to live.
A pupil says, “Earth is 70% water.” What is missing? The 70% describes how much of the surface area is covered by water. It does not tell us how much of Earth’s volume is water, because the oceans are only a thin layer compared with the whole planet.
Class activity
A 100-cup model, with a grown-up. Draw a grid of 10 × 10 squares on paper (100 squares) for all of Earth’s water. Colour 97 squares dark blue, 2 squares white and 1 square light blue. Or use 100 beans or rice grains in three heaps. Look at the light-blue square or the single bean: describe how small the usable fresh water is, and say what the heaps of 97 and 2 stand for.
Practice
1
Name the four spheres of Earth and say which one is the water.
Hydrosphere (water), atmosphere (air), lithosphere (rock and soil), biosphere (life). The water is the hydrosphere.
2
Sort into biotic or abiotic: a fish, sunlight, soil, a mushroom, air.
Biotic: the fish and the mushroom. Abiotic: sunlight, soil and air.
3
A model of Earth’s water uses 200 equal cups. How many cups hold salt water, ice and usable fresh water?
Salt water: 194 cups (97% of 200). Ice: 4 cups (2%). Usable fresh water: 2 cups (1%).
4
Why can’t people drink ocean water or water fields with it directly?
