Week 15 · Term 1 review (Units 1–2) and Darslab's own review tasks · 3 lessons
2
Review: heat, light, gases and pressure in water
Integrated Science Sec 1 Term 1, Units 1–2: Review
GoalUse the ideas of Unit 1, second half: gases in water, light zones, acidification, pressure with depth and ecological balance.
New words
dissolved oxygen · oxygen mixed in water, less when the water is warmeracidification · the fall in the pH of the sea as it takes in CO₂ecological balance · a steady state in which living things and their environment affect one another evenly
Explanation
This is a review: no new facts. Gases dissolve less in warmer water, so warm water holds less oxygen. When sea water takes in CO₂, carbonic acid forms (CO₂ + H₂O ⇌ H₂CO₃), the pH falls, and shell-builders such as corals find it harder to make calcium carbonate. Light is absorbed with depth and the colours disappear in a certain order, so there are sunlit, twilight and dark zones. Pressure in water grows with depth: P = Pa + Pwater, with Pwater = ρgh, and force F = P × A. About every 10 m of water adds about 1 atm. Ecological balance can be upset by changes such as overfishing, pollution, nutrient excess (algal bloom) and habitat destruction. A chain of cause and effect shows how a change travels through a food web. Sustainable development meets today’s needs without harming the future.
more CO₂ in the sea→more carbonic acid→lower pH→harder for corals to build shells
Worked examples
Find the water pressure at 20 m, using ρ = 1000 kg/m³ and g = 9.8 N/kg: Pwater = 1000 × 9.8 × 20 = 196 000 Pa. With the air above (Pa = 101 300 Pa), P = 101 300 + 196 000 = 297 300 Pa.
Find the force of the water alone on a window of area 0.02 m² at 20 m: F = P × A = 196 000 × 0.02 = 3920 N.
Cause and effect: a river brings extra fertiliser; algae grow quickly; when they die, bacteria use up the oxygen; fish suffocate. This chain shows an algal bloom upsetting the balance.
Class activity
Make a cause-and-effect puzzle, with a grown-up. Write one chain of four steps on four paper strips for each of: a warm summer lake losing oxygen, overfishing, and acidification. Mix the strips of one chain and swap with a partner, who puts them back in order. Check each other’s order and explain any step that is wrong.
Practice
1
Why do fish struggle in a warm lake in summer?
Warm water holds less dissolved oxygen.
2
Find Pwater at 5 m (ρ = 1000, g = 9.8).
1000 × 9.8 × 5 = 49 000 Pa.
3
Write the equation that makes the sea more acidic.
CO₂ + H₂O ⇌ H₂CO₃.
4
Name two human actions that can upset ecological balance.
For example: overfishing, pollution, habitat destruction.
5
Why are there fewer plants in the dark zone of the sea?
