Part 1: Multiple choice
The main source of dissolved oxygen in water is
Waves and water currents increase the amount of dissolved oxygen because they
Which organisms add oxygen to water by photosynthesis?
All of the following are sources of dissolved CO₂ in water EXCEPT
Aquatic plants obtain CO₂ mainly by
The balanced equation of photosynthesis is
Solubility of a gas is expressed as
When the temperature of water increases, the solubility of oxygen
Between 20 °C and 50 °C the solubility of CO₂ decreases
Compared with fresh water, the solubility of the gases in sea water is
The concentration of oxygen in air is about ... times that of carbon dioxide.
At which time of day is dissolved oxygen in a weedy pond usually lowest?
A high level of dissolved oxygen leads to
The main cause of oxygen depletion after an algal bloom is
When CO₂ combines with water it produces
An increase in dissolved CO₂ causes the pH of water to
The decrease in the pH of the oceans caused by CO₂ is called
Calcification means
High dissolved CO₂ converts calcium carbonate into
Which organism has an external skeleton of calcium carbonate?
The bony skeleton of fish consists mainly of
The exoskeleton of shrimp is made of
A decrease in dissolved CO₂ can cause
Reducing CO₂ in water decreases photosynthesis and therefore
A fall of the pH of sea water from 8.2 to 8.1 means the concentration of H⁺ rises by about
Visible light is part of the
Which colour of visible light has the longest wavelength?
The relation between speed, wavelength and frequency of light is
The correct order of light zones from the top to the bottom is
As depth increases, light intensity under water
The amount of light penetrating the surface is greatest when the rays hit the surface at an angle of
Which colours of light are absorbed first as light enters water?
At 100 m in clear tropical water, about what percentage of visible light remains?
Almost all infrared radiation is absorbed within the top
Photosynthesis in water occurs mainly in
In clear waters algae and aquatic plants may grow down to about
In turbid water, algae are found
Coral reefs grow best in
Tuna and barracuda live mainly in
The Gulf Stream moderates the climate of
Liquids, unlike gases, are
The formula for the force on a surface due to a pressure P is
The pressure due to water at depth h is
Water pressure increases by about ... for each 10 m of depth.
The total pressure at a depth of 100 m is about
At the same depth the pressure in the sea is greater than in a freshwater lake because sea water has
The swim bladder of bony fish at intermediate depths helps in
Rays at great depths avoid collapse because
Cartilage compared with bone is
Deep-sea fish membranes contain more unsaturated fatty acids in order to
Part 2: Written questions and problems
1. [Give reasons] Give a reason: a warm summer pond may contain too little oxygen for fish.
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Oxygen solubility falls as temperature rises, while the fishes' oxygen needs rise with metabolism.
2. [Give reasons] Give a reason: waves increase the dissolved oxygen in the sea.
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They break the surface, increase contact area with air and mix the water, increasing gas exchange.
3. [Give reasons] Give a reason: although air contains 500 times more O₂ than CO₂, water contains much more CO₂ than expected.
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CO₂ is far more soluble than O₂ and reacts with water to form carbonic acid.
4. [Give reasons] Give a reason: oxygen is less soluble in sea water than in fresh water.
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Dissolved ions in sea water reduce the space and interaction available for gas molecules (20–30 % lower).
5. [Give reasons] Give a reason: an increase in dissolved CO₂ lowers the pH of water.
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CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻; more H⁺ ions form.
6. [Give reasons] Give a reason: ocean acidification threatens corals and oysters.
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Extra CO₂ converts CaCO₃ into soluble Ca(HCO₃)₂ and reduces carbonate availability, so calcification of shells and skeletons decreases.
7. [Give reasons] Give a reason: eggs and larvae are the most sensitive to acidification.
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Their enzymes, shells and tissue are underdeveloped and work in a narrow pH range.
8. [Give reasons] Give a reason: fish growth is weakened by high CO₂.
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CO₂ disturbs the calcium balance in the body and therefore bone formation.
9. [Give reasons] Give a reason: a decrease in dissolved CO₂ affects food chains.
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Producers make less glucose, so less energy is available for consumers.
10. [Give reasons] Give a reason: algal blooms may lead to fish deaths.
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When the algae die, bacteria decompose them and consume the dissolved oxygen.
11. [Give reasons] Give a reason: photosynthesis occurs mainly in the surface layers of water.
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Light intensity falls rapidly with depth by absorption and scattering; only surface layers receive enough.
12. [Give reasons] Give a reason: the sea looks blue.
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Water absorbs red and orange strongly and scatters the short-wavelength blue light back to our eyes.
13. [Give reasons] Give a reason: coral reefs thrive in warm shallow water near the equator.
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Constant strong sunlight supports the symbiotic algae living in the coral tissue, which supply food, and the temperature is suitable.
14. [Give reasons] Give a reason: solar radiation is vital for ecological balance in aquatic environments.
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It drives photosynthesis (basis of food chains), warms water, shapes distribution of organisms and helps to drive currents.
15. [Give reasons] Give a reason: algae and phytoplankton are abundant in surface layers.
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They need light for photosynthesis, which is available there.
16. [Give reasons] Give a reason: photosynthesis in polar seas falls in winter.
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The sun gives little or no light, so the rate decreases greatly.
17. [Give reasons] Give a reason: pressure increases with depth in water.
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The weight of the water column above the point increases.
18. [Give reasons] Give a reason: rays and some deep-sea fish have no gas-filled bladder.
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A gas space would be crushed at the high pressure; a liquid bladder and an oily liver provide buoyancy.
19. [Give reasons] Give a reason: organisms living at great depth have compact bodies and special proteins and fluids.
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To resist crushing by high pressure and to keep enzymes working under it.
20. [Give reasons] Give a reason: a diver should never hold his breath when ascending.
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Pressure falls and the air in the lungs expands (Boyle's law), which may rupture the lungs.
Part C — Define / compare
21. [Define] Define solubility.
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The ability of a solute to dissolve in a solvent forming a homogeneous solution at a given temperature and pressure; the maximum amount dissolving in a given volume to give a saturated solution.
22. [Define] Define acidification of water.
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Lowering of the pH of water caused by dissolved CO₂ forming carbonic acid.
23. [Define] Define calcification.
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The process by which organisms deposit calcium carbonate to build shells or skeletons.
24. [Define] Define the photic (sunlit) zone.
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The upper layer of water receiving enough light for photosynthesis.
25. [Define] Define water pressure.
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The pressure exerted by water at a point below its surface, equal to the weight of the water column above per unit area (ρgh).
26. [Compare] Compare the effect of high and low dissolved CO₂ on acidification and photosynthesis.
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High CO₂: lower pH, less calcification, photosynthesis not limited. Low CO₂: higher pH, photosynthesis limited and food chains weaker.
27. [Compare] Compare O₂ and CO₂ in the effect of rising temperature on solubility.
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Both decrease; CO₂ solubility falls by a greater amount.
28. [Compare] Compare the sunlit, twilight and dark zones.
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Sunlit: strong light, photosynthesis. Twilight: dim light, no photosynthesis. Dark: no light.
29. [Compare] Compare bony and cartilaginous fish in skeleton and buoyancy organ.
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Bony: bone, swim bladder. Cartilaginous: cartilage, large oily liver (no gas bladder).
30. [Compare] Compare gases and liquids as fluids.
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Gases are easily compressible and fill their container; liquids are almost incompressible and keep a constant volume.
Part D — Problems
31. [Problem] Find the water pressure and total pressure at a depth of 20 m in a freshwater lake.
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2.0 × 10⁵ N/m²; 3.01 × 10⁵ N/m².
ρgh = 1000 × 10 × 20; add 1.013 × 10⁵.
32. [Problem] Find the pressure due to sea water at a depth of 500 m.
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5.125 × 10⁶ N/m² (≈ 50.6 atm).
1025 × 10 × 500.
33. [Problem] A window of area 0.5 m² is at 50 m in a lake. Find the force due to the water alone.
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2.5 × 10⁵ N.
P = 1000 × 10 × 50 = 5 × 10⁵; F = 5 × 10⁵ × 0.5.
34. [Problem] At what depth is the total pressure 5 atm, using 1 atm per 10 m?
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40 m.
Water pressure = 4 atm.
35. [Problem] A tank of 200 m³ has dissolved oxygen of 10 mg/L. How many kg of oxygen does it contain?
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2 kg.
10 g/m³ × 200 = 2000 g.
36. [Problem] A 250 mL sample contains 9.1 mg/L dissolved oxygen. What mass of oxygen is in the sample?
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2.275 mg.
9.1 × 0.25.
37. [Problem] Oxygen solubility is 14.6 mg/L at 0 °C and 9.1 mg/L at 20 °C. Find the percentage decrease.
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37.7 %.
5.5/14.6.
38. [Problem] If the O₂ solubility in fresh water at 20 °C is 9.1 mg/L, estimate it in sea water if it is 25 % lower.
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≈ 6.8 mg/L.
9.1 × 0.75.
39. [Problem] The pH of sea water falls from 8.2 to 8.0. By what factor does [H⁺] change?
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×1.58 (a 58 % increase).
100.2.
40. [Problem] Find the mass of CO₂ consumed and of Ca(HCO₃)₂ formed when 50 g of CaCO₃ dissolves completely.
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22 g CO₂; 81 g Ca(HCO₃)₂.
0.5 mol each; 0.5 × 44; 0.5 × 162.
41. [Problem] Algae absorb 132 g of CO₂ in photosynthesis. Find the mass of glucose formed and of oxygen released.
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90 g glucose; 96 g O₂.
3 mol CO₂ → 0.5 mol glucose (90 g) and 3 mol O₂ (96 g).
42. [Problem] Find the frequency of green light of wavelength 540 nm.
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5.56 × 10¹⁴ Hz.
ν = 3 × 10⁸/540 × 10⁻⁹.
43. [Problem] A light wave has frequency 6.0 × 10¹⁴ Hz. Find its wavelength in nm and its colour region.
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500 nm; blue-green.
λ = 3 × 10⁸/6 × 10¹⁴ = 5 × 10⁻⁷ m.
44. [Problem] In clear water light intensity halves every 5 m. What fraction remains at 15 m, and at 20 m?
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1/8 = 12.5 %; 1/16 = 6.25 %.
(1/2)³ and (1/2)⁴.
45. [Problem] Sunlight intensity at the surface is 1000 W/m². If 1 % reaches 100 m, what is the intensity there?
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10 W/m².
0.01 × 1000.
46. [Problem] A diver at 30 m (4 atm) fills the lungs with 3.0 L and ascends without exhaling. What volume would the air occupy at the surface?
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12 L.
V₂ = 4 × 3.0/1.
47. [Problem] A swim bladder has 25 mL at 2 atm. What is its volume at 6 atm (no gas added)?
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8.33 mL.
2 × 25/6.
48. [Problem] Find the force on an eardrum of area 0.6 cm² due to water alone at a depth of 10 m in fresh water.
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6 N.
P = 10⁵ N/m²; A = 6 × 10⁻⁵ m²; F = 6.
49. [Problem] Estimate the total pressure at 3000 m depth in atm using 1 atm per 10 m.
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301 atm.
300 + 1.
50. [Problem] Find the difference in water pressure at 100 m between sea water (1025 kg/m³) and fresh water.
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2.5 × 10⁴ N/m².
(1025 − 1000) × 10 × 100.