Week 7 · 1-7 Effect of Atmospheric Gases on Aquatic Environments: oxygen and carbon dioxide in water, solubility, acidification and calcification; 1-8 Effect of Light and Solar Radiation: spectrum, light zones, absorption, distribution of life · 3 lessons
1
Oxygen and carbon dioxide in water: sources and solubility
Integrated Science Sec 1 Term 1, Unit 1: pp. 43–55
GoalName the sources of dissolved oxygen and carbon dioxide in water and say how temperature and salt change how much gas dissolves.
New words
dissolved oxygen · oxygen gas mixed into water, which fish and other aquatic animals breathe through gillssolubility · how much of a gas dissolves in a given volume of water at a given temperature and pressurediffusion · the movement of a substance from where it is more concentrated to where it is less concentratedphytoplankton · tiny floating algae that make food by photosynthesis
Explanation
Oxygen enters water in two main ways: from the air, where waves and currents speed up the exchange of gas, and from photosynthesis by phytoplankton, algae and aquatic plants. Carbon dioxide comes from the air, from the respiration of living things, from the decay of dead matter and from human activities. Aquatic plants and algae take in CO₂ by diffusion. Photosynthesis in summary is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, and it releases oxygen only in light, while respiration goes on all the time. Air is about 21% oxygen and 0.04% carbon dioxide, about 500 times less, yet CO₂ dissolves much more easily than O₂. Both gases dissolve less in warm water and less in salt water. As an extra fact, fresh water holds about 14.6 mg of oxygen per litre at 0 °C, about 9 at 20 °C and about 7.5 at 30 °C. Enough dissolved oxygen supports breathing, metabolism and activity, and a stable ecosystem. Gases dissolve independently: more CO₂ does not “push out” the oxygen.
Worked examples
Compare the oxygen levels in cold and warm fresh water: 14.6 mg per litre at 0 °C and 7.5 mg per litre at 30 °C. The fall is 14.6 − 7.5 = 7.1 mg per litre, and 7.1 ÷ 14.6 ≈ 0.49, so warm water holds about half as much oxygen as ice-cold water.
How many times more oxygen than carbon dioxide is in the air? 21 ÷ 0.04 = 525, so about 500 times more. Even so, CO₂ is much more soluble in water, so the two gases are both present in water in useful amounts.
Why do fish often gather near a small waterfall or where a stream runs fast? Moving water mixes in more air and speeds up the exchange of gases, so there is more dissolved oxygen there, and cooler running water holds more as well.
Class activity
Fizzy water, with a grown-up. A grown-up opens two bottles of fizzy water slowly (never shaken): one kept in the fridge and one at room temperature. Pour the same amount of each into two clear glasses side by side and watch for a few minutes. Describe the bubbles and say which water seems to hold more dissolved gas. Link to the rule: gases dissolve less in warm water. Do not taste.
Practice
1
Name two sources of dissolved oxygen in water.
The air (helped by waves) and photosynthesis of algae, phytoplankton and aquatic plants.
2
Name two sources of carbon dioxide in water.
For example: the air, respiration of organisms, decay of dead matter, human activities.
3
Does a warm pond hold more or less dissolved oxygen than a cold one? Give the reason.
Less, because gases dissolve less in warm water.
4
Fresh water holds 9 mg of oxygen per litre at 20 °C. How many mg are in 5 litres?
