Marine Science (0697) | Dissolved gases

1. How Gases Enter the Sea Water

1.1 The Process of Dissolution

The gases that we find dissolved in sea water primarily come from the atmosphere.

Think about opening a can of soda. The fizzy bubbles are carbon dioxide that was forced into the liquid under pressure. In the ocean, a similar process happens naturally when the surface of the water meets the air.

• When air meets the sea surface, gas molecules move across the boundary and mix into the water. This process is called dissolution.
• Wind and wave action help increase the surface area and mixing, allowing more gas to dissolve quickly, especially in the surface layer.

1.2 Oxygen (\(O_2\)) and Carbon Dioxide (\(CO_2\))

We focus on oxygen and carbon dioxide because they are biologically essential:

• Oxygen (\(O_2\)) is needed by almost all marine organisms (like fish, crabs, and zooplankton) for aerobic respiration—the process of releasing energy from food.
• Carbon Dioxide (\(CO_2\)) is needed by producers (like phytoplankton and algae) for photosynthesis, the process of making food using light energy.

Key Takeaway: Gases dissolve from the atmosphere into the ocean surface, driven by wind and wave action. Oxygen has naturally low solubility, limiting the amount available for marine life.

2. Comparing Gas Concentrations

2.1 Atmosphere vs. Dissolved in Seawater

The concentrations of gases in the atmosphere are very different from the concentrations dissolved in the ocean.

Comparison Table (Approximate Relative Concentrations)

2.2 Why is Carbon Dioxide so Concentrated in the Ocean?

Unlike oxygen, which just floats around dissolved, carbon dioxide is chemically reactive with water.

When \(CO_2\) dissolves, it does not just stay as a gas; it forms carbonic acid, which then breaks down into bicarbonate and carbonate ions.

This means the ocean is a massive carbon sink—it stores carbon in these stable ionic forms, acting like a huge reservoir. This stored CO₂ is many times greater than the amount of CO₂ floating around in the atmosphere.

Key Takeaway: Oxygen levels are high in the atmosphere but low in the sea. Carbon dioxide levels are low in the atmosphere but very high in the sea because it reacts chemically to form stable ions.

3. Temperature and Gas Solubility: The Inverse Relationship

3.1 Describing the Relationship (Syllabus 2.3.3)

Of all the factors affecting dissolved gases, temperature is perhaps the most important environmental factor.

The relationship is simple and crucial:

An increase in temperature reduces the solubility of oxygen and carbon dioxide in water.

This is an inverse relationship. Hot water holds less gas; cold water holds more gas.

3.2 Explaining the Reduction in Solubility (Using Particle Theory)

To understand why warm water holds less gas, we use the particle theory (which you studied in Section 2.1).

1. Heat Energy: When water is heated, the water molecules gain kinetic energy and move around much faster and more vigorously.
2. Escaping Gases: These faster-moving water molecules collide with the dissolved gas molecules (\(O_2\) or \(CO_2\)).
3. Exit Strategy: These collisions transfer enough energy to the gas molecules that they can break the weak bonds holding them in the water and escape back into the atmosphere (or form bubbles).

Therefore, warm surface water in tropical regions tends to have less dissolved oxygen than cold deep water in polar regions.

3.3 The Investigation (Syllabus 2.3.4 PA)

You may be asked how to investigate this effect. The basic method relies on observing or measuring the gas lost as water is heated.

Method Outline:

• Prepare several samples of water (e.g., tap water or simulated seawater).
• Heat each sample to a different, fixed temperature (e.g., 5°C, 15°C, 25°C, 35°C).
• Measure the Dissolved Oxygen (DO) concentration using a DO probe or a chemical titration method (like the Winkler method) for each temperature.
• Expected Result: As the temperature increases, the measured DO concentration should decrease, confirming the inverse relationship.

Common Mistake to Avoid: Don't confuse the rate of chemical reactions (which increases with temperature) with the amount of gas water can physically hold (which decreases with temperature).

Key Takeaway: Solubility of gases decreases as temperature increases because faster moving water molecules force the gas molecules to escape the liquid. This is vital because marine organisms need dissolved oxygen to survive.