Cambridge IGCSE Marine Science 2: Sea water

Topic 2 connects the physical and chemical properties of seawater to ocean circulation and marine conditions. Cambridge 0697 requires particle theory, the water cycle, pH and salinity, dissolved gases, density, depth-related change, upwelling and El Niño. Practical procedures for testing these relationships belong to the separate practical hub.

Particle theory and the water cycle

In a solid, particles are closely packed in fixed positions and vibrate. In a liquid, particles remain close but move past one another, so the liquid flows and takes its container's shape. In a gas, particles are widely separated and move rapidly and randomly, filling the available space. Temperature measures the kinetic energy of particles and is measured in degrees Celsius in this syllabus.

Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration because of random motion. It does not require particles to stop moving once concentrations become equal; random movement continues, but there is no net movement in one direction.

During melting and evaporation, water absorbs energy. During freezing and condensation, it releases energy. The water cycle transfers water through evaporation, condensation, precipitation and surface run-off. Faster evaporation is promoted by higher temperature, stronger air movement, a larger exposed surface and lower humidity.

Melting floating ice causes little direct water-level rise because it already displaces water. Melting land-based ice adds water that was previously on land, raising sea level. The practical comparison belongs in the practical hub, but the displacement reasoning belongs here.

Elements, compounds, mixtures and solutions

An element contains one type of atom, such as oxygen. A compound contains elements chemically combined in fixed proportions, such as calcium carbonate. A mixture contains substances not chemically bonded in fixed proportions. Seawater is a mixture whose composition varies.

Water is the solvent in seawater. Dissolved salts are solutes, and together they form a solution. Sodium chloride and magnesium sulfate are soluble salts. Calcium carbonate has limited solubility and can form shells, skeletons and sediments. “Dissolved” does not mean a substance has ceased to exist; its particles are dispersed through the solvent.

pH and carbon dioxide

The pH scale describes acidity, neutrality and alkalinity. Values below 7 are acidic, 7 is neutral, and values above 7 are alkaline. Seawater is normally slightly alkaline, but the exact value can vary.

Carbon dioxide from air dissolves in water. Some reacts with water and lowers pH. Therefore, adding carbon dioxide to seawater can make it less alkaline. A lower pH is more acidic even if the new value remains above 7. Avoid saying that any pH decrease automatically makes a solution acidic.

Salinity and estuaries

Salinity is the concentration of dissolved salts, expressed here in parts per thousand, or ppt. The Pacific Ocean has a typical ocean salinity. The Baltic Sea has low salinity because freshwater input and precipitation are large relative to evaporation and exchange. The Red Sea has high salinity because evaporation is high and freshwater input is low.

Rock erosion releases mineral material, and run-off carries dissolved substances towards the sea. Precipitation and melting ice dilute seawater, while evaporation removes water and leaves dissolved salts behind, increasing salinity. Temperature can influence evaporation and therefore salinity indirectly.

An estuary is a partly enclosed tidal body of water where a river enters the sea. Fresh water and seawater mix, so salinity changes with position and through the tidal cycle. A single salinity reading cannot represent the whole estuary.

Dissolved gases

Oxygen and carbon dioxide from the atmosphere dissolve in water. Oxygen has low solubility, so aquatic organisms depend on a comparatively limited dissolved supply. Although oxygen is much more abundant than carbon dioxide in air, biological processes can change their relative dissolved concentrations in seawater.

Increasing temperature reduces the solubility of oxygen and carbon dioxide. Warm water can therefore contain less dissolved oxygen at saturation than cold water. Photosynthesis adds oxygen and uses carbon dioxide; respiration and decomposition use oxygen and release carbon dioxide. Explain a measured concentration using both physical solubility and biological activity where the data justify it.

Density and convection

Density is mass per unit volume:

$$\rho = \frac{m}{V}$$

If $m$ is measured in grams and $V$ in cubic centimetres, density is in $\text{g cm}^{-3}$. If kilograms and cubic metres are used, it is in $\text{kg m}^{-3}$

Heating usually causes water particles to move faster and occupy a greater volume, lowering density. Increasing salinity adds dissolved mass and generally increases density. Denser water tends to sink below less dense water. This movement can establish convection currents, with sinking dense water and rising less dense water. The syllabus expects the general conclusion that colder water lies beneath warmer water; avoid importing unnecessary exceptions beyond this boundary.

Changes with depth

Light penetration decreases with depth, and detailed wavelength absorption is not required. Pressure increases because a deeper point supports a taller column of water. Temperature generally falls from warmer surface water to colder deep water, though actual profiles vary. Salinity can also vary with water masses and mixing.

Dissolved oxygen does not follow one simple rule at every depth. Surface exchange and photosynthesis can raise it near the surface. Respiration and decomposition consume it below. Colder deep water can dissolve more oxygen, while circulation also transports oxygen. Interpret the particular profile supplied rather than memorising a single straight trend.

Upwelling and El Niño

Wind can move surface water away from an area, allowing cold, nutrient-rich deep water to rise. This upwelling supplies nutrients to surface producers and can support high productivity and fisheries.

During El Niño, trade winds reduce or reverse. Surface water becomes warmer in the eastern Pacific and eastern-Pacific upwelling weakens. Lower nutrient delivery can reduce productivity and affect food webs and fisheries. Rainfall patterns also shift, with effects in Australia and Asia differing from those in the eastern Pacific. Describe these as linked local effects, not as identical weather everywhere.

Seawater system map

Worked application

Two coastal samples have equal volumes of $100\text{ cm}^3$. Sample A has mass $102.4\text{ g}$ at $24^\circ\text{C}$, while sample B has mass $103.1\text{ g}$

Common misconceptions

• “Particles stop moving when diffusion reaches equilibrium.” Random movement continues without net transfer.
• “Melting floating ice raises sea level like melting land ice.” Floating ice already displaces water.
• “A dissolved salt disappears chemically.” Its particles remain dispersed in the solution.
• “Any fall in pH makes seawater acidic.” It may remain alkaline while becoming less alkaline.
• “Evaporation removes salt with the water.” It removes water and usually raises salinity.
• “Warm water holds more dissolved oxygen.” Gas solubility decreases as temperature rises.
• “Density and mass are interchangeable.” Density is mass per unit volume.
• “El Niño increases upwelling in the eastern Pacific.” Reduced or reversed trade winds weaken it.

Assessment guidance

Build explanations as linked mechanisms. For salinity, name the freshwater or evaporation process and its dilution or concentration effect. For density, state how temperature or dissolved salt changes mass per volume, then link density to sinking or rising. Treat depth profiles as data to interpret, especially for dissolved oxygen. El Niño responses should connect trade-wind change, eastern-Pacific warming, reduced upwelling, nutrient availability and local consequences. Show density substitutions with consistent units. Keep indicator, Secchi-disc and controlled-solubility procedures in practical-skills answers.

Retrieval practice

1. Explain diffusion and four changes of state using particle theory and energy transfer.
2. Compare floating-ice and land-ice effects on water level.
3. Predict how evaporation, rainfall, run-off and ice melt affect salinity.
4. Explain how temperature affects dissolved gases and density.
5. Interpret light, pressure, temperature and oxygen profiles with depth.
6. Trace an El Niño sequence from trade winds to productivity and rainfall effects.

Return to the Marine Science hub.