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.