Sea water
Explain how particle behaviour, the water cycle, dissolved substances and gases, density, depth and upwelling shape seawater conditions.
Exam scope and assessment notes
This original lesson maps Cambridge IGCSE Marine Science 0697 syllabus sections 2.1, 2.2, 2.3, 2.4, 2.5, 2.6 for examinations in 2024–2029. These are original StudyForA explanations and practice, not Cambridge questions, mark schemes or official grades. Paper 2 assesses practical skills in both familiar and unfamiliar contexts. This course can rehearse planning, measurement, data handling and evaluation, but it cannot provide real laboratory or coastal-field experience, verify equipment use, certify safety, or award a practical mark. Follow teacher supervision, local safety rules and environmental permissions for all practical work.
Before you begin
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before you start: What does salinity measure?
What you will learn
- Use particle theory to explain states, diffusion, temperature and changes of state
- Interpret water-cycle processes and factors affecting evaporation
- Describe pH, salinity, dissolved gases, density and their measurement
- Explain how depth and upwelling alter marine conditions, including El Niño effects
Water cycle and particle ideas
Solids, liquids and gases differ in particle arrangement and motion. Temperature measures average particle kinetic energy. Diffusion is the net movement from higher to lower concentration due to random particle motion. Evaporation, condensation, freezing and melting involve energy changes; wind, temperature, exposed surface area and humidity affect evaporation.
Trace evaporation, condensation, precipitation and surface run-off. A practical comparison of floating ice and land-based ice should control starting water level, ice mass, container and melt conditions, then compare repeated readings. Floating ice displaces water before melting; land ice adds water to the system.
pH, salts and dissolved gases
Use element, compound and mixture precisely. pH describes acidity or alkalinity; universal indicator can compare seawater, freshwater and rainwater. Carbon dioxide dissolving in seawater changes pH. Seawater is a solution containing dissolved salts such as sodium chloride, magnesium sulfate and calcium carbonate.
Salinity is dissolved-salt concentration, commonly recorded in parts per thousand (ppt). The Pacific has typical salinity, the Baltic is relatively low and the Red Sea relatively high. Run-off, precipitation, evaporation, ice melt, temperature and rock erosion can affect salinity; estuaries vary during tidal mixing. Oxygen and carbon dioxide dissolve from air; warmer water holds less of these gases.
Density, depth and upwelling
Density = mass ÷ volume. Temperature and salinity affect seawater density; density differences can drive convection. Colder water generally lies beneath warmer water. With depth, light falls, pressure rises and temperature, salinity and dissolved oxygen can change. A Secchi disc estimates water transparency; it does not measure every wavelength or directly measure all water quality.
Winds can drive upwelling of cold, nutrient-rich water. El Niño involves weakened or reversed trade winds, warmer eastern Pacific surface water and reduced upwelling there, with regional effects on temperature, nutrients and rainfall. Explain this linked pattern without claiming that every locality changes identically.
Sources for this lesson
Pause and explain
As water warms, what generally happens to the solubility of oxygen in it?
Worked example
Two seawater samples have equal volume. Sample A has mass 1.02 kg and Sample B 1.03 kg. Calculate each density if each volume is 1.00 L, and state which is denser.
Show the worked solution
- Use density = mass ÷ volume.
- For A: 1.02 kg ÷ 1.00 L = 1.02 kg/L.
- For B: 1.03 kg ÷ 1.00 L = 1.03 kg/L; compare using the same units.
Answer Sample B is denser at 1.03 kg/L, compared with 1.02 kg/L for A. The result is meaningful only if mass and volume were measured consistently and the samples were at comparable conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A student compares the effect of temperature on dissolved oxygen using one cold sample and one warm sample from different locations. Identify the main flaw and improve the method.
Give me a hint
Separate temperature from location and other water differences.
Compare my reasoning
- Identify location and composition as uncontrolled variables.
- Use the same source water, equal volumes and containers.
- Change temperature systematically, repeat readings and use the same oxygen-measurement method.
Location may change salinity, organisms and starting oxygen, so the comparison does not isolate temperature. Use one mixed batch of water, controlled volumes, multiple temperatures and repeats; measure oxygen with the same calibrated method.
Look for these in your work
- Identifies confounding variables
- Controls starting sample and volume
- Includes a range and repeats
An estuary sample is 32 ppt at high tide and 24 ppt at low tide. Suggest why salinity changes and calculate the difference.
Give me a hint
Relate tide to seawater and freshwater mixing.
Compare my reasoning
- Subtract 24 ppt from 32 ppt.
- Recognise that the tide changes the proportion of incoming seawater.
- Consider river flow, rainfall and sampling location before generalising.
The difference is 8 ppt. A greater proportion of seawater at high tide can raise salinity; river water can lower it. The pattern depends on river flow, rainfall and where and when the sample was taken.
Look for these in your work
- Calculates an 8 ppt difference
- Explains mixing
- Qualifies the local pattern
A researcher observes cooler surface water with elevated nutrients off a coast after strong winds. Explain a plausible mechanism and one further measurement.
Give me a hint
Connect wind direction, surface-water movement and deep nutrient-rich water.
Compare my reasoning
- Explain that wind can move surface water away from a coast.
- Describe colder, nutrient-rich deeper water rising to replace it.
- Measure nutrients and temperature at multiple depths with a consistent sampling plan.
The wind may have driven coastal upwelling: surface water moved away, and colder nutrient-rich water rose. A depth profile of temperature and nutrient concentration can test the pattern; one surface sample alone cannot establish upwelling.
Look for these in your work
- Explains upwelling
- Connects temperature and nutrients
- Proposes depth-resolved evidence
Common mistakes
- Using pH and salinity as if they measure the same property
- Claiming warmer seawater holds more dissolved oxygen
- Forgetting units in density calculations
- Describing El Niño as a single local weather event
What can you explain now?
As water warms, what generally happens to the solubility of oxygen in it?
Compare with the explanation
It decreases
Use the scientific relationship, relevant evidence and correct units. State the limit of any conclusion when the evidence does not support a universal claim.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
How can temperature affect both dissolved gases and the density structure of seawater?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.