Cambridge IGCSE Marine Science Practical Skills 1: Sea-water pH, salinity and dissolved substances

Cambridge 0697 Paper 2 assesses practical understanding in familiar and unfamiliar contexts. This chapter develops the marked Topic 2 activities on comparing water pH, adding carbon dioxide to seawater, temperature and solute solubility, and temperature and gas solubility. It also develops defensible water sampling and salinity evidence. Density measurements are owned by Practical Skills 2.

Collect representative water samples

Define the question before sampling. A comparison of estuary salinity should state locations, tidal stage, sampling depth and timing. Use clean labelled containers, rinse them with sample water where appropriate, and avoid sediment unless bottom water is the target. Record date, time, coordinates or site description, weather, tide and water temperature.

Take independent samples rather than repeatedly testing one bottle and calling them replicates. Collect the same volume and use the same depth and container at each site. If a gradient is expected, sample at predetermined intervals from river to sea. Avoid choosing sites because their water looks different.

Comparing pH with universal indicator

Cambridge specifies investigating seawater, freshwater and rainwater with universal indicator. Place equal sample volumes in clean tubes, add the same indicator volume, mix consistently and compare against the supplied colour scale under the same lighting. Record both colour and estimated pH.

Universal indicator gives an approximate value. A pH probe can provide finer resolution in an unfamiliar extension, but it must be calibrated with suitable buffer solutions, rinsed between samples and allowed to stabilise. Do not claim greater accuracy merely because a display has more digits.

Use a known buffer or reference sample to check the indicator or probe. Temperature can affect readings, so compare samples at the same temperature where possible. Coloured or turbid water can mask indicator colour, which is a limitation rather than evidence of a different pH.

Carbon dioxide and seawater pH

To investigate carbon dioxide addition, place equal seawater volumes in identical vessels and measure initial pH. Bubble carbon dioxide through the experimental sample at a controlled flow for set time intervals. Treat a sample exposed to ordinary air or the same handling without added carbon dioxide as the control. Measure pH after each interval.

Keep initial sample source, volume, temperature, mixing, gas-delivery depth and measurement method constant. A delivery tube should not be used by mouth. If exhaled air is used only under an approved school procedure, prevent liquid from entering the tube and avoid cross-contamination.

Plot pH against bubbling time or delivered gas volume. A fall supports the conclusion that dissolved carbon dioxide lowered pH under the tested conditions. It does not establish the exact atmospheric response of an ocean, because gas flow, buffering, mixing and timescale differ.

Temperature and solute solubility