Sea Change

Sea Change: pH, universal indicator and carbon dioxide for Cambridge IGCSE Marine Science

This free game is part of our Cambridge iGCSE Marine Science (0697) course for exams in EXAM YEARS 2027-2029.

Acidic, neutral and alkaline: the pH scale

In Sea Change you work in a marine chemistry lab on the research ship Coral Star. It covers the pH part of the Cambridge IGCSE Marine Science (0697) course: describing neutrality, acidity and alkalinity in terms of pH, testing sea water, fresh water and rain water with universal indicator, and investigating what adding carbon dioxide does to the pH of sea water.

• pH 7 is neutral. Below 7 is acidic: the lower the number, the more acidic. Above 7 is alkaline: the higher the number, the more alkaline.
• Universal indicator shows the pH as a colour. Red, orange and yellow are acidic, green is neutral, and blue and purple are alkaline.
• In the game the colour chart sits at the bottom of the screen and you tap the colour that matches each sample.

Testing sea water, fresh water and rain water

The first mission is the practical for testing the pH of three kinds of water with universal indicator.

• Put the same volume of each water in a clean test tube and add the same number of drops of universal indicator to each (3 drops in the game), so it is a fair test.
• Compare each colour with the chart against a white background.
• Sea water is about pH 8: slightly alkaline, so it turns universal indicator blue-green.
• Fresh water from a river is close to neutral, about pH 7, so it goes green.
• Rain water is slightly acidic, about pH 5.6, because carbon dioxide from the air dissolves in it and forms carbonic acid.

Then three tubes lose their labels, and you use the pH of each one to work out which water it is.

Adding carbon dioxide to sea water

In the Breath Test you blow gently through a straw into a beaker of sea water with universal indicator, next to a control beaker that gets nothing. Your breath is about 4% carbon dioxide, compared with about 0.04% in normal air.

• The indicator changes gradually, from blue-green (about pH 8) to green (pH 7) to yellow (about pH 6), while the control stays blue-green.
• A graph of pH against the number of breaths builds on the whiteboard: the more carbon dioxide dissolves, the lower the pH.
• Carbon dioxide + water ⇌ carbonic acid (CO₂ + H₂O ⇌ H₂CO₃). Carbonic acid is a weak acid that releases hydrogen ions (H⁺) and hydrogencarbonate ions, and more hydrogen ions means a lower pH.
• The independent variable is the number of breaths and the dependent variable is the pH. Keep the volume of sea water, the drops of indicator and the temperature the same, and repeat the test.

In the Gas Lab and Ocean Lab missions a carbon dioxide cylinder gives a steady flow. You open and close a gas valve on each beaker and stop it when its colour matches the target pH. Each step down the pH scale needs more carbon dioxide than the step before, and sea water needs more carbon dioxide than fresh water to reach the same pH.

The real ocean: ocean acidification

The game ends at the reef tank with a pH meter. The ocean absorbs about a quarter of the carbon dioxide people add to the air. Its pH was about 8.2 in 1750 and is about 8.1 today, and it could be about 7.8 by 2100 if carbon dioxide keeps rising. A fall of just 0.1 means about 26% more hydrogen ions.

The sea is still alkaline, though, and universal indicator still shows blue-green. Ocean acidification means the pH is falling towards neutral, not that the sea has become an acid. Even this small change makes it harder for mussels, oysters and corals to build their calcium carbonate shells and skeletons. The change is too small for universal indicator to show, so scientists use pH meters on ships and buoys.

The four missions

Test the Waters: add universal indicator to sea, fresh and rain water, read each pH on the chart, then identify three unlabelled samples. Breath Test: blow carbon dioxide into sea water next to a control and watch the colour and the graph change. Gas Lab: open the gas valves and stop each beaker on its target colour, and leave the controls alone. Ocean Lab: four beakers of sea, fresh and rain water, mystery samples to read and controls, against the clock. Every mistake shows a short explanation, and quick checks between missions test the key ideas.