Ocean acidification in a cup

This activity shows how water becomes more acidic when carbon dioxide is bubbled through it. Thus, it demonstrates the link between carbon dioxide in the atmosphere and a process called ocean acidification, a change in the pH, or acidity, of the ocean. This change in ocean chemistry has an impact on many living things in the oceans including coral polyps, clams, prawns, and other sea life.

Ages 10+

30 minutes

(10 minutes preparation)

Part of:

AXA Ocean Education

How to do it How to do it
What you'll need What you'll need
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It is best to use water that has been boiled and then cooled. This lessens the amount of calcium carbonate in the water, which can slow the process of acidification.

Activity steps - Blowing through water

Fill two clear cups two thirds full with water.
Take a drinking straw and blow through the water of one of the cups. This bubbles the carbon dioxide from exhaled breath through the water.
Taking a rest every 15 seconds, blow for 2 to 3 minutes through the water.
Before pouring your red cabbage indicator into each of the cups, try to predict how the colour of the indicator may change. Hint: the red cabbage indicator will turn pinker if the water is more acidic and bluer if the water is less acidic.
Pour 50ml of red cabbage water into each cup and observe the difference between the water in the two cups.

If you want to see a more dramatic difference between two cups of water, try this method.

Activity steps - Using still and sparkling water

Fill one clear cup with tap water and one with sparkling water.
Before pouring your red cabbage indicator into each of the cups, try to predict how the colour of the indicator may change. Hint: the red cabbage indicator will turn pinker if the water is more acidic and bluer if the water is less acidic.
Pour 50ml of red cabbage water into each cup and observe the difference between the water in the two cups.

Safety guidance

Each participant will need their own straw.
To avoid dizziness due to excessive exhalation, participants should swap who is blowing every 30 seconds.
Participants should carry their containers with two hands, carefully observing the environment around them. Spills and breaks should be reported to an adult immediately.

2 clear plastic cups
Drinking straw
pH indicator *
Tap water
Sparkling mineral water (optional)

* We suggest red cabbage indicator because you can make it at home by boiling up red cabbage in a little water. Be aware that this can get a little smelly so make sure to open a window. Make sure that the red cabbage water is cooled before it is handled by children. You can also use other pH indicators, such as store-bought indicator for testing water which is usually available from pet stores.

The other carbon problem

Ocean acidification is also known as the other carbon problem. It is the process by which carbon dioxide (CO₂) in the atmosphere dissolves in the upper waters of the ocean (H₂O) and through a chemical reaction becomes carbonic acid (H₂CO₃).

Activity Ocean Acidification Cup Figure 3 — Ocean acidification diagram

This increased acidity decreases the amount of carbonate ions available in the ocean. Many living things rely on carbonate to build body structures. Coral polyps use carbonate to create little 'cups' that they live in and use as protection from predators. Crustaceans, such as prawns or lobsters, and molluscs, such as clams and scallops, also rely on carbonate to build their exoskeletons, or 'shells'.

Gallery Coral Life Staghorn Coral — Coral polyps are soft-bodied animals that rely on carbonate to build 'cups' called corallite that protect their bodies from predators.

Lower levels of carbonate in the ocean means that coral polyps and other living things need to expend more energy taking what is left to create coral reefs and shells. As more energy is spent on building structures, less energy is available for other processes such as reproduction and growth and can even led to starvation. Ocean acidification happens in all areas of the oceans and may have ripple effects on ocean health.

Gallery Uk Marine Life Scallop — Scallops are soft-bodied animals that have hard shells that they build over time using carbonate from the ocean.

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