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Carbon storage and ocean acidification activity

In this activity you will see that extra carbon dioxide increases the acidity of the oceans.

You will need

  • Two small containers
  • Plasticine
  • A bendable drinking straw
  • A syringe
  • Sodium bicarbonate (bicarb soda)
  • Vinegar
  • A pH indicator – bromothymol blue, phenol red or you can make your own red cabbage indicator
  • A tiny amount of laundry detergent if you are using red cabbage indicator.

What to do


  1. Roughly chop some red cabbage and boil it for around 10 minutes.
  2. Drain and keep the liquid.


  1. Put a couple of spoons of sodium bicarbonate into one container.
  2. Make a cover to seal the container using the plasticine, with the short end of the bendable straw going into the container. Make sure it is well sealed.
  3. Put some water with a little indicator in the second container.
  4. If you are using red cabbage indicator, mix in a tiny amount of laundry detergent to make the mixture slightly basic and change the colour to green. This will give you a more obvious colour change when you do the experiment.
  5. Trim the long end of the bendable straw so it goes into the container of indicator and down to the bottom.
  6. Draw up a syringe full of vinegar and carefully work it through the plasticine seal on the first container, use the plasticine to make an airtight seal around it.
  7. Being careful of all your seals push the vinegar down into the container. Watch the end of the straw in the water to see what happens.
  8. To do it again, gently work the syringe out and put your finger over the hole while you refill it, then repeat step 7.

What’s happening?

Indicators are chemicals that change colour when they are in acids or bases. Many plants with strong purple or red colouring can be made into indicators. Red cabbage indicator goes green in a base or red in an acid. bromothymol blue is green in neutral solutions like water and yellow in acids, while phenol red goes from yellow in neutral solutions to pink in acids. What you are seeing in the experiment is the change from the neutral or slightly basic water to acidic when carbon dioxide (CO2) bubbles through it.

When you mix the vinegar and sodium bicarbonate together you produce CO2 gas through two reactions. The vinegar acts as a weak acid and the bicarbonate is a weak base, they react together to neutralise each other.



Acetic acid + sodium bicarbonate -> sodium acetate + carbonic acid

Then the carbonic acid breaks down:

H2CO3 -> H2O + CO2

Carbonic acid -> Water + Carbon dioxide

When the CO2 goes through the straw and bubbles up through the water, it reverses the second reaction and produces some carbonic acid again.

H2O + CO2 -> H2CO3

Water + Carbon dioxide -> Carbonic acid

This carbonic acid changes the colour of the indicator, eventually it will turn the whole solution acidic.


Much of the increasing levels of atmospheric carbon dioxide (CO2) will be absorbed into the oceans, but this will take several centuries. More CO2 in the ocean will make it more acidic – this is called ocean acidification. Some researchers are investigating carbon capture and storage (CCS) technologies that will inject CO2 deep into the oceans, which will remove it immediately and may stop it mixing with the surface water and making it more acidic. Currently the ocean is very slightly alkaline: making it more acidic is dangerous – for example, it can stop many animals from being able to make shells.

In the deep oceans there is very high pressure and low temperatures. Researchers are investigating the theory that if CO2 pumped is down there, whether it would stay as a liquid rather than a gas and not move much. However we don’t know much about how it interacts with the water around it. There is some evidence that it forms carbonic acid and affects some of the sea creatures around it.

We also don’t know how rising global temperatures will affect this type of storage – as the oceans become hotter it may melt the hydrates and make the liquid CO2 more likely to mix with the water around it.

We also don’t know how rising global temperatures will affect this type of storage – as the oceans become hotter it may melt the hydrates and make the liquid CO2 more likely to mix with the water around it.

We need to learn more about how the oceans will react to increasing amounts of CO2, both from the atmosphere and direct injections of captured CO2.

Ocean acidification for kids

Deep injection of CO2