Try this: Breathe in… breathe out…

New Scientist reports that our oceans may continue to absorb human emissions of greenhouse gas more rapidly and for longer than expected, which is bad news for the marine organisms that are already suffering from ocean acidification. Try this activity to see the effect of carbon dioxide in water.

You will need

• Two transparent cups or other similar containers (plastic cups work well if you are doing this in a large group)
• A drinking straw
• pH indicator such as bromothymol blue (available at most pet and aquarium stores – see below if you can't find it)
• Tap water

What to do

1. Half fill each cup with tap water.
2. Add a few drops of bromothymol blue to each of the cups. The water will appear a pale blue colour.
3. Insert a drinking straw and gently blow bubbles into the water for several minutes. Be careful not to blow too hard as the water might splash into your eyes. You might like to wear safety goggles.
4. After a short time, less than a minute, the water will change colour.
5. Compare the colour of the bubbled water with the colour of the water in the other cup.

If bromothymol blue or a similar indicator is unavailable you can make your own using purple cabbage. Prepare two small cups or containers of cabbage water. The initial purple colour of the cabbage water indicates a pH of around 7.0.

What's happening?

When you blew into the water you will have noticed that the water changed from the original blue colour to a green, possibly pale yellow colour. This indicates that the water has changed from neutral to acid.

We describe whether things are acidic, basic or neutral by using a scale called the pH scale. Pure water has a pH of 7 and is regarded as neutral. pH ranges from 0 for a very strong acid, 3 - 5 for weak acids, 8 - 9 for weak bases, and 13 - 14 very strong bases.

With every breath, we take in oxygen and exhale carbon dioxide. By blowing into the water you are adding carbon dioxide (CO₂), making the water slightly acidic.

When carbon dioxide is bubbled through water, some of it dissolves into the water. That is, some of the carbon dioxide goes into the spaces between water molecules. A small proportion of this dissolved carbon dioxide creates carbonic acid, a weak acid.

Applications

Carbon dioxide is one of the greenhouse gases , so named because it helps to keep Earth's heat from escaping into space. The amount of carbon dioxide in our atmosphere is increasing and this is causing significant changes to our climate.

We need to decrease our carbon dioxide emissions but we also need to find ways of storing or removing the carbon dioxide that we do produce. This is called carbon capture and sequestration. Forests and oceans are large carbon sinks and they serve to partially offset greenhouse gas emissions. The Australian Government and CSIRO are also investigating the ability to capture carbon dioxide emissions and then inject the gas deep underground, removing it from the atmosphere and avoiding its harmful greenhouse effect.

More information

• CSIRO carbon sequestration research

Try this too: voice activated chemical reaction

If you have access to a lab you might want to try this ‘science magic' activity. You can change the colour of a solution just by speaking to it.

WARNING: Have an adult present. Avoid flame and have good ventilation. Do not use concentrated NaOH. Avoid contact with skin and eyes.

1. In a 500 millilitre flask, place 250 millilitres of 95 per cent ethyl alcohol.
2. Add 5-6 drops of bromothymol blue indicator to the alcohol.
3. If the solution is not blue, add enough dilute NaOH (sodium hydroxide) to produce the blue colour.
4. Pass the flask around the class with each person talking into the flask “please change to yellow”. Don't inhale the alcohol fumes.
5. The carbon dioxide in our breath will change the colour of the solution.