Teacher Demonstration 29
Ocean Acidification
This demonstration provides a clear, visual introduction to one of the important
consequences of increasing atmospheric carbon dioxide concentrations on seawater
called ocean acidification. A student exhales through a straw into a solution of tap
water and aquarium pH indicator. The changing colour demonstrates that increasing
the amount of dissolved carbon dioxide causes water to become more acidic.
The Catalyst (ABC1) “Ocean Acidification” story can be viewed here:
www.abc.net.au/catalyst/stories/s2029333.htm
Materials:
• 2 glasses or beakers
• Baking soda
• Straw
1
2
• Tap water
• White vinegar
• Safety goggles
• Aquarium pH Indicator 1, 2
• Citric acid
Ordinary aquarium pH indicator is readily available from pet stores – the photographs
below show the results using this pH indicator (pH 6 = Yellow / pH7.6 = Blue) .
Alternatively, you can use phenolphthalein pH indicator for a more dramatic result. You
will need to add a few drops of limewater (builder’s lime dissolved in water) to the water
sample first to make a slightly basic solution. Blowing bubbles of exhaled air through this
solution will cause it to change from bright pink to perfectly clear.
Instructions
1 Add several drops of aquarium pH indicator to each
of two separate beaker or glass of water. The water
will appear green / blue indicating a pH of roughly
7.2 to 7.6 (use the colour chart provided with the
indicator solution).
2 Add a few drops of white vinegar (acetic acid) to one
of the beakers to demonstrate the effect of an acid on
the colour of the pH indicator solution. The solution
will turn yellow indicating a pH 6.0 or less.
Teacher Demo 29 – Ocean Acidification | Page 1
© 2010 | ABC Science Online
3 Next, sprinkle a few grains of baking soda to the same
beaker to demonstrate the effect of a base on the
colour of the pH indicator solution. The solution will
turn blue indicating a pH of 7.6 or more (note: the pH
of baking soda in water is 8.2 but this is outside the
range of the pH indicator solution).
4 Now sprinkle a few grains of citric acid into the same
beaker and the solution will change colour to yellow
again.
Before proceeding, discuss the reaction of the pH
indicator to acids and bases (ie it turns yellow when
the solution becomes acidic and blue when it
becomes basic.
5 Ask a student volunteer to blow into the second
beaker of water for thirty seconds. The solution will
slowly but visibly change colour from blue/green to
yellow/green.
Suggested discussion:
Ask the class to observe what has happened to the pH of the solution. They should
successfully deduce that the solution has become acidic.
Discuss what could be causing the solution’s pH to change and, if necessary, guide
them by asking them to recall the difference between exhaled air and fresh air.
Once you have established that exhaled air contains much more carbon dioxide than
fresh air, the class may deduce that it is this gas that causes the pH to change.
It is unlikely that your students will immediately link this demonstration with the
potential problems of rising atmospheric carbon dioxide concentrations and the impact
on marine organisms and ecosystems. The Catalyst (ABC TV) story on ocean
acidification summarises the evidence for ocean acidification and its potential
consequences.
Teacher Demo 29 – Ocean Acidification | Page 2
© 2010 | ABC Science Online
Teacher notes
The majority of scientists now agree that climate change as a result of increased
concentrations of carbon dioxide and other greenhouse gas from the burning of fossil
fuels is a reality.
Atmospheric concentrations of carbon dioxide have risen from 280 parts per million in
the pre-industrial era to about 380 parts per million today.
Another major concern that has only been realised in the last decade or so is the
potential impact of the increased concentrations of carbon dioxide on seawater. As this
demonstration shows, increasing the amount of carbon dioxide dissolved in water
decreases pH.
The rapid changes already being observed in ocean chemistry are likely to affect
organisms that rely on carbonate ions to build external calcium carbonate shells and
skeletons because a reduction in pH reduces the amount of raw materials available for
them to do so.
The surface waters of the world’s oceans are slightly alkaline. The average pH is 8.2
but this figure varies up and down by 0.3 pH units due to local, regional and seasonal
variations.
Scientists estimate that the average pH of surface waters has already fallen by 0.1 units
since pre-industrial times and predict further reductions of as much as 0.5 units or
more by the year 2100. More research is required to make accurate predictions but
changes in the thickness of shells is already being observed in some parts of the ocean
(see ABC TV’s Catalyst “Ocean Acidification” story).
While the ocean pH may have been lower before, it is the rate at which the pH is
changing that is of greatest concerns. During slow changes, the ocean’s chemistry
provides a buffer against a rapid loss of the carbonate ions many marine organisms
rely on to build external shells and skeletons.
References:
The Royal Society “Ocean acidification due to increasing atmospheric carbon dioxide”
June 2005
A PDF of this report can be downloaded at:
http://royalsociety.org/Ocean-acidification-due-to-increasing-atmospheric-carbondioxide/
Teacher Demo 29 – Ocean Acidification | Page 3
© 2010 | ABC Science Online