Earth and Environmental Science 11–12 · Year 12

Ocean acidification: carbon dioxide lowering the pH of water and attacking carbonate shells

Module 7: Climate Science

Practical, model not builtLow risk

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The idea

Carbon dioxide dissolving in water forms carbonic acid, which lowers pH and dissolves calcium carbonate, the chemistry that threatens shell-building marine life.

What you need

two beakers of tap water with bromothymol blue (or a pH meter), a straw
seashells or chalk pieces weighed to 0.01 g; white vinegar for an accelerated dissolution run
artificial seawater (35 g/L sea salt) for the buffered comparison
balance, stopwatch

How to do it

Record the starting pH of the tap water and the seawater.
Blow through the straw into the tap water for 60 s and record the colour or pH every 15 s; repeat in the seawater.
Place weighed shell pieces in vinegar and in the acidified water; reweigh after 24 hours.
Compare the pH drop in tap water with seawater and explain the difference (buffering by carbonate).
Run the model to see the pH pure water reaches in equilibrium with today’s air and with exhaled breath.

What you should see

The indicator turns from blue-green to yellow within a minute of blowing, and a meter shows the tap-water pH falling while seawater falls by a smaller amount. Pure water in equilibrium with 420 ppm carbon dioxide reaches pH 5.61 and with exhaled breath (about 4 % carbon dioxide) pH 4.62, from K_H = 3.4 × 10⁻² mol L⁻¹ atm⁻¹ (NIST WebBook lists 0.034 to 0.035 mol/(kg·bar)) and the standard 25 °C value K₁ = 4.3 × 10⁻⁷. Shell pieces in vinegar lose measurable mass in 24 hours and fizz. The Earthlearningidea sheet states that ocean surface pH has fallen from 8.21 to 8.10 since the Industrial Revolution, a fall of 0.11 and about a 30 % increase in acidity.

What changes

What you change: carbon dioxide added (time blowing) and water type
What you measure: pH; mass lost by the shell
What you keep the same: water volume
starting temperature
indicator amount
shell size

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

The ocean is becoming acid (it is becoming less alkaline; pH stays above 7).
Only warming harms coral.

Safety card

Low riskLearners carry it out

Hazards

blowing back through the straw (only blow, never suck)
vinegar in eyes

Controls

one straw per person
safety glasses

Note

Chemicals are used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1 (general information for all staff, including the CSIS 1.7 risk assessment requirement), read the Safety Data Sheet for each chemical and complete a RiskAssess risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES12-14 EES11/12-1 EES11/12-6 EES11/12-7
Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-12-03
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities