Science 7–10 · Years 9–10

Carbon dioxide into water: acidity, and why seawater resists it

Science understanding: Earth and space sciences (ACARA Year 9 carbon cycle and Year 10 climate; NSW Stage 5 focus areas Environmental sustainability and Reactions)

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Carbon dioxide dissolves in water and lowers its pH; water carrying dissolved hydrogencarbonate, as seawater does, resists the change, which is how the ocean takes up a large share of emitted carbon dioxide while slowly becoming less alkaline.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

hydrochloric acid 1 mol/L irritates eyes and skin (low hazard at this concentration per the RSC)
a blocked delivery tube can pop the bung from the generator

Controls

eye protection
delivery tube kept open and never clamped shut
generator flask stands on the bench, not held
neutralise spent acid with sodium hydrogencarbonate before disposal

Note

Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) of the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update), or through RiskAssess; keep the Safety Data Sheet for every chemical with the class register.

What you need

carbon dioxide generator: a 250 mL conical flask with about 10 g of marble chips and 50 mL of hydrochloric acid 1 mol/L, a one-hole bung and a delivery tube (the RSC method), with a wash bottle (a boiling tube a third full of water, the gas bubbling through it) between the generator and the beaker to catch acid spray
beaker A: 200 mL of deionised water in a 250 mL beaker
beaker B: 200 mL taken from a stock of 0.20 g of sodium hydrogencarbonate dissolved in 1.00 L of deionised water (2.38 mmol/L, a model buffered ocean), in a 250 mL beaker
universal indicator solution, 10 drops per beaker, and a pH meter or probe calibrated with pH 4 and pH 7 buffers
a stop clock and eye protection

How to do it

Calibrate the meter and record the starting pH and indicator colour of beakers A and B, both of which have stood open to room air.
Bubble carbon dioxide from the generator, through the wash bottle, into beaker A for 5 minutes, recording the pH every 30 seconds; stir gently and let each reading settle, because a glass electrode responds slowly in water with as few ions as deionised water.
Recharge the generator with fresh chips and acid and repeat with beaker B.
Plot pH against time for both beakers on one graph and compare the final values with the model's equilibrium limits.
Leave both beakers uncovered overnight and measure the pH again as the extra carbon dioxide escapes.
Repeat the whole run three times and report the mean final pH of each beaker.

What you should see

In equilibrium with air at 420 ppm the carbonate model gives pH 5.60 for deionised water and pH 8.56 for the 2.38 mmol/L hydrogencarbonate water; if the room air holds 1,000 ppm the starting values are 5.41 and 8.19. Bubbled with pure carbon dioxide at 1 atm and 25 degrees C, the equilibrium limits are pH 3.91 for A and 5.20 for B. So A falls from about 5.6 toward 4 within minutes (universal indicator moves from yellow-orange to red-orange), while B starts near 8.6 and stops more than one pH unit above A. Overnight both drift back toward their air values. In beaker A the meter lags and drifts, since a glass electrode responds slowly in nearly ion-free water, so A's readings are approximate and the indicator colour is the check on them; a reading well below 3.9 points to acid spray from the generator, which the wash bottle is there to stop. The learner knows it worked when B finishes above A in every trial and A's settled readings approach pH 3.9 without going clearly below it. NOAA states the ocean absorbs about 30 percent of the carbon dioxide released into the atmosphere; the Global Carbon Budget gives 2.9 of 11.1 GtC in 2023, 26 percent.

What changes

What you change: dissolved hydrogencarbonate (none or 2.38 mmol/L), and time bubbling
What you measure: pH
What you keep the same: water volume
gas supply (same generator charge)
temperature
amount of indicator

Common misconceptions

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

Emitted carbon dioxide all stays in the air; about a quarter of each year's emissions dissolves into the ocean.
The ocean is turning acidic; it is still alkaline and is becoming less alkaline.
Fresh water and seawater respond alike to carbon dioxide; the hydrogencarbonate in seawater buffers the change.

Curriculum references

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

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC5-ENV-01 SC5-RXN-01 SC5-WS-01 SC5-WS-05
Australian Curriculum v9AC9S9U03AC9S10U04AC9S9I03AC9S9I05

Sources

The pages the author read to write this activity.

All Concept Studio activities