Science 7–10 · Year 8

Chemical weathering: carbonic acid from your own breath on limestone

Science understanding: Earth and space sciences (NSW Stage 4 focus area: Change)

PracticalMedium risk

School laboratory, not for home

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

The idea

Carbon dioxide dissolving in water makes carbonic acid, and that weak acid dissolves limestone and marble; the same reaction shapes caves and weathers gravestones.

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

1 mol/L hydrochloric acid (irritant)
indicator splashes
blowing, not sucking, through the straw

Controls

safety glasses
one straw per learner, discarded after use
learners told to blow only
acid handled in small volumes over a tray

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

2 boiling tubes or 100 mL beakers per group, quarter filled with distilled or deionised water
universal indicator solution, a few drops per tube, and a colour chart
unused drinking straws (one per learner)
ground limestone or powdered chalk (calcium carbonate, not slaked lime), a small spatula; one limestone chip a few millimetres across for the indicator tube, and two similar limestone chips of about 1 g (about 10 mm across) for the rate comparison
1 electronic balance (0.01 g) and 1 mol/L hydrochloric acid, 20 mL, for the rate comparison

How to do it

Add indicator to the water and record the starting colour against the chart: distilled or deionised water that has stood open to the air has already taken up some carbon dioxide and reads about pH 5.6, yellow to yellow-orange rather than green (as in the ocean carbon dioxide practical); tap water may start green or blue because it is often made alkaline (Earthlearningidea). Blow steadily through the straw into the water for a minute and record the colour change.
Add a small spatula of powdered limestone, agitate, and record the colour a few seconds later.
Repeat in the second tube, but add a single limestone chip of about the same mass instead of powder; record the colour after one minute and again next lesson (tube stoppered).
Rate comparison: weigh the two 1 g limestone chips, drop one in 10 mL of 1 mol/L acid and one in 10 mL of water, and reweigh after 5 minutes (rinse and dry first).
Write the equations: H2O + CO2 gives H2CO3; H2CO3 + CaCO3 gives Ca(HCO3)2 (soluble).

What you should see

Exhaled breath moves the indicator from its starting colour toward orange, and with persistent blowing red-orange or pink (a weakly acidic solution of carbonic acid; the Earthlearningidea sheet describes the change from green to yellow, orange or even pink for water that starts neutral). Powdered limestone returns the colour to green or slightly blue within seconds and the solution goes cloudy with suspended powder; the single chip produces little change within the lesson because its surface area is far smaller, which is why the sheet has the tube kept until the next lesson. In the rate comparison the chip in 1 mol/L acid fizzes and loses mass the balance can read within five minutes while the chip in water loses none, showing the natural process at speed. The chip has to be about 1 g: 10 mL of 1 mol/L acid holds 10 mmol of hydrogen chloride and each carbonate consumes two of them, so the acid can dissolve at most 0.50 g of calcium carbonate and a 1 g chip always leaves a residue to rinse, dry and reweigh, while a chip a few millimetres across, about 0.07 g, would disappear altogether. The learner knows it worked when the powder, not the chip, neutralises the breath acid within the lesson.

What changes

What you change: surface area of the limestone (powder against chip) and acid strength
What you measure: time for the indicator colour to return and mass lost
What you keep the same: water volume
indicator amount
blowing time
limestone mass

Common misconceptions

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

Only strong acids dissolve rock; carbonic acid from rain and soil water is weak but dissolves limestone and marble over time.
Weathering moves rock away; weathering breaks it down in place, erosion moves it.
Breath is just used air; it carries enough carbon dioxide to acidify water in a minute.

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)SC4-CHG-01 SC4-WS-01 SC4-WS-04
Australian Curriculum v9AC9S8U04AC9S8I02AC9S8I05

Sources

The pages the author read to write this activity.

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