Science 7–10 · Year 9

Carbon locked in shells and limestone: carbonate and acid on a balance

Science understanding: Earth and space sciences (ACARA Year 9 carbon cycle; NSW Stage 5 focus area Reactions, content groups Law of conservation of mass and Chemical reactions)

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

Shells, coral and limestone store carbon as calcium carbonate; acid releases it as carbon dioxide, and the mass the flask loses on a balance measures how much carbon the sample held.

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 (the RSC lists it as low hazard at this concentration)
froth can carry acid spray out of the flask

Controls

eye protection throughout
cotton-wool plug in every flask; add the sample gently
wipe spills at once with plenty of water
neutralise the residue with sodium hydrogencarbonate before it goes to the sink

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

2.00 g (weighed to 0.01 g) of each of: marble chips, clean crushed seashell grit, and dried crushed eggshell
hydrochloric acid 1 mol/L, 50 mL per sample (0.050 mol, a 25 percent excess over the 0.040 mol that 2.00 g of pure calcium carbonate needs)
three 250 mL conical flasks, three cotton-wool plugs, a 50 mL measuring cylinder
an electronic balance reading to 0.01 g and a stop clock
eye protection and sodium hydrogencarbonate for neutralising the waste

How to do it

Put 50 mL of acid in a flask, rest the cotton-wool plug and the weighed sample paper beside it on the balance pan, and record the total mass.
Tip the sample into the acid, push in the plug at once, return the empty paper to the pan and start the clock.
Record the mass every minute until two readings a minute apart are equal.
Mass lost = carbon dioxide released. Calculate the calcium carbonate as mass lost x 100.09 / 44.01, its percentage of the 2.00 g sample, and the carbon as mass lost x 12.01 / 44.01.
Run three trials for each material and find the means.
Write the word and symbol equation (calcium carbonate + hydrochloric acid gives calcium chloride + water + carbon dioxide; CaCO3 + 2HCl gives CaCl2 + H2O + CO2) and explain where the carbon in a shell came from: sea animals build calcium carbonate from calcium ions and dissolved hydrogencarbonate taken from seawater.

What you should see

Pure calcium carbonate loses 0.879 g of carbon dioxide from 2.00 g (44.01 / 100.09 = 0.4397 g per gram), which is 0.240 g of carbon, 12.0 percent of the sample's mass. Marble chips come closest to this; shell grit and eggshell lose less because they also hold organic matter and water (the AACT eggshell practical measures the same carbonate by back titration). Results run low for two reasons. Some gas stays dissolved: 50 g of water can hold up to 0.075 g of carbon dioxide under pure carbon dioxide at 25 degrees C (NIST Henry's law constant 0.034 mol/(kg bar)), about 8.5 percent of 0.879 g. And the plug keeps carbon dioxide in the flask's 200 mL headspace, which is heavier than the air it replaced: at 25 degrees C and 101.3 kPa the headspace holds 0.00817 mol of gas, so filled with carbon dioxide (44.01 g/mol) instead of air (mean molecular weight 28.97 g/mol, NASA Earth Fact Sheet) it weighs up to 0.123 g more, 14.0 percent of 0.879 g; the two-equal-readings stop comes before that gas has diffused out. Together they can take a pure marble trial down to 0.681 g lost, 77.4 percent carbonate. Results run high if acid spray escapes past a missing plug. The learner knows it worked when three marble trials agree within 0.02 g and sit between 77 and 100 percent carbonate, with any shortfall accounted for by the two low biases.

What changes

What you change: carbonate source (marble, seashell, eggshell)
What you measure: mass of carbon dioxide lost
What you keep the same: sample mass 2.00 g
acid volume and concentration
particle size (crushed to similar pieces)
temperature

Common misconceptions

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

Carbon is stored only in living things and the air; limestone and shells lock it away in rock for millions of years.
The mass disappears in the reaction; it escapes from the flask as a gas, and a sealed flask would show no loss.
A shell is made of calcium alone; its carbon and oxygen came from carbon dioxide dissolved in seawater.

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-RXN-01 SC5-WS-01 SC5-WS-04
Australian Curriculum v9AC9S9U03AC9S9I03AC9S9I04

Sources

The pages the author read to write this activity.

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