Investigating Science 11–12 · Year 11

Reactions of calcium carbonate: limewater, carbon dioxide and cave formation

Module 2: Cause and Effect – Inferences and Generalisations

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 first precipitates calcium carbonate from limewater and then redissolves it as calcium hydrogencarbonate; heating drives the gas off and the solid returns, the chemistry that dissolves caves and builds stalactites.

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

limewater irritates eyes
1 mol/L hydrochloric acid irritates eyes and skin
hot water bath
suck-back of liquid into the generator

Controls

safety glasses
remove the delivery tube from the limewater before stopping the gas
handle hot tubes with tongs
dispose of solutions as the school’s CSIS procedures direct

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.

What you need

freshly filtered limewater (saturated calcium hydroxide solution), 20 mL in each of two boiling tubes
a carbon dioxide generator: conical flask with marble chips and 1 mol/L hydrochloric acid, bung and delivery tube
water bath at about 80 °C, test-tube rack, stopwatch
a limestone chip in dilute acid for comparison

How to do it

Bubble carbon dioxide from the generator steadily through the first tube of limewater; record the time to first cloudiness and to the most opaque stage.
Keep bubbling until the tube clears, and record the time.
Pour half of the clear solution into a clean tube and stand it in the 80 °C water bath for 5 minutes; record what happens.
Leave the other half open to the air overnight and record the result.
Write word and balanced equations for each stage and link them to rainwater dissolving limestone and to dripping water depositing calcite in a cave.

What you should see

The limewater turns milky within seconds to a minute as calcium carbonate precipitates (Ca(OH)₂ + CO₂ → CaCO₃ + H₂O). With continued gas it clears again as soluble calcium hydrogencarbonate forms (CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂); this second step needs as much carbon dioxide again as the first, one more mole per mole of calcium, so clearing takes longer than clouding at the same gas flow. Heating the clear solution turns it milky again as carbon dioxide is driven off, and the tube left open to the air slowly forms a crust, the reverse reaction that deposits stalactites.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

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

Once limewater turns milky it cannot change back.
Caves are dug out by flowing water alone, with no chemistry involved.
Stalactites grow because water freezes.

Curriculum references

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

Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-9 INS11/12-1 INS11/12-2 INS11/12-4
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities