Geography K–10 · Years 7–8
Weathering: freeze–thaw on wet sandstone and acid on chalk
Landscapes and landforms (ACARA Year 8 sub-strand); NSW Stage 4 focus area Landscapes and Landforms (2015 syllabus) and Landscapes and landforms (2024 syllabus)
The idea
Rock is broken down in place by physical stress (water expanding as it freezes) and by chemical reaction (acid dissolving calcium carbonate), and both can be measured as mass lost.
Safety card
Hazards
- vinegar in eyes
- rock dust
Controls
- safety glasses when pouring vinegar; rinse with water if splashed
- brush loose grains outdoors or over a tray
Note
White vinegar (about 4 % acetic acid) is the only chemical: check it against the NSW Department of Education Chemical Safety in Schools package and record the practical in RiskAssess (riskassess.com.au). The freezer part uses no chemicals.
What you need
- Part A: 6 pieces of soft sandstone or house brick, about 50 g each; balance (0.1 g); 2 zip bags; access to a freezer; paper towel for drying
- Part B: 6 sticks of blackboard chalk (calcium carbonate; check the packet, some chalk is calcium sulfate); 2 × 250 mL jars; 200 mL white vinegar (about 4 % acetic acid); 200 mL tap water; paper towel
How to do it
- Part A: air-dry all six pieces on paper towel in a warm, dry place, weighing each once a day until two weighings in a row agree within 0.1 g (constant mass); record that mass. Soak three pieces in water for 1 hour and bag them wet; keep three dry as controls. Freeze both bags overnight; thaw for the day; repeat for 5 cycles. Brush loose grains off, dry all six to constant mass the same way, and reweigh. Mass loss = start − end.
- Part B: weigh three chalk sticks; place them in the vinegar jar. Weigh three and place them in the water jar. Observe for 5 minutes and record what is seen.
- After 24 hours lift the sticks out and rinse each one under clean water over a sink before drying: the calcium acetate the reaction makes is soluble, and vinegar left in the porous stick dries to a solid residue that would be weighed as though it were chalk, biasing the loss low at exactly the 0.1 g the measurement turns on. Then pat dry, air-dry to constant mass (two daily weighings in a row within 0.1 g) and reweigh. Mass loss for each jar.
- Tabulate mean mass loss for wet-frozen, dry-frozen, vinegar and water.
What you should see
Part A: wet-frozen pieces may lose grains and crack; five cycles can give only a small loss, so the balance reading (0.1 g) and a close look for new cracks are both recorded, and dry controls are expected to lose little or none. Part B: chalk in vinegar fizzes at once (carbon dioxide bubbles) and loses a measurable mass over 24 hours; chalk in water stays intact with near-zero loss. The learner reports grams lost per treatment. Porous sandstone and chalk hold absorbed water for days, so a piece weighed before it reaches constant mass can show no loss or even a gain. Chalk of calcium sulfate does not fizz, which is itself a result to record. Part B is limited by the acid supplied rather than by the rock: 200 mL of vinegar at about 4 per cent holds about 8 g, or 0.133 mol, of acetic acid, and two moles of acid dissolve one mole of calcium carbonate, so the jar can dissolve at most about 6.7 g of chalk in total across its three sticks however long they stay in. Weigh the chalk at the start and compare it with that ceiling: where the chalk outweighs it, the measured loss reports the acid the jar was given and not a property of the rock, and a fresh 200 mL would dissolve about as much again.
What changes
- What you change
- treatment (wet or dry freezing; vinegar or water)
- What you measure
- mass loss (g)
- What you keep the same
- starting mass
- cycles or soaking time
- temperature
- same rock or chalk batch
- dried to constant mass before and after
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Rocks only break when something hits them.
- Water cannot break rock because it is soft.
- Weathering and erosion are the same process.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Geography K–10 Syllabus (2015), NESA; current and taught in 2026, replaced from 2027. The outcome codes are not printed on the NESA landing page for this syllabus, which only links the document, so the url given is the syllabus document itself (geography-k-10-syllabus-2015.docx); the code and its outcome text were read in that document on 23 September 2026.GE4-2GE4-7
- Geography 7–10 Syllabus (2024), NESA; implementation from 2027, not yet taught; code read on this page on 22 September 2026GE4-PRI-01GE4-TAP-01
- Australian Curriculum v9AC9HG8K01AC9HG8S02AC9HG8S04
Sources
The pages the author read to write this activity.