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Science and Technology K–6 · Year 5

Freeze-thaw weathering: ice cracks a plaster rock

Science understanding: Earth and space sciences

PracticalLow risk

The idea

Water in a crack expands by about 9 percent when it freezes and can split rock, a slow change that a plaster brick with a water-filled cavity reproduces in a freezer overnight.

Safety card

Low riskAn adult supervises

Hazards

  • plaster sets hot and must never be cast around a hand or body part
  • plaster dust
  • latex allergy (a balloon or a latex glove finger is natural rubber latex)

Controls

  • adult mixes plaster
  • cast only in cups
  • dust mask when handling dry powder
  • no plaster down the sink
  • check allergy records; use a nitrile glove finger instead if a learner has a latex allergy

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

What you need

  • plaster of Paris, 500 g, and water to mix
  • 2 disposable cups as moulds
  • a small balloon or a finger of a rubber glove, filled with 20 mL of water, all air squeezed out, and tied
  • access to a freezer
  • camera

How to do it

  1. Mix the plaster and pour it into two cups. Push the water-filled balloon into the middle of one cup before it sets; leave the other cup as the control.
  2. Let both set for a day and peel off the cups. Photograph both bricks.
  3. Put both bricks in the freezer overnight, take them out and let them thaw for a few hours. Photograph again.
  4. Repeat the freeze-thaw cycle up to five times, photographing after each.
  5. Describe when the first crack appears and how it grows; compare with the control.
  6. Compute the volume the 20 mL of water becomes as ice and explain the force on the plaster.

What you should see

Twenty millilitres of water becomes 21.8 mL of ice (density 0.9998 to 0.9167 g per cubic centimetre), a 9.1 percent expansion that pushes outward on the plaster from inside; an air bubble left in the balloon would take up that extra room instead, which is why the air is squeezed out. The brick with the water pocket is the one expected to crack, and the learner records the cycle at which the first crack appears and how it grows; the control brick, with no water pocket, is the comparison. Rock in alpine country cracks the same way over many winters.

What changes

What you change
water pocket present or absent; number of freeze-thaw cycles
What you measure
appearance and length of cracks
What you keep the same
  • plaster mix
  • brick size
  • freezer temperature

Common misconceptions

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

  • Ice is soft, so it cannot break rock.
  • Cold shrinks everything.
  • Rocks only break when something hits them.

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 and Technology K-6 Syllabus (2017), current, taught until 2026; a Stage 2 outcome, because NSW teaches gradual weathering and erosion in Stage 2 ('investigate why the Earth's surface changes over time as a result of natural processes and human activity'), while the Stage 3 surface-change content under ST3-10ES-S covers sudden geological change and extreme weather events, its other content group being Earth's place in our solar system; code read from the syllabus document on 22 September 2026ST2-10ES-S
  • Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; Stage 2 content: 'Explain how the forces produced by wind, water and living things weather rocks'; code read from the outcomes page and the content read from the Stage 2 content page on 22 September 2026ST2-SCI-01
  • Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-1WS-S
  • Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01
  • Australian Curriculum v9AC9S5U02AC9S5I01AC9S5I03

Sources

The pages the author read to write this activity.

  1. www.scootle.edu.au/ec/search?accContentId=AC9S5U02
  2. curriculum.nsw.edu.au/learning-areas/science/science-and-technology-k-6-2024/outcomes
  3. www.nsw.gov.au/education-and-training/nesa/curriculum/science/science-and-technology-k-6-2017
  4. primaryconnections.org.au/teaching-sequences/year-5/wear-on-earth
  5. edu.rsc.org/primary-science/freezing-and-the-intriguing-ice-experiment/4013619.article

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