Science 7–10 · Year 8

Freeze-thaw weathering of porous rock in a freezer

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

PracticalLow risk

The idea

Water expands by about 9 percent when it freezes, so water in pores and cracks levers rock apart a little more on each freeze, and porous rocks fall apart first.

What you need

2 plastic trays, each with the same set of six rocks: granite, basalt, porous sandstone, limestone, slate, gneiss (Earthlearningidea selection)
water to cover the rocks; access to a freezer for 5 to 10 freeze-thaw cycles over about two weeks
1 electronic balance (0.1 g); 1 sieve (1 mm) and paper towel for collecting loose grains
1 plastic syringe (20 mL) with its nozzle blocked with Blu Tack or clay, a few drops of food colouring and a millimetre rule, for the water-expansion measurement (the Earthlearningidea Ice power method)

How to do it

Dry and weigh each rock; place matching sets in the two trays and cover with water. Keep one tray at room temperature as the control.
Freeze the other tray overnight, thaw it by day, and repeat for 5 to 10 cycles; note on each thaw whether grains have collected on the tray floor.
After the last cycle pour off the water through the sieve, dry the loose grains and weigh them; weigh each rock again after drying.
Tabulate mass lost per rock for both trays and identify the rock that lost most.
Ice power: block the syringe nozzle, fill it with 10.0 mL of cold water tinted with food colouring and measure the length of the water column in millimetres; freeze it between lessons, measure the length of the ice column in millimetres, and compute the expansion as (ice length - water length) / water length x 100 percent, as the Earthlearningidea Ice power sheet does. The barrel's cross-section cancels in that ratio, so the two lengths give the volume change.

What you should see

The freezer tray holds many more loose grains than the control, and most of them are sandstone grains: the Earthlearningidea result is that the porous sandstone breaks up fastest, while the rocks that are not porous do not break up this way. The ice in the syringe takes more room than the water did: 10.0 mL of water should become about 10.9 mL of ice (Earthlearningidea states water expands by 9 percent on freezing; HyperPhysics gives ice at 0 degrees C 0.92 g/cm cubed against 1.00 for water). Measured by column length rather than by the 1 mL graduations, the reading is fine enough to pin the 9 percent: the uncertainty is the half millimetre a rule resolves divided by the water column's own length, so a column of 30 mm or more keeps it under 2 percent, against the 5 percent that half a graduation, 0.5 mL in 10 mL, would leave. Plunger friction, air coming out of the water as it freezes and ice gripping the barrel can each hold the reading low. The learner knows it worked when the freezer tray has clearly more loose grains than the control and those grains match the sandstone; the balance then shows whether the sandstone's loss after 5 to 10 cycles is above its 0.1 g resolution.

What changes

What you change: freeze-thaw cycles (frozen tray against control) and rock type
What you measure: mass of grains shed
What you keep the same: same rock types and sizes
same water depth
same number of days

Common misconceptions

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

Weathering needs wind or rivers; freeze-thaw happens with the rock standing still.
Hard rocks weather slowly and soft rocks fast; porosity, not hardness, decides freeze-thaw damage.
Ice is heavier than water because it is solid; it is less dense, which is the expansion that breaks rock.

Safety card

Low riskLearners carry it out

Hazards

freezer burns from long contact
wet floor when thawing

Controls

gloves when removing trays
trays on a towel while thawing

Note

No hazardous chemical and no flame or heating apparatus: a generic classroom risk assessment (CSIS 1.7 or RiskAssess) covers trips, spills, warm lamps and sharp edges.

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-03 SC4-WS-04 SC4-WS-05
Australian Curriculum v9AC9S8U04AC9S8I02AC9S8I05

Sources

The pages the author read to write this activity.

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