Science 7–10 · Year 8

Sediment settling in a jar: sorting, layers and Stokes' law

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

Practical, model not builtLow risk

This site has no interactive model of its own. Where a step or a material names a Concept Studio model, simulation or tool, it has not been built; an external simulation a step names (for example PhET) is not part of this site.

The idea

Grains settle at speeds set by their size, so one stirred mixture lays down a graded bed with the coarsest grains at the base, the pattern that lets a geologist read which way up a rock was deposited.

Safety card

Low riskAn adult supervises

Hazards

heavy jar of water
wet floor
fine dust from dry potter's clay powder, which contains crystalline silica

Controls

carry with two hands over a tray
mop spills at once
an adult wets the clay powder in a lidded jar before the lesson, away from the learners, so learners handle only wet clay; spills are wiped with a damp cloth, never swept dry

Note

Potter's clay powder contains crystalline silica, which the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update, Figure 1.8-1) lists as a substance requiring health surveillance; handled wet it raises no dust. Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) or through RiskAssess.

What you need

1 tall clear jar or 1 L measuring cylinder with a lid, per group
a mixture of 100 g fine gravel (2 to 4 mm), 100 g coarse sand (about 1 mm), 100 g fine sand (about 0.1 to 0.2 mm) and 50 g clay or silt (potter's clay powder, stirred into some of the water in a lidded jar by an adult before the lesson)
water to fill to 30 cm depth; 1 stopwatch; 1 ruler taped to the jar

How to do it

Add the gravel, the two sands and the pre-wetted clay to the jar, fill with water to the 30 cm mark, cap and shake for 10 seconds.
Stand the jar and start the stopwatch; record the time when the gravel has landed, when the coarse sand layer has stopped growing, when the fine sand has settled and how long the water stays cloudy.
After 24 hours measure the thickness of each layer and sketch the jar.
Repeat with a second jar to check the times.
Compare the settling times with the Stokes' law predictions from the simulation for a 0.1 mm grain and a 0.02 mm grain over 30 cm.

What you should see

Gravel lands within a second or two, the coarse sand within a few seconds, the fine sand within about a minute and the clay stays in suspension for hours (still cloudy after the lesson and clearing by the next day), producing a graded bed coarse at the base and fine at the top. Stokes' law with the NIST water values at 20 degrees C (density 998.21 kg/m cubed, viscosity 1.0016 mPa s) and a grain density of 2650 kg/m cubed (the value the model assumes for quartz sand; a class measurement can replace it) gives 9.0 mm/s for a 0.10 mm grain, so 33 seconds for 30 cm, and 0.36 mm/s for a 0.02 mm grain, so 14 minutes; grains of 1 mm fall at about 0.9 m/s in the model but that is outside Stokes' range (Reynolds number about 900), which is why the model is only applied to the fine sand and silt. The learner knows it worked when the layers appear in size order and the fine-sand time is within a factor of two of the 33 second prediction.

What changes

What you change: grain size
What you measure: settling time (and layer order)
What you keep the same: water depth
water temperature
shaking time
sediment masses

Common misconceptions

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

Heavier grains always land first whatever their size; settling speed depends on size and density together, and for the same mineral it is size.
Clay settles quickly because it is dense; the tiny grains fall slowly because drag dominates.
A thick pile of layers forms in one flood; each graded bed records one settling event, so a cliff of such beds records many events.

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-01 SC4-WS-04
Australian Curriculum v9AC9S8U04AC9S8I03AC9S8I04

Sources

The pages the author read to write this activity.

All Concept Studio activities