Science 7–10 · Year 8

Cooling rate and crystal size with molten salol

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

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

Slow cooling gives atoms time to join large crystals, fast cooling freezes many small ones; that is why granite is coarse and basalt is fine.

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

hot water bath and warm slides (scald and burn)
salol (phenyl salicylate), which the RSC lesson rates as minimal hazard; molten salol can splash
glass slides can crack

Controls

teacher runs the water bath and issues the warm tubes
safety glasses
handle warm slides by the edges
wipe spills with paper towel once set

Note

Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) of the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update), or through RiskAssess; keep the Safety Data Sheet for every chemical with the class register.

What you need

salol (phenyl salicylate), a test tube about a quarter full per group, melted in a water bath at about 45 to 50 degrees C (the Geological Society sheet uses about 45 degrees C; the RSC lesson about 50 degrees C)
1 glass rod or dropping pipette per tube, kept in the warm tube
4 microscope slides per group: two chilled in a fridge (the Geological Society sheet; the RSC lesson also allows a beaker of ice) and two warmed to about 60 degrees C in a beaker of hot water (the RSC lesson, drying them just before use) or on a radiator (the Geological Society sheet)
1 hand lens or low-power microscope, 1 stopwatch, 1 sheet of dark paper
1 water bath (electric, thermostatic) and tongs for the teacher

How to do it

Collect a warm slide pair at the last moment. Place one drop of molten salol on the lower slide and lower the second warm slide onto it to spread the drop; start the stopwatch.
Watch through the lens on the dark paper; record the time the first crystal appears and the time growth stops.
Repeat immediately with the chilled pair.
Sketch both slides at the same magnification and estimate the typical crystal width with the slide's 25 mm edge as the scale.
Compare with hand specimens of granite and basalt and state which slide models each; repeat a second time if the first attempt was too slow and the slides changed temperature.

What you should see

On the chilled slides crystals appear within seconds and stop growing quickly, giving many small crystals (the RSC lesson notes that the coolest slide produces crystals first). On the warm slides the first crystal appears later and radiating crystals grow larger before they meet, giving fewer, bigger crystals. The learner knows it worked when the warm pair shows visibly fewer and larger crystals than the cold pair; if both look alike the slides were not at different temperatures when the drop landed.

What changes

What you change: slide temperature (cooling rate)
What you measure: time to first crystal and final crystal size
What you keep the same: drop size
same salol batch
same observer and lens
time between collecting slides and placing the drop

Common misconceptions

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

Big crystals grow from more material; they grow from more time.
Basalt has no crystals; it has many crystals too small to see without magnification.
Molten rock cools like water freezing into one solid mass; it grows many crystals that meet.

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 v9AC9S8U04AC9S8I02AC9S8I06

Sources

The pages the author read to write this activity.

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