Earth and Environmental Science 11–12 · Year 11

Crystal size and cooling rate with salol on warm and cold slides

Module 1: Earth’s Resources

PracticalLow risk

The idea

Slow cooling grows large crystals and fast cooling grows small ones, which is why intrusive and extrusive igneous rocks of the same composition look different.

Safety card

Low riskAn adult supervises

Hazards

hot water from the kettle
warm slides
salol in the eyes

Controls

safety goggles (Earthlearningidea resource list)
water bath at about 45 °C, never boiling
handle warm slides with a cloth or tongs

Note

Heat or hot water is used: follow the NSW Department of Education Chemical Safety in Schools (CSIS) package, Section 1, and record a RiskAssess risk assessment before the lesson.

What you need

salol (phenyl salicylate), about 5 g for the class, in a boiling tube (Earthlearningidea resource list)
six clean microscope slides per group: two kept in a freezer, two at room temperature and two warmed in hot water and dried
250 mL beaker of hot water from a kettle as a water bath at about 45 °C, thermometer, stirring rod
hand lens or low-power microscope, millimetre scale, stopwatch, safety goggles
granite and basalt specimens for comparison

How to do it

Melt the salol in the boiling tube in the water bath at about 45 °C, only just enough to melt it; Earthlearningidea warns that overheating makes the liquid supercool and spoils the result.
With the stirring rod put a few drops of melted salol on a freezer-cold slide and cover it at once with the second cold slide; repeat with the room-temperature pair and then the warm pair.
Watch each pair with the hand lens; time how long crystals take to appear and to fill the slide, and sketch them against a millimetre scale.
Measure the longest crystal in each pair and tabulate slide temperature against crystal length and time to crystallise.
Match the results to obsidian or basalt (fast cooling) and granite (slow cooling), and repeat any run that crystallised out of order.

What you should see

Crystals appear first and grow fastest on the cold slides, as a mass of small crystals. Earthlearningidea’s stated outcome is that the largest crystals form on the room-temperature slides, and that on a warm slide the crystals are predicted not to form immediately but to grow slowly and be large; the learner’s own measurements are the result. Crystal size falls as the cooling rate rises. Because liquid salol can supercool and then crystallise at the wrong temperature, the sheet suggests doing the activity twice; a run that gives the wrong order is reported, not discarded.

What changes

What you change: cooling rate (slide temperature: freezer, room or warm)
What you measure: largest crystal length in mm and time to crystallise
What you keep the same: salol only just melted
same amount of salol per slide
clean slides of the same type
same scale for measuring

Common misconceptions

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

Big crystals mean the rock is old.
All igneous rocks form from lava at the surface.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Earth and Environmental Science Stage 6 Syllabus (2017), NESA; current, replaced by the 11–12 Syllabus (2025) from 2028EES11-8 EES11/12-3 EES11/12-4 EES11/12-5
Earth and Environmental Science 11–12 Syllabus (2025), NESA; to be implemented from 2028, not yet taughtEES-11-02
Investigating Science Stage 6 Syllabus (2017), NESA; currentINS11-10
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

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