Science 7–10 · Year 7

Cooling curve of stearic acid: temperature holds steady while a liquid freezes

Solutions and mixtures

Practical, model not builtMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

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

During a change of state the temperature stays constant even though the substance keeps losing heat, because the energy goes into rearranging particles rather than slowing them.

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 and Bunsen burner: scalds and burns
thermometer can break if forced through solidified stearic acid
molten stearic acid is hot

Controls

heat-resistant mat, tie back hair, stand while heating
never force the thermometer once the acid is solid; remelt to free it
run a burn under cool water for 20 minutes

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson. Stearic acid is low hazard; the risk is heat.

What you need

boiling tube about one-third full of stearic acid (about 15 g)
thermometer 0 to 100 °C or a temperature probe with data logger
250 mL beaker of water on tripod and gauze, Bunsen burner, heat-resistant mat
clamp stand and boss, stop clock, graph paper

How to do it

Stand the boiling tube in the beaker of water and heat the water; record the stearic acid temperature every minute, note the temperature at which the solid starts to melt, and carry on until the acid is fully melted and at least 5 °C above that temperature, about 75 °C. The RSC method stops at about 70 °C, which leaves no measurable liquid stretch before the plateau for a sample that freezes near the top of its range.
Lift the tube out, clamp it in air, and record the temperature every minute (or every 30 seconds with a probe) as it cools to about 50 °C. Do not stir once the acid starts to solidify.
Plot temperature against time for the cooling run and mark the flat section.
Read the freezing point from the plateau and compare with the class median.

What you should see

The cooling curve falls while the liquid cools, then flattens for several minutes while solid forms, then falls again. Pure stearic acid freezes at about 69 °C; the RSC notes that school samples usually melt and freeze somewhere between about 55 and 70 °C and, being impure, give a plateau that slopes gently rather than lying perfectly flat. Starting the cooling run at about 75 °C is what makes the first fall long enough to read: from a 70 °C start a sample freezing near 69 °C would drop into its plateau within one reading. The learner knows it worked when at least three consecutive readings lie within 2 °C of each other while the tube is clearly still losing heat to the room, and the class plateaus agree with one another.

What changes

What you change: time
What you measure: temperature of the stearic acid
What you keep the same: mass of stearic acid
room temperature
reading interval
thermometer position in the tube

Common misconceptions

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

Temperature must fall the whole time a substance is cooling.
Freezing and melting happen at different temperatures for the same pure substance.
The flat part of the graph means the heater or the room has stopped working.

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)SC4-SOL-01 SC4-WS-05
Australian Curriculum v9AC9S7U05AC9S7I04AC9S7I05

Sources

The pages the author read to write this activity.

All Concept Studio activities