Crystallization heat
To demonstrate an exchange of heat between a system and its environment — in this case the opposite of the dissolution of a salt.
What students learn
The science behind this experiment
Changes of state imply a transfer of energy between a substance and its environment. During melting, substances absorb heat and the attraction between the molecules or ions breaks down. This experiment examines the behaviour of a hot concentrated solution as it cools to room temperature.
As already seen in the experiment on the dissolving of a solute in water, a change of state is a thermodynamic matter which involves both enthalpy changes and entropy changes.
When the solute crystallises, there is a decrease in its entropy which contrasts with the decrease in enthalpy but is not enough to prevent the spontaneous crystallization of the melted salt.
Equipment used
- Test tube holder — 5423.30
- Spatula — 5460.34
- Thermometer — 2245.15
- Spirit burner — 5514.10
- Iron stand — 5404.51
- Universal clamp — 5416.20
- Right angle clamp — 5401.16
Chemicals and reagents
- Sodium acetate
Safety notes
- Take care when using the burner and hot glassware. Always use a test tube holder when heating test tubes in a flame.
Questions for students
- Describe what happens on inserting the thermometer in the liquid.
- Record the temperature during the crystallisation.
- State whether the crystallisation is exothermic or endothermic.
- Will dissolving sodium acetate in water give out or absorb heat?
What to expect
<p>There is a sudden crystallization (stirring or movement of the liquid helps the crystallization). If the crystallization does not happen, inserting some crystals can help the process.</p><p>Heat is evolved and the process is therefore exothermic.</p><p>According to this experiment, dissolving sodium acetate in water should produce a decrease of the temperature. This is easily confirmed by adding sodium acetate to a small quantity of distilled water.</p><p>This is because more energy is necessary to overcome the attraction between the ions than is produced when the ions coordinate with the water molecules.</p>
Disposal
Dispose of sodium acetate in the solid wastes container.
Run this experiment in your classroom
This experiment is part of the ATP Mobile Lab — a self-contained laboratory that turns an ordinary classroom into a working science lab, with over 200 experiments in physics, chemistry, biology, robotics and engineering. Gali, the AI tutor built into ATP Connect, guides students through each step and answers their questions at the bench.
See the Mobile LabTalk to our team