Cold Pack Chemistry

Studying the endothermic dissolution of ammonium nitrate

Thermochemistry Beginner (Middle School) 20 min ~$6.00

Objective

Measure the temperature change when ammonium nitrate dissolves in water, demonstrating an endothermic dissolution process used in instant cold packs.

Background

Ammonium nitrate dissolves endothermically in water, absorbing heat from the surroundings and causing a significant temperature drop. This principle is used in commercial instant cold packs for first aid. The dissolution has ΔH = +25.7 kJ/mol.

Safety Warnings

  • Ammonium nitrate is an oxidizer — keep away from organic materials and heat
  • Do not mix with anything other than water for this experiment
  • Wear goggles

Required PPE

goggles

Materials

  • Ammonium nitrate (NH₄NO₃) (30 g)
    Or from a commercial cold pack
  • Distilled water (100 mL)
    Room temperature

Equipment

Polystyrene cup calorimeter Thermometer Stirring rod Analytical balance

Procedure

1

Add 100 mL of distilled water to the polystyrene cup calorimeter. Record the initial temperature.

3 min
2

Weigh 30 g of ammonium nitrate.

2 min Ammonium nitrate is an oxidizer
3

Add the ammonium nitrate to the water. Stir gently to dissolve.

3 min
4

Record the temperature every 30 seconds as it drops. Note the minimum temperature.

5 min
5

Feel the outside of the cup — it should feel cold.

1 min
6

Calculate ΔH of dissolution per mole: q = mcΔT, n = mass/M(NH₄NO₃), ΔH = q/n.

5 min

Expected Results

Temperature should drop 15-20°C. 30 g NH₄NO₃ = 0.375 mol. q = 100 × 4.18 × ΔT. Calculated ΔH should be approximately +25 kJ/mol (±15%).

Cleanup

The ammonium nitrate solution is safe to dispose down the drain (dilute). Rinse the calorimeter.

Details

Category
Thermochemistry
Difficulty
Beginner (Middle School)
Duration
20 min
Est. Cost
$6.00
Steps
6
Materials
2

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Frequently Asked Questions

What is the objective of Cold Pack Chemistry?
Measure the temperature change when ammonium nitrate dissolves in water, demonstrating an endothermic dissolution process used in instant cold packs.
How difficult is Cold Pack Chemistry?
This experiment is rated as Beginner (Middle School). It takes approximately 20 minutes to complete.
What safety precautions are needed for Cold Pack Chemistry?
Key safety precautions include: Ammonium nitrate is an oxidizer — keep away from organic materials and heat; Do not mix with anything other than water for this experiment; Wear goggles.
What materials are needed for Cold Pack Chemistry?
The main materials required are: Ammonium nitrate (NH₄NO₃), Distilled water.
What results should I expect from Cold Pack Chemistry?
Temperature should drop 15-20°C. 30 g NH₄NO₃ = 0.375 mol. q = 100 × 4.18 × ΔT. Calculated ΔH should be approximately +25 kJ/mol (±15%).