Dissolving Is Not Always Exothermic

Comparing temperature changes of different salt dissolutions

Thermochemistry Beginner (Middle School) 25 min ~$8.00

Objective

Compare the enthalpy of dissolution of different salts (NaOH, CaCl₂, NH₄NO₃, KNO₃) to show that dissolution can be exothermic or endothermic.

Background

Students often assume dissolving always releases heat. This experiment tests four salts: NaOH and CaCl₂ dissolve exothermically (temperature rises), while NH₄NO₃ and KNO₃ dissolve endothermically (temperature drops). The results illustrate that lattice energy vs hydration energy determines the sign of ΔH.

Safety Warnings

  • NaOH is extremely corrosive
  • NH₄NO₃ is an oxidizer
  • Wear goggles and gloves

Required PPE

goggles gloves

Materials

  • Sodium hydroxide pellets (10 g)
    Exothermic
  • Calcium chloride (10 g)
    Exothermic
  • Ammonium nitrate (10 g)
    Endothermic
  • Potassium nitrate (10 g)
    Endothermic
  • Distilled water (400 mL)

Equipment

4 polystyrene cups Thermometer Stirring rod Analytical balance

Procedure

1

Add 100 mL water to each of four labeled cups. Record the initial temperature of each.

5 min
2

Add 10 g NaOH to cup 1. Stir and record the maximum temperature.

3 min Very exothermic — NaOH is corrosive
3

Add 10 g CaCl₂ to cup 2. Stir and record the maximum temperature.

3 min
4

Add 10 g NH₄NO₃ to cup 3. Stir and record the minimum temperature.

3 min
5

Add 10 g KNO₃ to cup 4. Stir and record the minimum temperature.

3 min
6

Create a table ranking the salts from most exothermic to most endothermic.

3 min
7

Discuss why some dissolve exothermically and others endothermically (lattice energy vs hydration energy).

5 min

Expected Results

NaOH: +15-20°C rise. CaCl₂: +10-15°C rise. NH₄NO₃: -15-20°C drop. KNO₃: -5-10°C drop. Clear demonstration that dissolution can be either exothermic or endothermic.

Cleanup

NaOH and CaCl₂ solutions should be neutralized before disposal. All solutions can be poured down the drain after dilution.

Details

Category
Thermochemistry
Difficulty
Beginner (Middle School)
Duration
25 min
Est. Cost
$8.00
Steps
7
Materials
5

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Frequently Asked Questions

What is the objective of Dissolving Is Not Always Exothermic?
Compare the enthalpy of dissolution of different salts (NaOH, CaCl₂, NH₄NO₃, KNO₃) to show that dissolution can be exothermic or endothermic.
How difficult is Dissolving Is Not Always Exothermic?
This experiment is rated as Beginner (Middle School). It takes approximately 25 minutes to complete.
What safety precautions are needed for Dissolving Is Not Always Exothermic?
Key safety precautions include: NaOH is extremely corrosive; NH₄NO₃ is an oxidizer; Wear goggles and gloves.
What materials are needed for Dissolving Is Not Always Exothermic?
The main materials required are: Sodium hydroxide pellets, Calcium chloride, Ammonium nitrate, Potassium nitrate, Distilled water.
What results should I expect from Dissolving Is Not Always Exothermic?
NaOH: +15-20°C rise. CaCl₂: +10-15°C rise. NH₄NO₃: -15-20°C drop. KNO₃: -5-10°C drop. Clear demonstration that dissolution can be either exothermic or endothermic.