Endothermic Experiments: Reactions That Absorb Heat and Feel Cold
Every chemical reaction involves breaking old bonds and forming new ones. Breaking bonds requires energy input, while forming bonds releases energy. If the energy required to break the old bonds exceeds the energy released by forming new ones, the reaction is endothermic and absorbs heat from its surroundings. If the reverse is true and more energy is released than consumed, the reaction is exothermic and releases heat. The difference between these two quantities is the enthalpy change of the reaction. Endothermic reactions have a positive enthalpy change because they gain energy, while exothermic reactions have a negative enthalpy change because they lose energy. You experience endothermic processes every time you use an instant cold pack, feel a cool breeze from evaporating sweat, or notice that dissolving salt in water makes the water slightly colder.
Understand Endothermic versus Exothermic Reactions
Energy diagrams provide a visual way to understand the difference between endothermic and exothermic reactions. In an exothermic reaction, the products have lower energy than the reactants, and the difference is released as heat to the surroundings. A burning match is exothermic: the products (carbon dioxide and water) have less chemical energy than the reactants (wood and oxygen), and the difference appears as heat and light. In an endothermic reaction, the products have higher energy than the reactants, and the difference must be absorbed from the surroundings as heat. Dissolving ammonium nitrate in water is endothermic: the dissolved ions have more energy than the solid crystal, and the missing energy is pulled from the water, making it colder. It is important to understand that endothermic reactions are not somehow less favorable or less real than exothermic ones. They proceed spontaneously when the increase in entropy (disorder) of the system is large enough to compensate for the energy absorption. Dissolving salt in water is endothermic, yet it happens spontaneously because the entropy increase of dispersing ordered crystal ions into disordered solution more than compensates for the energy cost.
Make a Baking Soda and Vinegar Cold Reaction
The reaction between baking soda and vinegar is mildly endothermic, meaning it absorbs a small amount of heat from the surroundings. Place a cooking thermometer or digital probe thermometer into a glass or beaker. Add half a cup of white vinegar and record the starting temperature. Add two tablespoons of baking soda all at once and stir while watching the thermometer. The vigorous fizzing produces carbon dioxide gas, and the temperature should drop by 3 to 8 degrees Celsius depending on the amounts used and the starting temperature. The drop occurs because the reaction between acetic acid and sodium bicarbonate absorbs energy from the surrounding liquid. The endothermic nature of this reaction is often overlooked because the dramatic fizzing draws attention away from the temperature change. To make the effect more noticeable, use a thin-walled metal cup instead of glass, as the metal conducts heat better and the outside of the cup will feel noticeably cold to the touch. Record the lowest temperature reached and calculate the total temperature drop. If you repeat the experiment with different amounts of baking soda while keeping the vinegar constant, you should find that more baking soda produces a larger temperature drop (up to a limit determined by the amount of vinegar available to react).
Dissolve Ammonium Nitrate in Water
Ammonium nitrate dissolving in water is one of the most dramatically endothermic processes you can perform at home. Ammonium nitrate is the active ingredient in many commercial instant cold packs and is also sold as a fertilizer at garden supply stores (look for cold packs at the pharmacy if the fertilizer form is not available in your area). Measure the temperature of one cup of room temperature water in a sturdy container. Add three to four tablespoons of ammonium nitrate and stir until dissolved. The temperature drops rapidly, often by 15 to 25 degrees Celsius, making the container cold enough to feel genuinely uncomfortable to hold. In commercial cold packs, a thin inner bag of water is surrounded by ammonium nitrate crystals. When you squeeze the pack and rupture the inner bag, the water contacts the crystals and the endothermic dissolution begins. The reaction absorbs so much heat because breaking apart the crystal lattice of ammonium nitrate requires more energy than is released when the ammonium and nitrate ions are hydrated by water molecules. The large temperature drop makes this the most effective demonstration of endothermic processes available with household materials. Handle ammonium nitrate with clean, dry hands and store it away from heat sources.
Observe Citric Acid and Baking Soda in Water
Citric acid (available in the canning or baking section of grocery stores) reacts with baking soda in water to produce a notable temperature drop. Dissolve two tablespoons of citric acid in half a cup of water and record the temperature. Add two tablespoons of baking soda and stir. The mixture fizzes vigorously as carbon dioxide is produced, and the temperature drops by 5 to 15 degrees Celsius. This combination is more endothermic than vinegar and baking soda because citric acid is a triprotic acid (it can donate three hydrogen ions per molecule), making the overall reaction absorb more energy per gram. The same citric acid and baking soda combination is used in fizzy bath bombs: when the bath bomb hits water, the endothermic reaction produces the tingling cool sensation along with the fizzing bubbles. You can make a simple bath bomb by mixing three parts baking soda with one part citric acid, adding a few drops of essential oil for fragrance, and pressing the mixture firmly into a mold (a silicone ice cube tray works well). Let it dry for 24 hours, then drop it into warm bath water to experience the endothermic reaction combined with the fizzing carbon dioxide release.
Compare Endothermic and Exothermic Reactions
Running an endothermic and exothermic reaction side by side makes the contrast vivid and memorable. For the endothermic reaction, use the baking soda and vinegar mixture in a thin-walled metal cup. For the exothermic reaction, dissolve a tablespoon of calcium chloride (sold as ice melt or moisture absorber at hardware stores) in half a cup of water in an identical metal cup. Calcium chloride dissolution is strongly exothermic, with the solution warming by 10 to 20 degrees Celsius. Record the starting temperature of both cups, add the reactants simultaneously, and measure the temperature every 30 seconds for five minutes. Plot both temperature curves on the same graph. The endothermic curve goes down while the exothermic curve goes up, creating a clear visual representation of the two categories of reaction energetics. Feel both cups at the point of maximum temperature difference and note the dramatic contrast between the cold endothermic cup and the hot exothermic cup. This comparison directly demonstrates that chemical reactions can either release or absorb energy, and that the direction of heat flow depends entirely on the specific bonds being broken and formed in each reaction.
Build a DIY Cold Pack
Apply your understanding of endothermic reactions to build a functional cold pack. You will need a sealable plastic bag (quart size), a smaller sealable bag or a thin-walled plastic pouch that can be burst by squeezing, ammonium nitrate or a mixture of baking soda and citric acid, and water. Place three tablespoons of ammonium nitrate (or a mixture of two tablespoons baking soda and two tablespoons citric acid) in the larger bag. Fill the smaller bag or pouch with a quarter cup of water and seal it tightly. Place the water pouch inside the larger bag with the solid chemicals and seal the outer bag. To activate the cold pack, squeeze firmly to rupture the inner water pouch. Knead the bag to mix the water with the chemicals. The bag should become cold within seconds. Measure the temperature of the outside surface with an infrared thermometer or by touch. Test how long the cold effect lasts by recording the temperature every two minutes. Commercial cold packs stay cold for 15 to 20 minutes. Your homemade version may have a shorter or longer duration depending on the amounts used and the insulation provided by the bag. Experiment with different ratios and amounts to optimize the temperature drop and duration for practical use as a first aid cold pack.
Endothermic experiments let you feel thermodynamics in action, demonstrating that some chemical reactions absorb heat from their surroundings, and that this energy absorption is a fundamental consequence of the balance between bond breaking and bond forming in every chemical process.