Explanation: Endothermic reactions are chemical reactions that absorb heat from their surroundings, causing a decrease in temperature. In this demonstration, we will be mixing two substances that undergo an endothermic reaction, resulting in a noticeable decrease in temperature.

Materials needed: - Ammonium nitrate (NH4NO3) - Water - Thermometer - Two small cups - Stirring rod

Safety precautions: - Wear safety goggles and gloves to protect your eyes and skin from any potential splashes. - Perform the demonstration in a well-ventilated area to avoid inhaling any fumes. - Do not touch the reaction mixture with bare hands as it may cause frostbite.

Step-by-step guide: 1. Measure out equal amounts of ammonium nitrate and water in separate cups. 2. Place the thermometer in one of the cups containing water to measure the initial temperature. 3. Slowly add the ammonium nitrate to the water while stirring continuously with the stirring rod. 4. Observe the temperature change using the thermometer. You should notice a decrease in temperature as the reaction takes place. 5. Feel the outside of the cup to see if it feels cold to the touch, indicating that heat is being absorbed from the surroundings. 6. Discuss with the students the concept of endothermic reactions and how they differ from exothermic reactions.

Key principles: - Endothermic reactions absorb heat from their surroundings, causing a decrease in temperature. - The reaction between ammonium nitrate and water is an example of an endothermic reaction. - The decrease in temperature is a result of the energy required to break the bonds in the reactants and form new bonds in the products.

By following these steps and safety precautions, high school students can safely observe and understand the principles of endothermic reactions through this simple chemical demonstration.