Final answer:
The hazardous waste characteristic of the 50 million lithium batteries recalled due to the risk of overheating and explosion is reactivity. Lithium's high reactivity poses risks of instability, toxic gas release, and potential detonation.
Step-by-step explanation:
The hazardous waste characteristic of the 50 million lithium batteries installed in cell phones which were at risk of overheating and exploding is reactivity. According to the information provided, hazardous wastes are materials that may be dangerous due to their toxicity, corrosiveness, flammability, or reactivity. These lithium batteries are reactive because they can be unstable under normal conditions, possibly reacting with water or other substances, and can release toxic gases, or even detonate or explode when heated. Lithium, used in these batteries, is known for its high reactivity, which means that it can spontaneously react with many substances, leading to the hazards mentioned.
Additionally, lithium batteries provide a limited supply of reactants for energy and can accumulate byproducts that interfere with the reaction. This reactivity can be exacerbated at increased temperatures, further increasing the risk of overheating and potential explosions, as shown by the Nernst equation term that is proportional to temperature.