A Styrofoam cup keeps drinks warm because the Styrofoam offers excellent insulation, which keeps the liquid warm inside. While a Styrofoam cup can keep drinks warm, a thermos container captures heat to keep drinks hotter for longer.
According to Michigan State University, heat is created when molecules in the liquid move in different directions and bang into one another. These fast moving particles hit the side of the container where they are located. Heat conduction causes the heat from the liquid to be transferred to the container. The container gets hotter while the liquid gets colder. The liquid also loses heat as the surface area is exposed to air. The air gets heated while the container and the cup cool down.
A thermos container keeps liquids hot because the tight lid prevents heat from escaping the container. The core of the thermos is also filled with insulation, which does not conduct heat as well, so the liquid inside the cup does not cool down as quickly. Most thermos containers also feature reflective exteriors that limit the heat lost to radiation. A Styrofoam cup is made up of 95 percent air. This air conducts heat, which draws the warmth from the liquid into the cup.