Density differences explain why Styrofoam floats and wood sinks in water. Styrofoam's lower density compared to water causes it to float, while wood's higher density makes it sink.
The concept of density explains why Styrofoam floats and wood sinks when put in water. Density is defined as mass per unit volume. If two objects have the same mass but different volumes, they will have different densities. The wood probably has a higher density than water, which is why it sinks. Styrofoam, on the other hand, is designed with many air pockets, which give it a much larger volume for a given mass as compared to wood, resulting in a lower density than water and thus it floats. Objects with densities lower than water will float due to Archimedes' principle, which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid displaced by the object.
When a 0.5 kg mass is placed on the Styrofoam, its overall density increases as the additional mass causes more water to be displaced, and as a result, a smaller percentage of the block remains above water. However, the exact percentage will depend on the total volume of the block. If the container is filled with ethyl alcohol, which has a lower density than water, the same block of Styrofoam would displace less of the alcohol and thus float even higher, as it is less dense in comparison.