Final answer:
An object will float or sink when placed on the surface of a liquid based on its density compared to the density of the liquid.
Step-by-step explanation:
When an object is placed on the surface of a liquid, it will either float or sink depending on its density compared to the density of the liquid. If the object's average density is less than that of the liquid, it will float.
This is because the buoyant force, which is equal to the weight of the fluid displaced, is greater than the weight of the object. On the other hand, if the object's average density is greater than that of the liquid, it will sink.