Final answer:
An object with a density of 0.8 g/cm³ will float in water because it's less dense than water, while an object with a density of 1.2 g/cm³ will sink because it's denser than water.
Step-by-step explanation:
When objects are placed in water, whether they float or sink is determined by their density in comparison to the density of water. Water at 4°C has a density of approximately 1 g/cm³. An object with a density of 0.8 g/cm³ is less dense than water, therefore it will float due to the buoyant force being greater than the object's weight. Conversely, an object with a density of 1.2 g/cm³ is denser than water, meaning it will sink because its weight is greater than the buoyant force exerted by the water.
Therefore, when an object of 0.8 g/cm³ and an object of 1.2 g/cm³ are placed on a container of water, the object with a density of 0.8 g/cm³ will float, while the object with a density of 1.2 g/cm³ will sink. This is in accordance with the Archimedes' principle, which relates the buoyant force to the weight of the fluid displaced by an object.