Virginia 7th Grade Science SOL Practice Test

Which statement about why an object sinks in water is correct?

It sinks because it is cold. It sinks because it has a small volume. It sinks when its density is less than the water's density. It sinks when its density is greater than the water's density.

Correct Answer:

It sinks when its density is greater than the water's density.

Explanation:

Buoyancy depends on density relative to water. The upward buoyant force equals the weight of the water the object displaces. If the object's density is greater than water's density, its weight is larger than the buoyant force, so it sinks. If the object's density is less than water's, the buoyant force can support it and it floats; if the densities are equal, it can hover with neutral buoyancy. So the correct idea is that sinking happens when the object's density is greater than the water's density. Cold temperature, or having a small volume, doesn't by itself determine sinking, and an object with density less than water would not sink.

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