When the density of the object is equal to the density of the fluid the object will sink.
Explanation:
Suppose when the density of the object is less than the density of the fluid then the object will float in the liquid, although if the object is submerged it will keep on floating as forces will act upon the object other than buoyancy. But on the other hand if the density of the object is more than as well as equal to the density of the fluid then the object will sink into the liquid, no part can be seen above the surface .
