If we put an empty closed plastic bottle in water, it floats but same filled with sand, sinks in water. Why?

An empty closed plastic bottle is filled with air, so it floats upon water. A light substance whose density is less as compared to that of water floats in water. This is why it is seen that lighter things cannot easily sink in the water rather it floats on the water. 

The density of sand is more as compared to that of water, hence the plastic bottle filled with sand sinks in water.