Ribonucleic acid, the basic unit of RNA, has a hydroxyl (-OH) on the second carbon atom. On the other hand, the deoxyribonucleic acid, the basic unit of DNA, has a single hydrogen in the second carbon. This chemical structure difference makes RNA heavier than DNA due to the extra carbon atom.
The RNA/RNA-DNA hybrids can be separated from double-standard DNA using a high-resolution centrifuge due to this difference in weight. The image belows shows the extra oxygen atom in the 2' carbon