D2O

D₂O, or deuterium oxide, is a form of water where the hydrogen atoms are replaced with deuterium, a heavier isotope of hydrogen. Unlike regular water (H₂O), which contains hydrogen atoms with just one proton, D₂O has deuterium atoms with an extra neutron, making it about 10% denser. It appears similar to ordinary water but has different physical and chemical properties. D₂O is often used in scientific research, especially in studying biological systems and nuclear reactors, because it behaves differently than regular water, allowing scientists to track or analyze processes involving hydrogen.