Final answer:
The statement is true; both 16O and 18O isotopes are found in water molecules. The lighter 16O is preferentially evaporated, while the heavier 18O condenses more easily during the water cycle, which affects the isotope ratios in oceans, especially during cold climates.
Step-by-step explanation:
Both the 16O and 18O isotopes occur in water molecules. This statement is true. Oxygen-16 (16O) is the most common isotope of oxygen, containing 8 protons and 8 neutrons. Oxygen-18 (18O) is an isotope of oxygen that has two extra neutrons, making it heavier than 16O. In the water cycle, 16O is preferentially evaporated from the oceans because it is lighter. Conversely, when water vapor condenses into rain or snow, the heavier 18O isotopes preferentially do so. During colder climates, 16O gets trapped in ice sheets on continents, leading to oceans becoming relatively enriched in 18O. The chemistry of shells built by certain organisms reflects these isotope ratios, which can be analyzed to interpret historical climatic conditions.