Grade 9 Environmental Chemistry Practice Test 2026 – Your Complete All-in-One Guide to Exam Success!

Dissolved oxygen measures the amount of oxygen gas that is physically dissolved in water. This oxygen is what aquatic organisms breathe to respire, so its presence is essential for aerobic life. The level is usually reported as milligrams of O2 per liter or as a percentage of saturation, and it changes with water temperature, salinity, movement, and biological activity (photosynthesis adds oxygen, while decomposition and respiration use it).

So the statement that describes DO as the amount of oxygen dissolved in water and its importance for aerobic life is the correct one. Other options misidentify what DO measures: nitrogen dissolved in water, mineral content, or oxygen in the air above the water are not what dissolved oxygen refers to.