A man bought a goldfish in a pet shop. Upon returning home, he put the goldfish in a bowl of recently boiled water that had been cooled quickly. A few minutes later the fish was found dead. Explain what happened to the fish.

Understanding the physiological needs of fish is paramount for maintaining their health and survival. Fish extract oxygen from water through their gills, where blood vessels are close enough to the water for oxygen to diffuse into the bloodstream. Unlike humans who breathe atmospheric oxygen, fish rely on the levels of dissolved oxygen in their aquatic environment. The dissolved oxygen is crucial for them to perform basic metabolic processes, including growth, movement, and digestion.

Dissolved oxygen enters water through two primary methods: absorption from the surrounding air and photosynthesis by aquatic plants. However, several factors influence the amount of dissolved oxygen that fish can extract from the water, such as the presence of aquatic plants, the activity level of the fish, and more importantly, the temperature and aeration of the water.

Water temperature has a significant impact on the solubility of oxygen in aquatic environments. This is because water molecules move more rapidly at higher temperatures, disrupting the bonding between water and oxygen molecules and thus making it easier for oxygen to escape. In essence, warm water holds less dissolved oxygen compared to cold water.

Additionally, as water temperature increases, metabolic rates of aquatic organisms, including fish, also increase. This leads to higher oxygen consumption, which could lead to oxygen depletion if the dissolved oxygen is not replenished. Moreover, the difference in water temperature layers can also result in stratification, which prevents the mixing of surface oxygen-rich water with deeper, oxygen-poor layers, further affecting the availability of oxygen to fish.

When water is rapidly cooled after boiling, the process does not allow sufficient time for oxygen to dissolve back into the water. Boiling water expels much of the dissolved oxygen, and without proper aeration, the oxygen levels will not return to amounts suitable for fish survival.

Rapid cooling might create an illusion of a suitable environment due to acceptable temperature ranges for fish life. However, it's the invisible oxygen content that becomes dangerously low. Proper aeration techniques, such as using a water pump or an air stone, can help reintroduce oxygen into the water. Without these measures taken after cooling boiled water, the reoxygenation process is too slow to sustain the fish's metabolic demands, which could lead to asphyxiation and ultimately, fatality, as in the case of the man's goldfish from the described exercise.