Understanding Why Fish Die in Boiled Water

When a fish is placed in an aquarium filled with water that has been previously boiled and then cooled down to room temperature, it often dies. This happens because the process of boiling significantly changes the composition of the water in a way that is harmful to aquatic life.

The Effect of Boiling on Water

Water from natural sources like rivers, lakes, or even tap water contains various dissolved substances. Importantly for aquatic life, it contains dissolved gases, including oxygen. Fish, like many other aquatic animals, need to extract this dissolved oxygen from the water to breathe and survive.

Boiling water involves heating it to its boiling point ($\text{100}^\circ\text{C}$ at standard atmospheric pressure). When water is heated, the solubility of gases in it decreases. As the water boils vigorously, most of the dissolved gases, particularly oxygen and carbon dioxide, escape into the atmosphere.

Why Dissolved Oxygen is Crucial for Fish Survival

• Fish breathe by taking water into their mouths and passing it over their gills.
• Gills are specialized organs with thin membranes and a rich blood supply.
• Dissolved oxygen from the water diffuses across the gill membranes into the fish's bloodstream.
• This oxygen is then transported to the fish's cells for respiration, which is essential for generating energy.

Without sufficient dissolved oxygen in the water, fish cannot respire properly, leading to suffocation and death.

Analysing the Options

Let's look at the given options in the context of boiled and cooled water:

1. Depleted in minerals: Boiling can affect the concentration of some minerals (e.g., scale formation from calcium carbonate), but a slight depletion in minerals is generally not the immediate cause of death for a fish. Minerals are important for long-term health but lack of dissolved oxygen is a rapid killer.
2. Depleted in oxygen: As explained above, boiling drives out dissolved gases, including oxygen. Water that has been boiled and cooled will have a significantly lower level of dissolved oxygen compared to fresh, unboiled water. This lack of oxygen is fatal for fish.
3. Unfit for drinking by fish: Fish don't typically 'drink' water like mammals do; they absorb water and salts through their gills and skin, managing their internal salt balance. The primary issue isn't the drinkability but the breathability.
4. Unfit for swimming by fish: Boiled and cooled water is physically the same as unboiled water in terms of its properties for swimming (density, viscosity). It does not become physically unsuitable for swimming.

Therefore, the primary reason a fish dies in previously boiled and cooled water is the severe depletion of dissolved oxygen necessary for its respiration and survival.