Describe an experiment to show that water has maximum density at 4°C. What important consequences follow from this peculiar property of water? Discuss the importance of this phenomenon in nature.

Hope’s apparatus can be used to demonstrate that water has maximum density at 4°C. The apparatus consists of a tall metallic cylinder with two side openings P, near the top and Q near the bottom, fitted with thermometers T₁ and T₂ respectively.

The central part of the cylinder is surrounded by a cylindrical trough containing a freezing mixture of ice and salt. The cylinder is filled with pure water at room temperature. The temperature recorded by both the thermometers is observed at regular intervals of time.

Observations —

1. Initially both thermometers T₁ and T₂ show same temperature (i.e., room temperature).
2. First the temperature recorded by lower thermometer T₂, starts decreasing and finally it becomes steady at 4°C, while the temperature recorded in upper thermometer T₁ remains almost unchanged during this time.
3. While the temperature recorded by lower thermometer T₂ remains constant at 4°C, the upper thermometer T₁ shows a continuous fall in temperature up to 0°C and then it also becomes steady.
4. At this stage, the lower thermometer T₂ shows the temperature 4°C at which water has the maximum density while the upper thermometer T₁ shows the temperature of water and ice at 0°C.

Important consequences that follow due to this peculiar property of water (i.e., anomalous expansion of water) are:

1. It is responsible for bursting of water pipelines and destruction of crops during very cold nights.
2. It helps in preserving aquatic life during very cold weather.

In nature, during winter when the atmospheric temperature starts falling below 0°C, water at the surface of a pond initially at temperature above 4°C, begins to radiate heat to the atmosphere, so the temperature of water near the surface starts falling.

When temperature of water at the surface falls below 4°C, water contracts and it’s density increases and therefore, it sinks to the bottom.

This continues till temperature of entire water reaches to 4°C. Now, further cooling of top layers below 4°C results in expansion of water and so it’s density decreases.

As a result, water does not sink further, but it remains on the surface. When the temperature of atmosphere falls below 0°C, water on the surface looses further heat to the atmosphere and gradually freezes into ice, but water below the ice layer remains at 4°C.

The water layer just below the ice in contact with it will be at 0°C, as shown in figure. Since, ice is a poor conductor of heat, so ice now prevents the flow of heat from water of the pond to the atmosphere.

Thus, temperature of water in contact with ice is at 0°C and that of layers below the ice gradually increases to 4°C. As a result, fish and other aquatic creatures remain alive in water of the pond (or lake), though water on the surface has frozen into ice. Nature thus protects the aquatic life during the winter season.