Question

Describe the experiment to show that water has maximum density at \[{4^ \circ }C\]. The important consequences follow from this peculiar property of water? Discuss the importance of this phenomenon in nature.

Answer 1

Hint: The experiment to observe the anomalous property of water using an instrument in order to measure the water level at different temperatures and its corresponding changes. The peculiar property of water and the reason due to which this anomalous behavior is observed needs to be probed upon in detail and from this concept the importance of this property is to be determined.

Complete answer:
The above problem revolves around the concept of the anomalous behavior of water which is a unique property of water when it is brought in contact with a freezing cold temperature or weather. We are asked to explain an experiment to show this peculiar property of water.

Let us first look into the experiment describing that shows that water has maximum density at \[{4^ \circ }C\].The construction of the apparatus is such that there are two openings that is one to measure temperature that the top of the cylindrical apparatus and one at the bottom in order to fit two thermometers through these openings so that the temperatures at the top and bottom surfaces of water present in the apparatus can be measured respectively.

The experiment makes use of an instrument known as Hope’s apparatus to measure the volume of water by attaching two different thermometers to two different depths of water and record and observe the changes in its behavior while the middle portion of the cylindrical instrument is attached to a mixture of salt and ice.

The lower portion, that is, the water present at a depth reaches \[{4^ \circ }C\] and remains constant while the upper portion does not change.However, once the water starts to cool entirely, that is, once the temperature starts to spread uniformly throughout the cylindrical tube, the upper portion of water also starts to decrease. However, it would be observed that the temperature value at the thermometer present above, in-turn reaches a value of \[{0^ \circ }C\].

We know that the freezing point of water is \[{0^ \circ }C\] which means that at this temperature the water freezes completely and hence only the upper portion of water is observed to be frozen.At the end of the experiment it would be clearly seen that the temperature recorded by the thermometer at the top surface of water will be at \[{0^ \circ }C\] while the temperature recorded by the thermometer at the bottom surface of water will be at \[{4^ \circ }C\].

Let us now look into the reason for the same. We know that the quantity known as density is given by the equation:
$density = \dfrac{{mass}}{{volume}}$
From this it can be seen that the density and volume of a substance is inversely related to each other and hence this means that when density increases, volume decreases and when density decreases, volume increases and vice-versa. This is the property which is mainly followed by water when its temperature is lowered continuously.

The above stated experiment can be explained by the anomalous behavior of water. In this experiment what happens is that when the ice mixture comes in contact with the water in the cylindrical apparatus the ice tends to sink below to the bottom surface of the water and hence the thermometer attached at this portion tends to record a change in temperature as the temperature decreases to \[{4^ \circ }C\] while the thermometer attached at the top portion remains unchanged.

Since the temperature is decreasing at this point the molecules of water or any substance for instance tends to come closer together and hence it tends to contract which means that the volume decreases thus increasing its density and hence the water becomes denser and denser till \[{4^ \circ }C\]

After \[{4^ \circ }C\] when the temperature tries to reduce further than the water exhibits a peculiar property wherein it expands instead of contracting between the temperature range \[0 - {4^ \circ }C\]. This results in the volume of water to increase thus decreasing its density and when any substance becomes less dense than the substance tends to float and rise upwards and this is what happens here as well. Thus, the top portion of the water encounters a temperature decrease and due to this sudden expansion of water the temperature reduces to \[{0^ \circ }C\].

This causes the top layer of water to freeze since it reached a temperature of \[{0^ \circ }C\] and hence the water has a maximum density at \[{4^ \circ }C\] because below this temperature the volume starts to increase decreasing the density.

This anomalous property of water is of great significance in nature. This behavior of water preserves the aquatic animals that reside in the water during extreme winters when the temperature drastically decreases, freezing only the top layer of the water into ice while the water below it remains at maximum \[{4^ \circ }C\] saving aquatic life beneath the surface of water. This proves to be the importance of this unique phenomenon of water in nature.

Note: An alternative experiment can be done using an instrument known as a pycnometer which is used to measure the level of water and hence also used to compare the densities of any liquid through a pipe. The density of water is recorded at different temperatures and can be observed that at \[{4^ \circ }C\] the density of water is maximum since the volume of water is minimum which is measured by the pycnometer.