The concept of pH, a measure of acidity or alkalinity, is a key aspect of Chemistry Topics, particularly in acid-base reactions.
How does the formation of clouds take place?
The mass of tiny droplets formed by the condensation of water vapour which we see floating high in the atmosphere is called a cloud. The formation of clouds involves the process of condensation. The changing of water vapour into liquid water on cooling, is called condensation. We will now describe how clouds are formed. The formation of clouds will become clear from the following activity.
Take a glass half filled with water and add some ice to it (see Figure). Wipe the glass from outside with a piece of clean, dry cloth so as to make its outer surface completely dry. Allow this glass containing ice-cold water to stand undisturbed for about five minutes. We will see that a large number of tiny drops of water appear on the outer surface of the glass (see Figure). The appearance of tiny water drops on the outer side of glass can be explained as follows : The air around the glass contains water vapour in it. When the water vapour present in air come in contact with the cold, outside surface of glass, they get cooled and condense to form tiny drops of liquid water. Thus, water drops appear on the outer surface of a glass containing ice-cold water due to condensation (of water vapour present in air).
Condensation is the reverse of evaporation. We can see the process of condensation of water vapour to form liquid water in many instances of our daily life. For example, if we take out a bottle of ice-cold water from the refrigerator and keep it on a table, then a puddle of water is formed around it after some time. This is because when the water vapour present in air around the bottle comes in contact with the cold outer surface of bottle, it gets cooled and condenses to form liquid water which then forms a puddle of water around the bottle.
We can see tiny drops of dew on the leaves of grass on the cold winter mornings. The formation of dew is also due to the condensation of water vapour present in air. During cold winter mornings, many times we see a thick white cloud like thing near the ground which reduces our visibility. This is called fog. Fog consists of a cloud of tiny water droplets suspended in the air near the ground. Fog is formed by the condensation of water vapour present in air near the ground during very cold winter mornings.
In order to clean their spectacles, people often breathe out on spectacle glasses to make them wet before wiping with a piece of cloth. The spectacle glasses become wet because the water vapour present in our breath condenses on the glasses to form tiny droplets of water.
The process of condensation is involved in the formation of clouds in the sky and bringing back water to the surface of earth (in the form of rain, etc.). This happens as follows: Water vapour formed by the process of evaporation (from oceans, rivers, lakes, ponds and soil), as well as by transpiration (from plants and trees), goes into air.
The air containing water vapour is heated by the sun. Hot air, being lighter, rises high up in the atmosphere (or sky). Now, as we go higher and higher from the surface of earth, the temperature decreases, it becomes cooler and cooler. So, when the air containing water vapour rises to a high altitude in the atmosphere, the water vapour present in it gets cooled so much that it condenses to form tiny droplets of water. It is these tiny droplets of water, formed by the condensation of water vapour, which remain floating in air in the sky and appear to us as clouds (see Figure).
The tiny droplets of water in the clouds join together to form bigger drops of water. These drops of water fall down on the earth in the form of rain. During winter in very cold regions, the water drops in the sky freeze to form snow (ice). So, water also falls down to earth in the frozen state called snow. This is called snowfall. Snowfall occurs during winter in the extremely cold, hilly areas of the earth. When snow lying on high mountains melts, it forms water. Sometimes the frozen water from the clouds also falls on the earth in the form of small, round pieces of ice called ‘hail’.
From the above discussion we conclude that water present on the earth forms water vapour by the processes of evaporation and transpiration, rises high up in the sky, gets cooled, condenses and comes back to earth mainly as rain and snowfall. As we will discuss after a while, rain and melting of snow replenishes (puts back) water in rivers, lakes, ponds, wells, soil as well as in oceans (from where it was taken originally).
Back To The Oceans
Only some of the water which falls on earth in the form of rain and snow is available to us in rivers, lakes, ponds and as groundwater. Most of the water that falls on earth as rain and snow sooner or later goes back to the oceans. Almost all the places on land are above the level of oceans (called sea-level). Since land is at a higher level, water can flow from land into the ocean (or sea). We will now describe how the water which falls on land as rain or snow reaches the oceans.
The snow which falls on high mountains melts slowly to form water. This water then flows down the mountains in the form of streams and rivers (see Figure). Some of the rain water which falls on land also goes into rivers. Most of the rivers cover long distances on land and ultimately fall into a sea or an ocean. This is how most of the water which falls on the earth as rain and snow goes back to the oceans. All the rain water, however, does not go back to oceans.
The rain water also flows into lakes and ponds to fill them. It is also held by the soil. Some of the rain water which falls on earth seeps through the soil and goes under the surface of earth. Ultimately this water is stopped by some hard rocks and collects there. This water becomes available to us as ground water (or rather underground water). It is this ground water which we take out for our use by digging wells and tube-wells or by installing hand pumps.
From this discussion we conclude that water which was initially taken away from rivers, lakes, ponds, oceans, and soil, etc., by evaporation and transpiration is finally put back into them. This happens over and over again leading to a water cycle in nature.