Contents
The study of chemical kinetics is essential in Chemistry Topics as it examines the rates of chemical reactions and the factors that influence them.
The Three States of Water are Interchangeable
In order to understand water cycle, we should first know the various physical states in which water can exist under different conditions. This is discussed below. Although we normally think of water as a liquid but water can also be a solid or gas under appropriate conditions. The ice which we take out from the freezer of a refrigerator is ‘frozen water’ (or ‘solidified water’). So, ice is solid water. In other words, ice is the water in solid state [see Figure (a)]. The water which falls from the sky as white flakes of ice during winter in extremely cold regions is called snow. Snow is also solid state of water. The water which flows from the tap is a liquid.
Thus, tap water is the liquid state of water [see Figure (b)]. Most of the water around us is in the liquid state. Water vapour is the ‘gas state’ (or ‘gaseous state’) of water. Water vapour is present in air. Since water vapour is a colourless gas, we cannot see water vapour in air. Thus, water vapour in air is the water in gas state. Very hot water vapour is called steam. Steam is also gaseous state of water. We can make steam ourselves. If we heat some water in a beaker continuously by keeping the beaker over a burner, then after some time the water starts boiling to form steam [see Figure (c)].
From the above discussion we conclude that ‘ice’, ‘water’ and ‘water vapour’ are the same substance but they are in different physical states : ice is a solid, water is a liquid whereas water vapour is a gas. Thus, water can be solid, liquid or gas. In other words, water can exist in three states : solid, liquid and gas.
Water can be changed from one state to another by heating or cooling. For example :
- When ice (or snow) is heated, it melts and changes into water.
- When water is cooled too much, it freezes and changes into ice (or snow).
- When water is heated, it evaporates and changes into water vapour.
- When water vapour is cooled, it condenses and changes into liquid water.
During water cycle in nature, water goes through the changes of state. Actually, the water cycle in nature involves many physical processes such as evaporation, transpiration, condensation, freezing and melting, etc. So, before we describe the complete water cycle in nature, we will first study the various processes involved in it. Please note that the heat energy required to run water cycle in nature is provided by the sun.
Disappearing Trick Of Water ; Evaporation And Transpiration
When we spread wet clothes on a rope or wire (called clothes line), they dry up after some time (see Figure). The water present in wet clothes seems to disappear. Actually, water present in wet clothes evaporates by receiving heat from the sun (or surrounding air) to form water vapour. This water vapour goes into air.
Gradually, all the water present in wet clothes evaporates and the clothes become dry. The changing of water into water vapour is called evaporation. The water spilled on the floor dries up and disappears due to evaporation. Similarly, water from wet roads and puddles near our homes and rooftops (formed during rains) also dries up and disappears by forming water vapour by the process of evaporation when the rain stops. If we wipe a blackboard with wet cloth, it dries up after a while.
The wet blackboard dries up after wiping it due to evaporation of water from its surface. When we iron (press) wet clothes, we can see steam (hot water vapour) arising from them. So, the steam arising from wet clothes, while they are being ironed, is also a process of evaporation of water.
Water needs heat to evaporate into water vapour. In the evaporation of water in nature, heat is provided by the sun directly or by the surrounding air which has been heated by the sun. This will become more clear from the following activity.
Activity
Take two similar plates and pour equal amounts of water in them. Place one plate containing water in sunshine and the other plate containing water in shade in a verandah (see Figure). Observe the two plates after every 15 minutes. We will find the water in the two plates disappearing slowly and becoming less and less. The water taken in the plate which is kept in direct sunshine disappears first. In other words, the water kept in sunshine evaporates first.
The heat needed by the water for evaporation which is kept in sunshine is provided by the sun directly [see Figure (a)].
In other words, the source of heat for the evaporation of water kept in the sunshine is the sun itself. We also observe that the water kept in shade also evaporates ultimately though the sunshine does not fall on it directly [see Figure (b)]. The heat required for the evaporation of water kept in shade is also provided by the sun but indirectly. This happens as follows : During daytime, the heat given out by the sun heats all the air around us. This hot air (or warm air) is also present around the plate containing water which is kept in shade.
So, it is the warm air produced by the sun which provides heat for the evaporation of water kept in shade. From this we conclude that though evaporation of water (to form water vapour) takes place in shade as well as in sunshine but it is faster in sunshine. Evaporation is faster in sunshine because sunshine produces a higher temperature. Evaporation is slower in shade because of lower temperature in shade. Thus, evaporation takes place continuously from all the open surfaces of water at all the temperatures. As a result, water vapour gets added to air continuously.
If we want to dry our washed and wet school uniform quickly, we should spread it near a heater (or an angithi). The heat produced by heater (or angithi) will evaporate the water present in wet clothes at a faster rate due to which the uniform will get dry quickly.
The evaporation of water can be made very fast by heating it on a burner. We have already studied in seperation of substances that the common salt dissolved in water can be separated by the process of evaporation. When water having salt dissolved in it is heated on a burner, water evaporates (turns into vapour) rapidly and goes into the air, but the salt is left behind. This is because though water forms vapour easily on heating, salt does not form vapour (or gas) at all. From this observation we conclude that the process of evaporation of salt water does not carry away the salt with it, the salt is left behind. We know that ocean water (or sea water) is saline which contains a lot of salts dissolved in it. So, when ocean water (or sea water) gets heated by the heat of the sun, then some of the ocean water evaporates to form water vapour which go into air. The salts present in the ocean water (or sea water) remain behind.
From the above discussion we conclude that when the sun shines, the heat of sun evaporates water from oceans, rivers, lakes, ponds and even soil continuously to form water vapour. This water vapour goes into the air (or atmosphere). The water vapour becomes a part of the air but it cannot be usually seen. In addition to evaporation, there is another process which puts a lot of water vapour into air. It is called transpiration. This is described below.
Loss of Water by Plants
Plants need water to grow. Plants take this water from the soil. Plants use a part of this water to make their food and retain some of it in their different parts. The remaining water is released by the plants into air as water vapour through the small pores in their leaves (called stomata). The loss of water from plants as water vapour through the pores of their leaves is called transpiration. The process of transpiration puts a large amount of water vapour into the air.
Before we end this discussion, we can say that water vapour gets added to the air (or atmosphere) by the processes of evaporation and transpiration. Evaporation of water takes place from oceans, rivers, lakes, ponds and soil whereas transpiration occurs from the leaves of plants and trees. Thus, water vapour is always present in the air.