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Analytical chemistry plays a vital role in Chemistry Topics, as it involves the identification and quantification of chemical substances.
Explain in Brief About Physical Change and its characteristics
We have many substances around us. All these substances have certain properties such as state (solid, liquid or gas), size, shape, colour, smell, temperature, composition and structure, etc. When one or more properties of a substance become different, we say that a change has taken place in it.
Here is an example. We know that ice melts to form water. Melting of ice is a common change around us. Now, ice is a solid whereas water is a liquid. So, the melting of ice involves a change in state: from solid state to liquid state. Changes are taking place all around us. Some of the changes observed by us in our everyday life are : Formation of curd from milk, Cooking of food, Burning of fuels, Drying of clothes, Ripening of fruits, and Rusting of iron.
Some changes are beneficial to us whereas some are harmful to us. For example, ripening of fruits is a beneficial change. So, we try to make the ripening of fruits faster by artificial methods. On the other hand, rusting of iron is a harmful change.
So, we try to prevent rusting of iron objects by various methods (such as painting the iron object, etc.). The change in a substance does not occur on its own. There is always a ‘cause’ which brings about a change in a substance. For example, ice does not melt on its own to form water. Ice must be given some heat to melt and change into water. Thus, ‘heat’ is the cause of the change of state of ice from solid to liquid.
Types of Changes
Changes can be of two types :
- Physical changes, and
- Chemical changes
We will now discuss physical changes and chemical changes in detail, one by one. Let us start with the physical changes.
Physical Change
Those changes in which no new substances are formed, are called physical changes. The changes in state, size, shape and colour of a substance are physical changes. The properties such as state, size, shape and colour of a substance are called its physical properties.
So, we can also say that: Those changes in which a substance undergoes a change in its physical properties are called physical changes. The physical changes are temporary changes which can be easily reversed to form the original substance. Very little energy (in the form of heat, etc.) is either absorbed or evolved in physical changes. Thus, the important characteristics of a physical change are as follows :
- No new substance is formed in a physical change.
- A physical change is a temporary change. A physical change can be easily reversed.
- Very little energy (heat, etc.) is either absorbed or evolved in a physical change.
- A temporary change in colour may take place in a physical change.
We will now take an example to understand a physical change clearly. Let us take a sheet of paper and cut it into a number of pieces. Now, we cannot join the cut pieces of paper to make the original sheet of paper. But each small piece of paper is still paper, it has not changed into something else.
So, during the cutting of a sheet of paper into pieces of paper, only the size and shape of paper has changed but no new substance has been formed. So, the cutting of paper (or tearing of paper) is a physical change.
Some more examples of physical changes are : Melting of ice (to form water) ; Freezing of water (to form ice); Boiling of water (to form steam) ; Condensation of steam (to form water); Evaporation of water (to form water vapour) ; Condensation of water vapour (to form liquid water); Cutting of cloth ; Breaking of a chalk stick ; Conversion of chalk stick into chalk dust ; Breaking of a glass tumbler ; Breaking of a wooden stick ; Cutting of a log of wood (into pieces of wood) ; Melting of wax ; Formation of clouds ; Drying of wet clothes ; Dissolving salt in water (to make salt solution) ; Dissolving sugar in water (to make sugar solution) ; Making soda water by dissolving carbon dioxide ; Glowing of an electric bulb (or tube-light) ; Stretching of a rubber band ; Grinding of a substance ; Hammering of metals to form thin sheets (like beating aluminium metal to form aluminium foil); Stretching metals to form wires ; and Conversion of liquefied petroleum gas (LPG) from liquid form in cylinder to gaseous form when it comes out of cylinder into gas stove. We will now describe some of these physical changes in detail.
Melting of Ice and Freezing of Water
Take some ice in a beaker and keep it aside for some time. We will see that ice melts to form water (see Figure 1). Actually, the ice kept in beaker receives heat from the surrounding air to melt and form water. Though ice and water look different, they are both made of water molecules.
This means that no new substance is formed during the melting of ice, only a change of state (from solid to liquid) takes place during the melting of ice. So, the melting of ice (to form water) is a physical change. The change which occurs during the melting of ice to form water can be reversed easily by freezing the water to form ice again. This can be done as follows.
Let us keep the beaker containing water in the freezer compartment of a refrigerator. After a few hours, the water kept in the freezer of a refrigerator gets cooled too much, freezes (or solidifies) to form ice.
In this case, the liquid water changes into solid water called ice. Only a change in state (from liquid to solid) takes place during the freezing of water to form ice, but no new substance is formed. So, the freezing of water (to form ice) is a physical change.
Boiling of Water and Condensation of Steam
Take some water in a hard glass beaker and heat it by using a burner till it starts boiling. When the water starts boiling, we can see the steam rising from the surface of hot water (see Figure). Now, water is a liquid whereas steam is a gas. So, during the boiling of water, only a change in state (from liquid to gas) has taken place. Though water and steam look different, they are both made of water molecules.
This means that no new substance is formed during the boiling of water. So, the boiling of water (to form steam) is a physical change. The change which occurs during the boiling of water (to form steam) can be reversed easily by condensing the steam to form water again. This can be done as follows.
Hold an inverted frying pan by its handle over the rising steam at some distance above the beaker of boiling water. Now, if we look at the inner surface of the frying pan, we will see drops of water sticking to it.
Actually, when hot, rising steam comes in contact with the inverted frying pan, then some of the steam gets cooled and condenses to form condensation of steam, there is only a change in state from gaseous state to liquid state but no new substance is formed. So, the condensation of steam (to form water) is a physical change:
Steam is very hot water vapour. So, we can also say that the conversion of water into water vapour is a physical change. In other words, evaporation of water is a physical change. The conversion of water vapour into liquid water (on cooling) is also a physical change. In other words, condensation of water vapour is a physical change.
The Case of Chalk Stick and Chalk Dust
Let us take a chalk stick and go on writing on the blackboard. Soon the whole chalk stick will be converted into small chalk particles called chalk dust. The conversion of chalk stick into chalk dust is a physical change because both the chalk stick and the chalk dust are just the same substance, only their size is different.
No new substance is formed during the conversion of chalk stick into chalk dust. The change which occurs during the conversion of chalk stick into chalk dust can be reversed easily as follows.
We take the chalk dust (or chalk powder) and add a little water to it to make a thick paste of chalk dust. This thick paste of chalk dust can be moulded into a chalk stick and then dried. In this way, we can get back the original chalk stick from the chalk dust. Thus, the physical change from chalk stick to chalk dust is a temporary change. It can be easily reversed by converting chalk dust back into chalk stick.
Making of a Solution
We take some water in a porcelain dish and dissolve some common salt in it. The salt disappears in water and forms a salt solution. So, a change has taken place in making salt solution. Let us now heat this porcelain dish containing salt solution on a burner till all the water evaporates.
A white powder is left behind in the porcelain dish. If we taste this white powder, we will find that it is common salt. It is the same common salt which we had dissolved in water earlier. This means that no new chemical substance has been formed by dissolving common salt in water to make salt solution.
So, the dissolving of salt in water (to make salt solution) is a physical change. Similarly, dissolving sugar in water (to make sugar solution) is also a physical change.
Breaking of a Glass Tumbler
When a glass tumbler breaks, it forms many pieces. Each broken piece of glass tumbler is still glass. So, during the breaking of a glass tumbler, only the size and shape of glass has changed but no new substance has been formed. So, the breaking of a glass tumbler is a physical change.
Heating a Hacksaw Blade
Hacksaw blade is a long, toothed blade which is used to cut metal objects. Take a used hacksaw blade and hold its one end with a pair of tongs. Keep the other end of hacksaw blade over the flame of a gas burner to heat it. After some time, we will see that the tip of hacksaw blade gets heated too much and turns red in colour (it is said to become red-hot).
Now, remove the tip of hacksaw blade from the burner and allow it to cool. On cooling, the tip of hacksaw blade gets back to its original grey colour. Now, only a temporary change in colour of hacksaw blade takes place during heating, no new substance is formed in this process.
So, the heating of hacksaw blade to the red-hot stage is a physical change (which can be easily reversed on cooling). (Caution: Heating the tip of hacksaw blade to red-hot stage is a very dangerous activity. It should be demonstrated only by the teachers).