Changes Around Us: science class six
What is change?
When a small plant grows into tree, we call it change. When ice melts into water, we call it change. When water freezes into ice, we call it change. When a bulb is lighted, we call it change. When you put a new dress, it is called change. When you polish your shoes to get shine, we call it change. The sun rises in the east and sets in the west everyday. We see the different sizes of moon in the sky during night.
Thus, we see many things changing around us continuously. In fact without change life is possible. Thus it becomes necessary to study the changes scientifically.
To study changes in systematic way, changes are divided into various category.
Classification of Change
Fast and slow changes
Some changes are slow means they take a longer time, while some are very fast i.e. happened in a short time.
Fast changes
Changes takes place in short period of time are called fast changes. Some changes happens very quickly, and very fast. Such as bursting of crackers, melting of ice, burning of match stick etc. are some examples of fast changes.
Slow changes
Changes those take more time to be happened are called slow changes. Some changes like, ageing of man, ageing of animals, growth of plant, rusting of iron, etc. takes more time to occur; these are some examples of slow changes
Reversible and Irreversible changes
Reversible change
Changes which can be reversed are called Reversible Changes. This means we can get the matter back after change.
Example: Change of water into vapour. We can get water back from vapour. Thus, change of water into vapour is a reversible change.
Dissolution of sugar in water. This is a reversible change because we can get sugar back from water.
Distillation of water is a reversible change. In this process water is converted into water vapour because of heating and vapour is converted into water by cooling to get distilled water. In this first water is changes into vapour and then vapour is changes into water. Thus, vaporization of water and condensation of water, both are reversible changes.
Similarly, melting of wax, folding of paper, folding of cloth etc. are some examples of reversible changes. Because in all these changes we can get the changed matter back.
Irreversible Change
Some changes are irreversible means we cannot get the matter back after change.
Burning of coal, burning of paper, burning of fuel, ageing of man, growth of plant, growth in animals, change of milk into curd, etc. are some examples of irreversible changes.
Physical and Chemical Changes
Physical Change
If only the physical state of a substance goes under change and no new substance is formed then it is called a physical change.
Vaporisation of water, condensation of water, tearing of paper, tearing of cloth, freezing of water into ice, heating of a iron rod, etc are some examples of physical changes
Most of the physical changes are reversible.
Chemical changes
When the composition or chemical properties of the matter change and a new substance formed after change, it is called chemical change.
Chemical Changes are irreversible. This means we cannot get matter back after chemical change.
Example
Burning of coal. In burning of coal, heat, light, ash, etc. are formed. We cannot get coal back from coal ash. Thus, burning of coal is a chemical change.
Burning of anything is a chemical change.
Natural and man-made changes
Natural Changes
Some changes happens naturally. This means that there are many changes done by nature. For example, ripening of food, growth of plants, change of day and night, rising of sun, setting of sun, change of river water into vapour, change of cloud into rain, etc. are some examples of Natural Changes. Man has no control over the natural changes. Thus, change occur due to nature is called Natural change
Man made changes
Some changes are done by man, such as burning of wood for fuel, making of curd from milk, giving the desired shape to woods, making of dam, digging of well, producing of electricity, etc. are some examples of man made changes. These changes are in control of man.
Desirable and Undesirable changes
Some changes are desirable while some changes are undesirable.
Desirable changes
Change of milk into paneer or curd, formation of cloud, digestion of food, emission of oxygen by plants, absorption of carbon dioxide by green plant, etc. are some examples of desirable changes. Desirable change means we get benefited by these changes. Thus changes which give us benefit are called Desirable Changes
Undesirable changes
Changes which are harmful for mankind and man do not want such changes are called undesirable changes. For example, eruption of volcanoes, breach of embankments of river, rotting of food, pollution of environment, etc.
Causes of Change
There is always cause for a change. Some of the common causes for change are given here.
(a) Temperature
(b) Pressure or force
(c) Biological process
(d) Mixing of two or more substances
(e) Burning of something
Changes because of Temperature
Many changes take place due to change in temperature.
Example: On heating an iron rod expands and on cooling an iron rod shrinks. On heating water changes into vapour. On cooling water vapour changes into liquid (water). On cooling water changes into ice.
Changes because of pressure or force
Some changes occurs because of pressure.
Example: When air is blown into a balloon, because of pressure of air, balloon gets inflated. And when mouth of balloon is released, it gets deflated. And when more pressure is applied to a inflated, balloon get burst.
Changes because of biological process
Some changes occurs because of biological processes. Like growth of plants, growth in animals, ripening of fruits, etc are the changes happen because of biological processes.
Changes because of mixing of two or more substances
Some changes occurs due to the mixing of two or more substances. For example, when sugar is added into water, after mixing this sugar disappears in water and taste of water become sweet. This change happens because of dissolution.
Making of paneer (cheese) is another very good example of change because of mixing of two substances. When some lemon juice is poured into milk, curdles and paneer (cheese) get separated from milk. The paneer (cheese) cannot be turned back into milk.
Changes because of Burning
Some changes occurs because of burning. For example, after burning of wood, wood turn into ashes. Most of the Changes because of burning are irreversible.
Summary
Changes occurs continuosly in nature.
There are several types of changes.
Slow and Fast changes: Changes which happens very fast are called slow changes. Changes which happen quickly, are called fast changes. For example, growth of tree and animal are slow changes. Burst of crackers is a fast change.
Reversible and Irreversible changes: Changes which can be reversed are called reversible changes. While changes which cannot be reversed are called Irreversible changes. For example: Melting of wax is a reversible change while burning of wax is an irreversible change.
Natural and manmade changes: Changes which are done by nature are called natural changes while which are done by man are called man–made changes. For example: Change in season is a natural change. Making of dam is a man–made change.
Physical and chemical changes: Changes in which only physical nature, i.e. only shape, size, etc. are changes are called physical changes. Changes in which chemical property of a matter change are called chemical change. For example: Melting of ice is a chemical change. Burn
Desirable and undesirable changes: Changes which are beneficial for us or we want for that change, are called desirable changes. And changes which we do not need or do not want to be happened are called undesirable changes. For example: Change of milk into curd, lowering of temperature in summer, change of height of embankment while making of dam to check the flood, etc. are desirable changes as we need these changes. And flood, very low temperature in winter, very high temperature in summer, etc. are undesirable changes, as we do not want such changes to be happened.
Some causes of changes: There are many causes of changes. Some of the causes are listed below.
(a) Temperature
(b) Pressure or force
(c) Biological process
(d) Mixing of two or more substances
(e) Burning of something
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