Chapter 2 Phenomena Around Us Changes Around Us
Changes Around Us:
Changes involve different kinds of alternations in different things. You can see many incidents or phenomena that take place around you every day.
In such incidents, you must observe that some changes take place in different things either permanently or temporarily. There are a lot of examples of such changes.
For example:
Ice melts and forms water; water evaporates; cloud forms and it rains; Germination of seeds takes place; flowers blossom; fruits ripen; Burning of wood produces ash; etc.
You see that some of these changes take place on their own and some others are made by human beings. Changes that take place around us can be of different types which we are to learn in this chapter.
Read and Learn More WBBSE Notes For Class 6 General Science and Environment
Chapter 2 Phenomena Around Us Reversible And Irreversible Changes
Do you know: Why does a substance undergo a change?
When a substance is subjected to certain external agents like force, heat, light, electricity, etc. then it changes into a new substance. Now, if the external agent is removed, the new substance may or may not undergo a change in the reverse direction.
From this point of view, all changes around us can be broadly classified into two categories:
- Reversible changes and
- Irreversible changes.
Under these two categories, there are more types of changes, such as periodic and non-periodic changes, desirable and undesirable changes, natural and man-made changes, slow and fast changes, and physical Hands-on Activity and chemical changes.
With the help of an adult, heat some wax in an earthen bowl, and then cool it. In a similar way, heat some sugar in a metallic spoon (with a plastic handle) for a long time, and allow it to cool.
You will see that wax melts on heating and on cooling it comes back to its original state. So in this case, the opposite change can be brought about. We can say that the change can be reversed or the change is reversible.
On the other hand, while heating sugar you can see that the colour of sugar first changes to brown and then to black when cooled. The blackish substance is carbon and it is completely different from sugar.
So in this case, the opposite change can not be brought about. We can say that the change cannot be reversed or the change is irreversible.
Observe the following changes :
| Name of the change | Is it possible to return back to the previous state? | Reversible / Irreversible |
| Melting of butter, chocolate | Molten butter and chocolate can be changed back to their solid forms. | Reversible |
| Burning of firecrackers | A new substance (ash) is left after burning. | Irreversible |
| Ripening of fruits | Not possible to get back the raw fruits. | Irreversible |
| Freezing of water | Can be reversed state on heating. | Reversible |
| Cooking of food | We cannot get back the ingredients in their original forms. | Irreversible |
| The dissolving of salt in water | On evaporation, water is eliminated and salt is left behind. | Reversible |
| Inflation of a cycle tube | Can be reversed. | Reversible |
| Souring of milk | Cannot be reversed | Irreversible |
| Blooming of flowers | Flowers cannot change back into buds. | Irreversible |
Definition:
Changes in which the substances can return back to their previous state by reversing the conditions are called reversible changes and those which cannot return back to their previous state are called irreversible changes.
What Happens When A Candle Is Lighted?
You know that a candle is made of wax. On lightning, immediately you see that the wax under the wick gets heated and starts melting which solidifies on cooking.
A major part of the melted wax actually burns on the wick-but the wick itself does not burn. See Fig. Note that the burnt part cannot be reversed.
Observe the shown below and identify in this case what type of phenomenon (reversible or irreversible) taking place :
The phenomena of cyclic nature that has been taking place are :
Rivers, ponds and other water bodies are getting filled up by the ice-melted water and rainwater coming from the clouds. Green plants grow by absorbing water from the soil.
Again, water from the soil and different water bodies evaporates in sunlight and rises up in the air-which on being cooled condenses to be changed into small droplets of water to form clouds and also ice on the hills. The total phenomena that has been occurring is cyclic in nature.
Chapter 2 Phenomena Around Us Periodic And Nonperiodic Changes
Changes that are repeated after regular intervals of time are called periodic changes. The occurrence of day and night or seasons are examples of periodic changes.
Similarly, the revolution of the earth or other planets around the sun, the rotation of a blade of a ceiling fan, the motion of the hands of a wall clock, tide and ebb, the change of periods in a school, the leap year, etc. are such changes.
Conversely, changes that do not repeat themselves at particular intervals are called non-periodic changes. The occurrence of storm or rain at some place or flood in a year does not repeat at fixed intervals.
So these are non-periodic phenomena. Similarly, lightning and thunder, earthquakes, tsunami, cyclones, rusting of iron, waves of the sea, observation of a comet in the sky, etc. are such changes.
Chapter 2 Phenomena Around Us Desirable And Undesirable Changes
Changes which occur naturally and which do not harm nature or human beings or animals are called desirable changes. These changes are useful for us and we want these to occur.
For example germination of seeds, growing of plants from seeds, ripening of fruits, season changes, etc. are desirable phenomena. Changes which harm nature or humans or animals and that we do not want to occur are called undesirable changes.
For example, cutting of trees, use of plastic carry bags, and bottles, loud noises for long periods of time, air pollution from vehicles and factories, etc. are undesirable phenomena.
Chapter 2 Phenomena Around Us Natural And Man Made Changes
Some phenomena happen on their own in nature and these are not controlled by human beings. These are called natural phenomena. For example, green plants while preparing food to absorb carbon dioxide from the air and release oxygen into the air.
The process is natural. On the other hand, there are some phenomena which are carried out by human beings. These are called man-made phenomena. These types of phenomena occur due to the careless attitude of man.
For example, vehicular exhaust enhances the amount of carbon dioxide in the air and makes the air polluted.
Forest fire:
In nature sometimes we see that a fire can break out in trees in the forest. It happens due to the rubbing of dry twigs among one another. It is called a Forest fire. It is a natural phenomenon because man has no hand behind it.
On the other hand, when we light a fire for cooking or other needs, then it is a man-made phenomenon.
Natural disasters:
Floods, earthquakes, tornadoes, tsunamis, volcanoes, etc. are natural phenomena. On the other hand, man-made disasters like pollution, deforestation, killing of insects by spraying pesticides, combustive fire, etc. are caused due to careless human activities.
Now let us take more examples from our surroundings:
- Germination of seeds: Desirable and Natural.
- Blooming of flowers: Desirable and Natural.
- Cutting of trees and making of residences: Undesirable and Man-made.. 4. Death of wild animals by poachers
- Undesirable and Man-made.
- Cooking on the hearth by man: Undesirable and Man-made.
- Hatching of eggs: Desirable and Natural.
Chapter 2 Phenomena Around Us Worksheet
All of us know that human life and its growth depend on different plants and animals in different ways such as food, shelter, medicine, fuel, commercial goods, etc.
But increasing human population damage the bio-diversity in many ways such as deforestation, environmental pollution, indiscriminate use of natural resources, etc.
Farmers often use chemical fertilizers and pesticides to yield more production (crop). Do you know how the excessive and unwise use of these things creates environmental hazards?
Effect on soil:
It makes soil either acidic or basic and also kills beneficial micro-organisms of the soil. The fertility of the soil also decreases.
Effect on the aquatic system:
The unused fertilizers and pesticides ultimately reach to the nearby ponds, rivers, lakes, etc. by rainfall, making the water unfit for human consumption and destroying aquatic life of fish and other aquatic animals and plants.
These also damage several representatives of the food chain example frogs, birds, snakes, butterflies, etc.
Effect on man:
The harmful chemicals directly or indirectly pass to human beings by vegetables, fruits, cereals, etc. and accumulate inside the tissues of different organs like lungs, kidneys, liver, etc. These remain in the body for a long time and cause several diseases.
Chapter 2 Phenomena Around Us Slow And Fast Phenomena
There are some phenomena which happen very quickly (within seconds or minutes) which are known as fast phenomena whereas the phenomena which happen gradually and very slowly (may take hours or days or months to happen) are known as slow phenomena.
Examples of fast phenomena:
Burning of paper, bursting of crackers, lightning, thundering, evaporation of spirit, the reaction of quick lime with water, change of boiled milk on adding lime juice, etc.
Examples of slow phenomena:
Growth of plants, germination of seeds, change of tadpole into a frog, formation of coal over millions of years, change of the caterpillar into a butterfly, the reaction of muriatic acid with marble pieces, change of coconut from coconut sapling, etc.
Chapter 2 Phenomena Around Us Physical And Chemical Changes
There are so many changes which continue to occur around us in nature. Some of these are spontaneous and some are made to occur. What happens to the spirit if it is kept exposed to air or a piece of iron is left exposed to moist air?
You see spirit evaporates quickly and after a few days the piece of iron rusts. Thus in each case, the substance undergoes a change.
However, the changes that a substance undergoes are of two types:
- Physical change and
- Chemical change.
Physical changes :
In some of the changes no new substances are formed, i.e., the original substance remains unchanged throughout. In such a case, the original substance can be reversed by reversing the conditions. This is a physical change.
You can guess that all changes in the state of matter (such as the melting of ice, freezing of water, vaporization of water, condensation of water vapour, sublimation of naphthalene or camphor, etc.) are physical changes.
Note that on melting, ice absorbs heat and on freezing, water evolves heat. Here the original substance is ice and the final product is also ice.
Similarly on boiling, water absorbs heat and on cooling, steam evolves heat and ultimately condenses back of water. Here the original substance is water and the final product is also water.
Thus, if you remove the condition (or cause) of a physical change, you can get back the original substance. Besides this, if you take the weight of the substances before and after the change, you will see no change in weight occurs.
That’s why a physical change is temporary and reversible.
More Examples Of Physical Change
1. Take some table salt or sugar in a spoon and add it to a glass containing water. Stir the contents gently till the salt or sugar dissolves in the water. Now evaporate the above solution slowly.
You will observe that the water evaporates and ultimately the salt or sugar remains in the glass.
Is the salt or sugar can be got back ?—Yes.
Is any new substance formed ?-No.
So this is a physical change.
2. On heating, the platinum wire becomes red-hot and emits white light. On cooling, the wire regains its original form and colour. The change that platinum undergoes is temporary. So this is a physical change.
3. On heating wax melts into a liquid and solidifies on cooling. So it is a physical change.
4. You know that iron is attracted by a magnet. Due to this property, iron can be converted into a magnet by appropriate property.
If you remove the main cause of the transformation of iron into a magnet, you can get back iron. This is also a physical change.
5. Similarly glowing of a bulb or heater is a physical change because as soon as the current is cut off, the bulb or heater returns to the original substance and no new substance is formed. That is, the current flow does not bring about a change in the material by which the wire of the bulb or heater is made.
6. Glacier from the top of the mountain melts into liquid water. This is an example of the natural phenomenon of physical change.
7. During some physical changes, there might occur evolution or absorption of heat.
For example:
1. On adding concentrated sulphuric acid to water, the solution becomes diluted and hot a little but no new substance is formed.
2. On adding solid ammonium chloride crystals to water in a beaker, a solution of the salt is produced and it seems cold-but no new substance is formed. So 1 and 2 are examples of physical change.
8. Heating or cooling of iron or any other metal by which expansion or contraction might occur. But no new substances are formed. On reversing the cause, the original metal can be got back. So, it is a case of physical change.
9. Breaking of glass is an irreversible physical change.
10. Tearing of paper is an irreversible physical change.
Physical Change Definition
A change in which no new substances are formed and which can be reversed by reversing the main condition is called a physical change.
Chemical changes :
There are some changes in which the original substance changes to form one or more new substances with different properties. No physical method (or easy means) can convert the new substances into the original substance.
This type of change is called a chemical change. This change is permanent and irreversible.
Examples Of Chemical Change
1. Weigh out a piece of iron nail and leave it in moist air. Examine its outer surface after a few days. You will see a brown crust what we call rust has developed on its surface.
Take the weight again and see that the rusted nail weighs more than the original nail. Separate the rust from the nail with the help of a knife. Hold a magnet over the rust and see that the rust is not attracted by the magnet.
So, iron and rust are completely different. The rust cannot be converted into iron by easy means. Thus, rusting of iron is permanent and it is a chemical change.
2. Heat a piece of magnesium ribbon. It burns with white fumes. These fumes get deposited over a cold surface as a white solid. The white solid is magnesium oxide, which is different from the original substance. It weighs more than magnesium. So it is a chemical change.
Note: This is an example of a chemical change where heat is needed.
3. Sugar is white, sweet to taste and soluble in water. Take some sugar in a test tube and heat it.
On heating, water vapours evolve and a black mass is left in the test tube. Mix water into this black mass and shake well. You see that it does not dissolve in water. It is tasteless also. So, the black solid is definitely different from the sugar.
Its name is charcoal. Original sugar can by no means be obtained from this charcoal and water vapour. Therefore, it is a permanent, irreversible and chemical change.
4. Burning of coal, wood, paper, straw, candle, kerosene, and petrol are examples of chemical change.
When these are burnt in the air, immediately carbon dioxide gas is formed leaving a small amount of ash behind. The weights of the original substance and ash are different.
By no easy means, ash can be converted back into the original substance. So the change is permanent and irreversible. Burning of firecrackers are also a chemical change.
Note: Heat is needed to catch fire in the substances and heat is evolved also when they burn.
5. When you explode caps by pressing the trigger of your toy gun, then also a chemical change takes place. In this case, pressure energy is required to bring about the change.
6. Green plants make their food by a process called photosynthesis. In this process, carbon dioxide (from air) reacts with water in presence of sunlight and chlorophyll in the green parts of the plant to form glucose.
Here, glucose is a new substance. So, the change. is permanent and a chemical one.
7. Cooking of food is a chemical change.
For example:
- Boiling of rice grains in the water contained in a vessel for some time changes it into boiled rice: This change is permanent because boiled rice can by no means be converted into raw rice grains. So this is a chemical change.
- Digestion of food.
- Curdling of milk.
- Change of a tree from a seed. Growth of animals.
- Change of an unripe mango into a ripe mango.
- Forest fire (a natural chemical change).
Chemical Change Definition
A change in which new substances with different properties are formed and which cannot be reversed by reversing the main conditions is called a chemical change.
Can physical and chemical changes take place together?
When a candle burns in the air, a portion of it melts. These molten part when cooled returns to solid wax (the material of the candle). This is a physical change.
At the same time, a major portion of the candle converts into carbon dioxide and water vapour-which are different from the wax. This change is a chemical change. So, in this case, both physical and chemical changes take place at the same time.
The Distinction Between Physical And Chemical Changes :
| Features | Physical changes | Chemical changes |
| 1. Nature of change | Temporary and reversible. | Permanent and irreversible. |
| 2. Formation of new substances | No new substances are formed. | New substances with different properties are formed. |
| 3. Loss or gain in weight | No loss or gain in weight takes place. | There occurs a loss or gain f in weight. |
| 4. Energy change | Evolution or absorption of energy may or may not take place. | Energy is either absorbed or evolved throughout the change. |