CBSE Class 6 Science Changes Around Us Exam Notes

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Changes Around Us
WHY DO CHANGES OCCUR ?
Do you know why these changes take place around us ? Weather changes due to the heat of the Sun. Day and night alternate because of the rotation of the Earth. In the same way every change has a cause. These causes can bring out the changes in shape, size, position, colour or state of the things.

CLASSIFICATION OF CHANGES
You have already learnt how to group substances on the basis of similarities and dissimilarities of their properties. Similarly, you can group or classify the changes taking place in the following five categories.
(1) Slow and fast changes
(2) Reversible and irreversible changes
(3) Natural and man-made changes
(4) Periodic and non-periodic changes
(5) Physical and chemical changes

REVERSIBLE CHANGES
If a change can be reversed on removing the cause, it is called a reversible change. In this change the product formed can be converted back into their original form. A reversible change can take place in both the directions. For example, ice changes into water on heating whereas water changes into ice on cooling.

SOME MORE EXAMPLES OF REVERSIBLE CHANGES
1. An electric bulb can be made to give light by switching on the power supply. The bulb returns into its original state and does not give light when it is switched off.
2. When the leaves of the touch-me-not plant is touched, they close. After sometime they open out again as before.
3. What happens to your heartbeat when you do vigorous physical exercise? You must have noticed an increase in your heartbeat. Your heartbeat becomes normal after you take a rest.
4. What happens when you blow air into a balloon? It increases in size. It comes to its original form when you allow the air to escape from the inflated ballon.

IRREVERSIBLE CHANGES
If a change cannot be reversed, it is called an irreversible change. In this change products cannot be converted to their original form. The reversible change can take place only in one direction.

SOME EXAMPLES OF IRREVERSIBLE CHANGE
1. Ripening of fruits is an irreversible change because it is not possible to get unripe fruits from ripened ones.

2. Milk often gets curdled during summer if not kept in a refrigerator. Thus curdling of milk is a irreversible change because it is not possible to get back milk from curdled milk.
3. Observe your mother when she cooks rice for you. Can you get back raw rice in its original form from the cooked rice? No, it is not possible. Thus, cooking of rice or baking of a cake are examples of irreversible changes.
4. When a cracker is set on fire during Deepawali or on other festivals, it burns with a loud sound. The products thus formed cannot be converted back into a cracker.

PHYSICAL AND CHEMICAL CHANGES
Physical Changes
A physical change is a change in the physical properties of a substance without a change in its chemical composition. As a result of physical changes, a substance changes its form, but not its chemical composition. Physical changes are usually reversible and temporary.

Everyday Examples of Physical Changes
1. Drying of wet clothes
2. Tearing paper into pieces
3. Melting of wax
4. Chopping of wood
5. Freezing water to get ice

Chemical Changes
A chemical change is a change in which new and different substances are formed. The substance or substances formed have properties different from those of the materials that combined during the change. The change is usually irreversible and permanent.

Everday Examples of Chemical Changes
1. Digestion of food.
2. Rusting of iron
3. Respiration
4. Cooking of food
5. Formation of compost

CHANGES INVOLVE INTERACTION
Almost all changes around us involve two or more materials which interact with each other during the change. Take an example of a woodcutter splitting a piece of wood. Upon being hit with the axe, the wood gets split. You should also note that with repeated use, the axe gets a bit blunt. Thus the change affects both the materials. Another example would be burning of a matchstick by striking on the sides of a matchbox. The matckstick burns as a result of rubbing. At the same time, a rubbing mark is also left on the matchbox. Therefore, the change or interaction has influenced both the objects.

CONDITIONS FAVOURING A CHEMICAL CHANGE
Some of the conditions that favour a chemical change are given below.

Contact
A chemical reaction between two substances takes palce only when they are brought in close contact with each other. The closer they contact, the better is the chemical reaction. For example, washing soda reacts vigorously when it is mixed with hydrochloric acid. However, both these substances do not react with each other when kept separately. Similarly, a small quantity of curd is added to warm milk to get curd. Here change is brought about by mixing curd with milk.

Heat
Heating is necessary for some chemical changes to take place. For example, when iron and sulphur are mixed, they do not react. On heating a mixture of iron and sulphur in a test tube a black substance of iron sulphide is formed. The iron in iron sulphide cannot be separated physically with a magnet. This indicates that an irreversible change has taken place because of heating. Other conditions which favour changes are types of solutions, light, electricity, pressure, and catalyst.

QUESTIONS :

I. Answer the following in one word or very briefly.
(1) What is a change? Explain with the help of a suitable example.
(2) What type of changes are classified as reversible ?
(3) What type of change produces an entirely new product ?
(4) List two conditions which help in bringing about a change
(5) Name four different types of changes
(6) What type of change is involved in burning of a fuel ?
(7) Identify two changes that can be reversed while preparing dishes at home.

II. Answer the following in brief
(1) Give four examples of reversible changes occuring around you
(2) What is a reversible change? Can all the changes be reversed ?
(3) Differentiate between a physical change and a chemical change
(4) What is an irreversible change? Give two examples
(5) Give two examples each of physical and chemical change

III. Answer the following in appropriate detail
(1) Give four examples each of reversible and reversible changes
(2) List three characteristics of reversible changes
(3) How can you say that burning of a candle is an reversible change ?