1.write 4 points to show that changes takes place around us
2.what are the effects of changes
give two example to show change in position
4.name the different types of changes that takes place. give two examples of each type of change
5.what is periodic and non periodic changes give an example
6.give two examples of change which can be both desirable and undesirable
7.give 4 differences between physical and chemical change
8.explain 3 example of effect of heat which changes the size of an object
9.give two examples where object become soft on heating
10.give two examples where object become hard on heating
11.explain with an activity that metal expands on heating
12.why is mercury used in liquid thermometer
Answers & Comments
1. INTRODUCTION
In our daily life, we observe many changes around us. Everything in this universe undergoes a change. These changes may be observed by us at school, home, playground, garden or any other place. For example, sudden change in weather, flowering of plants, melting of ice, ripening of fruits, drying of clothes, milk changing into curd, germination of seeds, cooking of food, rusting of iron, bursting of firecrackers etc., are the common changes that occur in day-to-day life.
The changes can bring about different kinds of alterations in the things around us. Some of the alterations brought about are permanent in nature, and hence, cannot be reversed. However, some alterations are brought about in position, shape, size or state of the things. These alterations are temporary in nature, and hence, can be reversed.
Classification Of Changes
The changes taking place around us can be classified as under :
(1) Slow and fast changes.
(2) Reversible and irreversible changes.
(3) Desirable and undesirable changes.
(4) Periodic and non-periodic changes.
(5) Physical and chemical changes
2. SLOW AND FAST CHANGES
Some changes are very fast. These changes occur within seconds or minutes.
Examples: A boy blowing a balloon, bursting of a balloon, burning of a match stick, bursting of a cracker, lighting of an electric bulb, spinning of a top, etc., are examples of fast changes
Some changes take place very slowly. These changes may take hours, days, months or years to complete.
Examples:
1. Germination of a mustard seed into a seedling may take a few days. The growth of a seedling into a full fledged mustard plant may take three months or more.
2. A baby grows into an adult in over 18 years.
3. Rusting of an iron nail in damp air may take a couple of days.
4. Milk changes into curd in 6 to 8 hours.
5. The water changes into ice in a fridge in a few hours.
The change of seasons from summer to winter takes months.
3. REVERSIBLE AND IRREVERSIBLE CHANGES
A change which can be reversed is called a reversible change. In this change, the products formed can
be converted back into their original forms.
Examples
1. Water can be changed into ice by placing it in the freezing chamber of the fridge. The ice so formed can be converted back into water by placing the ice outside the fridge.
2. An electric bulb can be made to give light by passing electric current through it. The bulb returns to its original state and does not give light when the electric current is switched off.
3. A piece of steel can be converted into a magnet by rubbing with a powerful magnet. The magnet so formed can be converted back into steel by hammering it or heating it strongly.
A change which cannot be reversed is called an irreversible change. In this change, the products cannot be converted back into their original form.
Examples:
1. When a paper is burnt, it changes to ash and smoke. From ash and smoke, we cannot get back paper.
Thus, the change is irreversible.
2. A candle, on burning, forms carbon dioxide gas and water vapour. These products cannot be converted back into a candle.
3. When a sparkle is set on fire, it burns with a bright flame. However, on cooling, the products do not change back into sparkle.
4. Similarly, curdling of milk, weathering of rocks, grinding of grains, cooking of food, falling of leaves from a tree, the ageing of human beings, animals and plants are irreversible changes.