Changes Around Us
Change is a universal phenomenon. Every moment several changes occur around us. Changes that occur in a substance may alter their colour, shape, size, composition, position, etc.
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 firecracker, etc. are some common changes that occur in our 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 states of the things. These alterations are temporary in nature and hence, can be reversed. A change always has a reason. Things change for greater stability. The change could be in position, shape, size, state, colour, temperature or composition of one or more bodies.
We can bring a change in a substance by doing one or more of the following:
(i) Heating
(ii) Applying force
(iii) Mixing it with other substances
Changes Caused By Heating
When an object is heated, it gets affected in one or many possible ways.
(i) Some objects get hot but do not change in any other way.
(ii) Some objects get hot and also expand in size.
(iii) Some objects get hot and begin to burn.
(iv) Some objects get hot and change their state.
Changes by applying pressure
When we apply force to an object, it gets affected in one or many possible ways.
(i) We can change its shape and size.
(ii) Air can be compressed.
(iii) Metals can be hammered into thin sheets.
(iv) Elastic can be stretched.
(v) Cotton can be spun into thin threads.
1.0Slow and fast changes
(a) Fast Change :
Some changes are very fast. These changes occur within a few seconds or minutes.
Examples
(1) A boy blowing a balloon
(2)bursting of a balloon
(3) burning of a match stick
(4) bursting of a cracker
(5) lighting of an electric bulb
(6) spinning of a top
(b) Slow Change :
Some changes take place very slowly. These changes may take hours, days, months or years to complete.
Growth in Plants
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. It is a slow natural change.
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.
6. The change of season from summer to winter takes months.
2.0Reversible and Irreversible changes, Temporary and permanent changes
Reversible Change :
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.
Breaking of glassware
Irreversible Change :
A change which cannot be reversed is called an irreversible change. In this change, the products formed cannot be converted back into their original forms.
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 sparkler is set on fire, it burns with a bright flame. However, on cooling, the products do not change back into sparkler.
4. Similarly, curdling of milk, weathering of rocks, grinding of grains, cooking of food, falling of leaves from a tree, ageing of human beings, animals and plants are some more examples of irreversible changes.
3.0Expansion and Contraction, Natural and man-made changes
(a) Expansion:
When an object is heated, it increases in size. The increase in size on heating is called expansion. The expansion of an object on heating, is a reversible change. This is because when the hot object is cooled, it decreases in size and comes back to the original size.
(b) Contraction:
The decrease in size of an object on cooling, is called contraction. Thus, in most simple terms, expansion means increase in size and contraction means decrease in size.
Expansion occurs on heating whereas contraction occurs on cooling.
The reversible change of expansion is used:
(i) in fixing of iron rim on the wooden wheel of a cart.
(ii) in fixing of iron blade of a digging tool (like a spade) to a wooden handle.
Change of Season
(c) Natural change:
Changes that take place without human intervention are natural changes.
For example, Change of seasons.
(d) Man-made change:
Changes that are carried out by human beings are called man-made or artificial changes.
For example, Construction of buildings.
Construction of Buildings
Periodic changes and non-periodic changes:
Changes which itself repeat after fixed period of time interval are called periodic changes whereas changes which do not repeat after fixed period of time interval are called
non-periodic changes. Ex. Rotation of earth is a periodic change whereas earthquake is a non-periodic change.
4.0Physical change
A change in which no new substance is formed is called physical change.
In this change, the composition of the substance remains unchanged.
Characteristics of physical change
Cutting of vegetables
(i) No new substances are formed during physical change.
(ii) Physical changes can be generally reversed.
(iii) There is no change in mass during physical change.
(iv) Only a little heat (if any) is absorbed or given off during physical change.
Condensation of water
Examples
Some of the very common examples of physical changes are given below:
(i) Melting of ice or wax or butter or ghee
(ii) Freezing of water to ice or solidification of liquid wax to solid wax
(iii) Changing of water into steam by boiling
(iv) Evaporation of water by the heat of sun
(v) Condensation of water vapours, such as formation of clouds, mist, fog, etc.
(vi) Glowing of an electric bulb on the passage of electric current
(vii) Magnetization of iron
(viii) Production of sound when two materials hit together
(ix) Expansion or contraction of metals on heating
(x) Formation of solutions of soluble substances in water
(xi) Crystallisation of salts from their solutions
(xii) Change of colour due to heat as in case of zinc oxide or lead monoxide
(xiii) Beating of metals into sheets or drawing metals into wires
(xiv) Shaping of glass by heat
Heating of copper wire is a physical change
5.0Chemical change
A change in which at least one new substance is formed is called chemical change.
Ripening of fruits
In this change, the composition of the substance changes.
Characteristics of a chemical change
(i) When a chemical change occurs, new substances with entirely new properties are formed.
(ii) Chemical change cannot be easily reversed.
(iii) There is usually a change in mass during chemical reaction.
(iv) A lot of heat is usually given off or absorbed during, a chemical change.
Examples
Following are the common examples of chemical changes
(i) Cooking of food
(ii) Food turning bad after a few days
(iii) Curdling of milk
(iv) Fading of colours of clothes
(v) Germination of seeds
(vi) Ripening of fruits
(vii) Lighting of a matchstick by striking
Chemical reaction
The process in which a substance or substances undergo a chemical change to produce new substances, with entirely new properties is known as chemical reaction.
During a chemical change, we may observe that mass is changing, but in reality, mass always remains conserved. So net change in mass is zero in a chemical change. It is also known as the law of conservation of mass.
Examples
Magnesium burns in air to combine with oxygen to form magnesium oxide.
2Mg(s) + O2(g) 2MgO(s)
Magnesium Oxygen Magnesium oxide
Chemical reactions can be of two types, exothermic and endothermic. In exothermic reaction heat energy is released.
For example, freezing of water, rusting of iron, mixing of salt and water. In endothermic reactions heat energy is absorbed.
For example, melting of ice cube, evaporation of water, cooking of egg, baking bread
6.0Differences between physical and chemical changes
Table of Content
- 1.0Slow and fast changes
- 2.0Reversible and Irreversible changes, Temporary and permanent changes
- 3.0Expansion and Contraction, Natural and man-made changes
- 4.0Physical change
- 4.1Characteristics of physical change
- 5.0Chemical change
- 5.1Characteristics of a chemical change
- 6.0Differences between physical and chemical changes
Frequently Asked Questions
Take a pencil and a sharpener. Sharpen half the pencil. How long did it take you to sharpen the pencil? Sharpening a pencil takes place within seconds, so it is a fast change.
Bring a picture of yourself when you were two years old. Many years we take to grow to our present age. Growth is a slow change.
Take a piece of paper and fold it as shown in figure. You have changed the sheet of paper into a toy aeroplane. You may have lots of fun in flying this plane. Once you are tired of it, unfold the paper again. Folding a paper into a toy aeroplane is reversible change. It can be reversed, we can unfold a paper again. Now another example, take some dough and make a ball. Try to roll out a roti figure. May be you are not happy with its shape and wish to change it back into a ball of dough again. Rolling of roti through ball of dough is a reversible change. We can convert rolled roti into ball dough again.
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