6th Science: Changes around us Notes and solved Questions

Changes around us are continuous process. 

Some changes occur naturally and some changes can be brought about by us. Such changes are: Rusting of iron, change of the state of water, Burning of wood, Growth of nail and hair, Blooming of flower and change in day and night ,season change,

What are different types of changes?

1. Reversible changes: Changes that can be reversed is called Reversible changes. Example: water changes into ice on cooling, wax change into liquid on heating, blowing of balloon 

2. Irreversible changes: Changes that cannot be reversed is called irreversible changes. Like Milk change in to curd, Iron changes to rust, Burning of paper, cooking of food, Growth of plant and animals, weathering of rocks, wear and tear of tires.

3. Physical changes: Changes in which physical properties like shape, size and state changes without formation of new substance is called Physical changes. It is a type of reversible changes. e.g. Melting of wax, solution of sugar and water, Boiling of water, Expansion of iron on heating.

4. Chemical changes: A change in which new substance is formed is called chemical changes. it cannot be reversed .Burning of paper, growth of nail, Rusting of iron etc.

5. Exothermic changes: Change in which heat is given out is called exothermic changes.

Carbon (c) + oxygen (O) ---- burning----> Carbon dioxide (Co₂) + Heat

Quick lime (CaO)   + Water       ------> Calcium hydroxide {Ca (OH)₂ } + Heat

6. Endothermic Changes: The changes takes place by absorption of heat is called Endothermic Changes For example

Iron + sulphur      ----Heat------->     Iron sulphide

Milk + sugar        --- Heat ------>  Sweet milk

Q. what is fast and slow change?

Ans: Fast changes: Changes that occur quickly is called Fast changes e.g burning of match stick, burning of fire cracker, Evaporation of petrol, burning of magnesium ribbon.

Slow Changes: Changes that occur slowly is called slow change like growth of nail, rusting of iron.

Q. What is combustion?

Ans: The burning of fuel on heating is called combustion. Heat and light is produced during combustion.e.g.Burning of Wood, burning of petrol, burning of LPG etc. Some substance catch fire on heating easily is called inflammable substance like paper, petrol, LPG, CNG etc.

Q. What is melting?

Ans: Changing of ice into water on heating is called melting. A constant temperature at which ice melt into water is called melting point.

Q. what is evaporation?

Ans: Changing of water into gas on heating is called evaporation. A constant temperature at which water changes into water vapour is called latent heat of evaporation.

Q. What is freezing?                      

Ans: Changing of water into ice on cooling is called freezing.

Q. What is condensation?               

Ans: Changing of water vapour into water is called condensation.

Q. What are sublimation and decomposition?

 Ans: The process of converting solid directly into gas is called sublimation. Such types of substance are called Sublime substance. Like petrol, camphor. The process of converting gas directly into solid is called decomposition.

Q. what is expansion?

Ans: When any substance is heated expand this is called expansion. When any substance cool contract.

Q. Give two examples occur on heating?

Ans: State change and expansion

Q. What happen when solid changes into liquid?

Ans: When solid is heated its molecules gain energy and move apart from each other and change into liquid.

Q. What changes you notice when you burn candle? Discuss the reason of the shortening of candle.

Ans:  When candle burn, Wax converted into liquid wax that is physical change. Some of the wax converted into gas that is Chemical change.

Q. What are the various forms of changes you notice around you?

Ans: physical change, Chemical change, state changes, Reversible changes, Irreversible changes

Q. How expansion and contraction are use full in fitting iron rim on wooden wheels?

Ans: When iron ream is heated it expands and easily fitted on wooden wheel. When cold water is poured on rim it contracts and tightly fixed on rim.

Q. What happen when things are heated beyond certain temperature?

Ans: when things are heated beyond certain temperature they change their states It is a physical changes.

Q. Why does milk in saucepan overflow when heated over gas?

Ans: On heating, particles of milk absorb heat and stats moving away and overflow from saucepan.

Q. why you are not able to hold metal rod when other end of rod is heated?

Ans: when one end of rod is heated, heat passes from one end to other end because iron is good conductor of heat. So we cannot hold metal for long  rod when is heated.

Q. What are periodic and non-periodic changes? Give examples.

Ans: Periodic change- A change that occurs during a definite time interval is known as periodic change, e.g., phases of moon, heart-beat, etc.

Non-periodic change- A change that does not repeat again and again after a regular interval of time is called non-periodic change, e.g., earthquake, flood

Q. What is a solution?   

Ans: Mixture of solid in liquid is called solution is formed. Like sugar and water solution.

Q. Define pasteurization.

Ans: Heating of milk for boiling to kill microbes is known as pasteurization. Milk is prevented from spoilage for several days by pasteurization.

Q. The shape of a paper sheet changes when you make an aero plane from it. Can you reverse this change?

Ans: Yes, this change can be reversed by unfolding the paper aero plane because it is physical change.

Q. Can deforestation be considered as a reversible change?

Ans: Deforestation is practice of the cutting down of tree or clearing forest. No, deforestation cannot be considered as a reversible change because it cannot take place in reverse direction. Forests cannot be grown quickly.

Q. Why do we keep vegetables or fruits in refrigerator?

Ans: Vegetables or fruits are spoiled by bacteria and other microbes. To prevent them from spoilage, they are stored at low temperature in refrigerator where microbes are unable to survive.

Q. Why dissolving sugar in water is a reversible and physical change?

Ans:  Dissolving sugar in water is a reversible and physical change as sugar can be obtained back by applying a physical method called evaporation.

Q. Why rotation of a fan is a periodic change.

Ans: The rotation of a fan is a periodic change because the change occurs after a fixed interval of time.

Q. Why the cooking of food is a chemical change.

Ans: The cooking of food is a chemical change as after cooking the food, new product is formed that cannot be reversed into previous food.

Q. Why  the melting of an ice candy is a reversible change.

Ans: The melting of an ice candy is a reversible change because ice candy melts due to increased temperature (room temp.) Melted ice candy can be solidified by freezing it. Hence, this is also a physical change.

Q. Why making statues out of plaster of Paris is an irreversible and chemical change.

Ans:  Making statues out of plaster of Paris is an irreversible and chemical change because plaster of Paris has a property of setting into a hard mass on getting mixed with water due to the formation of a new compound. The hard mass cannot be converted back into the previous plaster of Paris. Hence, it is an irreversible chemical change.

Q. What are desirable & undesirable changes?

Ans. Desirable changes: Those changes which are desired to occur that is we want them to take place are known as desirable changes. Example: ripening of fruits, germination of seeds etc.

Undesirable change: Those changes which we do not want to take place are called undesirable changes.

Example: rusting of iron, souring of milk, rottening of fruits etc.

Q. What are slow & fast changes? Give example.

Ans. Slow change: Those changes which take place in a long period of time are called slow changes. Example: rusting of iron, germination of seeds etc.

Fast change: Those changes which take place in a short time are called fast changes. Example: burning of paper, bursting a balloon.

Q. How is inflating a balloon different from bursting an inflated balloon?

Ans. Inflating a balloon is an example of a reversible change whereas bursting a balloon is irreversible change.

When we blow air into a balloon, its shape and size changes. But it regains its original form when the air is allowed to escape. On the other hand a brusted balloon cannot be changed back to the original form.

Q.How does a blacksmith change a piece of iron into different tools?

Ans.  A blacksmith first heats a piece of iron till it becomes red hot. It then becomes soft and can be beaten into desired shape.

Q. You must have seen that construction workers heat a black material called coal tar for repairing a road. State whether the change which has occurred in coal tar on being heated is reversible or irreversible. 

Ans. When coal tar is heated, it melts to form a thick dark liquid. The melting of coal tar is a reversible change as it solidifies again on cooling.