Dr. Homi Bhabha Competition Test Series

Transcription

1 Changes around us: Types of changes: 1. Natural Changes: Changes which are brought about by nature itself. Example: Changes of season, aging, tides in the sea, landslides. 2. Man made Changes: Changes which are brought about by man. Example: Formation of curd, cutting nails, burning wood. 3. Useful Changes: Changes which benefit us Example: The changing of milk into curd, Baking roti 4. Harmful change: Changes that harm us. Example: rotting of cooked food, Tooth Decay 5. Quick changes: Changes which occurs very fast. Example: Burning of cooking gas,cutting of an apple, explosion of crackers 6. Slow changes: Changes which occurs very slowly. Example: Rusting of iron, Growing of mango tree from seedling, changing milk into curd 7. Reversible Changes: If a change can be reversed Example: Ice changes into water, water changes into vapour

2 8. Irreversible changes: If a change cannot be reversed. Burning of coal,falling of leaves from tree, aging,cutting apple into pieces 9. Periodic Changes: Events that repeat themselves after a certain amount of time.eg. The Earth s rotation, Revolution of Earth, Full moon day. 10. Non periodic Changes: Changes that do not occur again after a specific amount of time. Example: Earthquake, volcanic eruption 11. Physical Changes: Are temporary changes in which no new substance is formed. Only physical property changes (i.e. shape, color, size ) Example: Dissolving sugar in water, Cutting mango into pieces, Switching on fan, salt pan. 12. Chemical Changes: Are permanent changes in which entirely new substance is formed with different properties. Example: burning of coal, growing seedling from seed. 13. Exothermic change: In which energy is released Example: changing water vapor to liquid molecule. In this water vapor looses energy to change from vapor state to liquid state. 14. Endothermic Change: In which heat is absorbed. Example: Melting of ice, Dissolution of common salt in water In this water in solid state (Ice) absorb energy from surrounding and changes into liquid molecule.

3 Above figure helps you understand the different states of matter. 1. Evaporation: Change of state of matter from liquid to Solid.Examples are clothes line will dry, Water in a boiling pot changing to a gas 2. Condensation: Change of state of matter from Gas to Liquid. Examples are dew forming on grass in the early morning, eye glasses fogging up when you enter a warm building on a cold winter day, or water drops forming on a glass holding a cold drink on a hot summer day. 3. Melting: Change of state of matter from Solid to Liquid. Examples are melting of ice, Burning Candle 4. Freezing: Change of matter from Liquid to Solid. Examples: change of water to ice if kept in freezer Corrosion & Prevention of Corrosion: It is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides, is a well-known example of electrochemical corrosion. Copper reacts with carbon di-oxide to form yellowish green patches Hydrogen sulphide reacts with silver. Silver turns black.

4 Prevention of Corrosion: To prevent contact between metal and air. This can be done by 1. Layer of grease or oil is applied 2. Coated with another metal Example: Galvanization (Iron is given coating of zinc) 3. Use enamel paint for ships 5. Combustion The act of burning something; "the burning of leaves A process in which a substance reacts with oxygen to give heat and light Examples: Burning of fuel in car For better practice write at least 10 examples of each type of changes. 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.

5 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. Example: 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. 5. Exothermic changes: Change in which heat is given out is called exothermic changes. Carbon (c) + oxygen (O) ---- burning----> Carbon dioxide (Co 2 ) + Heat Quick lime (CaO) + Water > Calcium hydroxide {Ca (OH) 2 } + Heat

6 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 human, rusting of iron. Q. What is combustion? Ans: The burning of fuel on heating is called combustion. Heat and light is produced during combustion. Example: Burning of Wood, burning of petrol, burning of LPG. Some substance catch fire on heating easily is called inflammable substance like paper, petrol, LPG, CNG 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.

7 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 deposition or desublimation? 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 deposition. Q. What is expansion? Ans: When any substance is heated it expands. This is called expansion. When any substance cools it 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.

8 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 useful 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 happens when things are heated beyond certain temperature? Ans: When things are heated beyond certain temperature they change their state. It is a physical change. Q. Why does milk in saucepan overflow when heated over gas? Ans: On heating, particles of milk absorb heat and starts moving away and overflow from pan. 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 when the rod 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, Example: phases of moon, heart-beat

9 Non-periodic change- A change that does not repeat again and again after a regular interval of time is called non-periodic change, Example: 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?

10 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.

11 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. Undesirable change: Those changes which we do not want to take place are called undesirable changes. Example: rusting of iron, souring of milk, fruits getting rotten. 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. 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 bursted 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.