CHEMISTRY ICSE NOTES OF CLASS 8 CHAPTER PHYSICAL AND CHEMICAL CHANGES

CHAPTER : PHYSICAL AND CHEMICAL CHANGES

1. what are slow and fast changes?

Ans- The changes that take longer time to complete are called slow changes.

Eg-Rusting of iron, change of seasons.

The changes that take place in very short interval of time are called fast changes.

Eg:- Burning of a cracker ,  lighting of an electric bulb.

2. what are natural and man made changes ?

Ans- Some of the changes take place in nature by themselves, which are called natural changes.

Eg: change of day and night, earthquakes

Any change that occurs due to efforts of human beings is called man made changes .

Eg : Cooking of food, Formation of Steel from iron.

3.What is periodic and non periodic change?

Ans- Changes that are repeated at regular intervals of time are called periodic changes.

Eg: change of day and night, change of seasons ,etc.

Changes that are not repeated at regular intervals are called non periodic changes.

Eg: earthquakes ,landslides.

4.What are reversible and Irreversible changes ?

Ans- When change the substance can be reversed by changing the conditions it is said to be reversible changes.

If a substance cannot be brought back to its original state after it change it is said to be an Irreversible change.

5. Explain physical change with examples.

Ans- It is the temporary change in which no new substances formed and the chemical composition of the original substance remains the same. Its physical properties like colour, state, shape may change. 

Eg: formation of dew ,melting of ice, sublimation of iodine and camphor.

6. What are the characteristics of physical change?

Ans- The characteristics of physical change are as follows:-

(a) No new substance is formed.

 (b)  There is no change in the chemical composition of the original substance. 

(c) The change is temporary and it can be reversed by reversing the conditions

(d) There is no change in the masses of the substance involved in the physical change.

(e) There is usually no loss  or gain of energy as a result of physical change.

7.Explain chemical change with examples.

Ans- A chemical change is a permanent change in which new substances are formed whose chemical composition and physical and chemical properties are different from those of the original substance.

Eg: Burning of paper ,Burning of wood. 

8. What are the characteristics of a chemical change.

Ans-

(a)  New substances are formed.

(b)  The change is permanent and Irreversible.

(c)  There is usually a change in the mass of the original substance.

(d)   Exchange of energy takes place.

(e)  The composition of the original substance changes completely.

9. Classify the following as a physical or a chemical change:-

(a)  Drying of wet clothes- Physical change

(b)  Manufacture of Salt from seawater- Physical  change

(c)   Butter getting rancid- Chemical change

(d) Boiling of water -Physical change

(e) Burning of paper- Chemical change

(f) Melting of wax- Physical change

(g) Burning of coal -Chemical change

(h) Formation of Clouds -Physical change

(i) Making of a sugar solution -Physical change

(j) Glowing of an electric bulb-Physical change. 

10.Fill in the blanks:-

(a)  The process of a liquid changing into a solid is called freezing. 

(b) A change which alters the composition of a substance is known as a chemical change.

(c)  There is no change in the composition of a substance during a physical change.

(d)  The reaction in which energy is evolved is called exothermic reaction. 

11. Give reason:-

(a) Freezing of water to Ice and evaporation of water are physical changes.

Ans- Freezing of water and evaporation of water are physical changes because if the freezed water or ice cubes are taken out of refrigerator it melts again into water, this indicates that the properties of water and ice are the same (the chemical compositions are same )and in the same way the vapour can be condensed to water again so these changes are called as physical changes which can be reversed. 

(b) Burning of a candle is both a physical and chemical change.

Ans- When a candle is lighted some of the solid wax first melts and turns into liquid then turns into vapour to produce a flame .New substances CO2 and H2O vapour are formed along with the evolution of light and heat energy this shows a chemical change.

When some of the molten wax drops to the floor it again solidfiles which shows a physical change. Thus the melting of a candle wax is a physical change and the burning of wax to produce CO2 and H2O is a chemical change.

12.Give four differences between physical and chemical changes.

Ans- PHYSICAL CHANGE

        -  No new substance is formed.

        -  Change is temporary.

        -  The change can be reversed by simple physical methods.

        - Heat or light energy may or may not be released or absorbed.

         CHEMICAL CHANGE

         - Substance with entirely different chemical composition and properties is formed.

       -  Change is permanent.

       -   Change cannot be reversed by simple physical methods.

      -  Heat or light energy or both are given out or absorbed.

13. Write the product and balanced equation of these reactions:-

(a)  Fe + S  ---------> 

(b)  Mg + O2 --------->

(c)  Pb + S --------->

(d)  Fe + Cl2 -------->

Ans-

(a) Fe + S --------> FeS

(b) 2Mg + O2 ---------->2MgO

(c) Pb + S --------->  PbS

(d)  2Fe + 3Cl2 ----------> 2FeCl3

14. What is a reagent ?

Ans- A substance used to start a chemical reaction is called a reagent. 

It is also one of the reactants .

Common reagents used in the laboratory are hydrochloric acid sulphuric acid and sodium hydroxide.

15. What are qualitative and quantitative properties ?

Ans- Descriptive properties of a substance which cannot be given a mathematical value are called qualitative properties.

Eg-smell,  taste, colour.

The properties which can be measured and given a specific mathematical value are called quantitative properties.

Eg- melting point , boiling point mass , solubility and density.

***********************************THE END**************************************

Physics ICSE class 8 notes of chapter Matter.

CHAPTER 1:MATTER

*Short/Long Answer questions.*

1. Define the term matter. What is it composed of ?

Ans- Matter is something which occupies space ,has mass and can be perceived by our senses .

 Example- air, water, iron ,etc

Matter is composed of tiny particles called molecules. In past it was considered to be made up of five tatva.

2. State three properties of molecules of a matter ?

Ans-  a. They are very small in size.

 b. They have spaces between them.

 c. They are in constant motion and they posses kinetic energy.

3. What do you mean by the inter-molecular spaces ?describe.

Ans- The spacing between the molecules of matter is called inter-molecular spacing .It is less in solids ,more in liquids and still more in gases.

4. What is meant by the inter-molecular forces of attraction? How do they vary in solids liquids and gases

Ans- The molecules of a matter exert a force of attraction on each other . This force of attraction is called the inter-molecular force.

Inter-molecular force is very strong in solids , less strong in liquids and is negligible in the molecules of gas.

5. Which of the following are correct?

(a) Solids have definite shape and definite volume.

(b) Liquid have definite volume but do not have definite shape

(c) Gases have definite volume but do not have definite shape.

(d) Liquids have definite shape and definite volume.

Ans-(a) and (b) are correct. 

6. Discuss the three States of matter : Solid, Liquid and Gas on the the basis of molecular model .

Ans- SOLID

Each solid is made up of tiny particles called molecules . Inter-molecular spacing is very small. Inter-molecular force is very strong. The molecules in a solid can only vibrate to and fro about the mean position.

LIQUID

The molecules are not rigid. Inter-molecular spacing in liquids is more than that in solids. Inter-molecular force in liquid are weak and the liquid molecules can move within the boundary of the liquid.

GAS

 Inter-molecular space is large as compared to liquids and solids. inter-molecular force in them are very weak .The molecules of gas can move about freely in space. 

7. What do you mean by the change of state ?Show the complete cycle of change of state.

Ans- The process of change from one state to another state either by absorption or rejection

 of heat at constant temperature is called the  change of state.

8. Differentiate between melting point and boiling point,giving at least one example of each.

Ans- The temperature at which solid changes into liquid without further  increase in temperature is called the melting point of solid. 

example -Ice (solid ) at 0 degree melts to form water( liquid) at zero degree celsius by absorption of heat.

The temperature at which liquid changes into vapour without further  increase in temperature is called boiling point of the liquid.

ex-water at 100 degree channges to 100 degree by absorption of heat.

9. Describe the process of condensation and sublimation with examples?

Ans- The change from the vapour state to the liquid state at a constant temperature on

 rejecting heat is called condensation.

Example -Steam at 100 degree condenses to water at 100 degree. 

The change from the solid state to the gaseous state directly is called sublimation.

Example- Camphor when burnt changes from solid directly to the gaseous state.

10. Explain the term melting and melting point?

Ans-

Melting- The change from the solid state to the liquid state on heating at a fixed temperature is called melting.

Melting point-The temperature at which solid changes into liquid without further  increase in temperature is called the melting point of solid. 

example -Ice (solid ) at 0 degree melts to form water( liquid) at zero degree celsius by absorption of heat.

11.Describe an experiment to demonstrate that  substance absorbs heat during melting without the change in its temperature. Ans- Take a test tube. Put some wax in the test  tube. Clamp the test tube in a vertical stand and  place the test tube in a beaker of water placed on  a wire gauze at a tripod stand. Clamp a  thermometer in the same vertical stand and insert  the thermometer in the test tube such that the bulb  of thermometer is inside the wax. Heat the beaker  over the flame of a burner and record the  temperature of the wax after every minute.  record your observations. Add caption From the above observations you will know that  the wax melts at 55 degree Celsius during which  heat is supplied the temperature does not rise  does the melting point of wax is 55 degree Celsius after melting of the whole wax the temperature begins to rise. 12. Some ice is taken in a beaker and its  temperature is recorded after each 1 minute.   The observations are listed as below  From the above observations what conclusion  do you draw about the melting of ice. Ans- From the above the observations  we will not  that the ice melts at zero degree celsius during  which heat is supplied and the temperature does  not rise. Thus the melting point of ice is zero  degree celsius after melting of the whole ice only   the temperature begins to rise 13.Explain the terms evaporation and boiling. Ans-.Vaporization- The change from the liquid  state to the gaseous state on heating at a constant temperature is called vaporization. Boiling point-The temperature at which liquid  changes into vapour without further  increase in  temperature is called boiling point of the liquid. 14. A liquid can change into vapour state : a. At a fixed temperature . b. At all temperatures  Name the processes involved in the two cases. Ans- At fixed temperature -Boiling  At all temperatures-Evaporation

15. Describe an experiment to demonstrate that the water absorbs heat during boiling  at a constant temperature . Ans- Take a beaker. Pour some water in the  beaker .Place the beaker on a wire gauze placed  over the tripod stand . Clamp a thermometer in a  vertical stand and insert in the beaker. Heat the  beaker over the flame of a burner and record the  temperature of water every minute . you will notice that the temperature of water rises  continuously till the water starts boiling at hundred  degree centigrade once the water starts boiling its  temperature does not rise any further .Although  the heat is still being supplied .Now the bubbles formed through the water are seen .At this  temperature water begins to boil and changes into the steam .Thus the boiling point of water is  100 degree centigrade. 16. A certain quantity of water is heated from  20 degree Celsius to hundred degree Celsius.  Its temperature is recorded after each 1  minute .The observations are:  What conclusions do you draw from the above  table about the boiling point of water . Ans- We will notice that the temperature of water  rises continuously till the water starts boiling at  hundred degree centigrade .Once the water starts boiling, its temperature does not rise any  further .Although the heat is still being supplied .Now the bubbles formed through the water are  seen .At this temperature water begins to boil and changes into the steam .Thus the boiling point of water is 100 degree centigrade.

?

17. State a the melting point of ice be the boiling point of water. Ans- The melting point of Ice is zero degree  celsius The boiling point of water is hundred degree Celsius. 18. What is evaporation? Ans-The change from the liquid state to the gaseous state on heating at a constant  temperature is called evaporation.  19. State the three factors on which the rate of evaporation of a liquid depends. Ans- The three factors on which the rate of evaporation mainly depends are as follows : (a)The temperature of liquid . (b) The area of the exposed surface  (c) The nature of liquid. 20. Wet clothes dry more quickly on a warm day than on a cool day .explain Ans-  Wet clothes dries up on a hot day much faster than a cool day because the rate of  evaporation is higher if the temperature is high. 21. Water in a dish evaporates faster than in a bottle. give reason Ans- In a dish water gets larger area than the bottle and the rate of evaporation increases if  the area of surface exposed increases. 22. Volatile liquid such as alcohol and spirit are stored and tightly closed bottles  .explain the reason Ans- Volatile liquids such as alcohol ,spirit ,Ether, etc evaporate much faster than water. This  is why the volatileliquids are stored in the tightly closed bottles. 23.Why is cooling produced on evaporation of a liquid?  Ans-During evaporation heat is absorbed and this heat is supplied by the surroundings of  liquid. This results in fall of temperature in surroundings. 24.Give an example which demonstrates that a liquid takes heat from its surroundings  when it evaporates. Ans-Evaporation of sweat from our body helps to maintain the body temperature at 37 degree  celcius.When sweat evaporates it requires heat which it takes away from our body.As a result  temperature falls to keep at 37 degree celcius. 25.Give two applications of evaporation.  Ans- (a)   Evaporation of sweat from our body helps to maintain the body temperature at 37  degree Celsius. When  sweat evaporates it requires heat which it takes away from our body.As a result  temperature falls to keep at 37  degree celsius. (b) We often pour tea in a saucer to cool it faster. In the saucer the surface area increases  and hence it cools faster.  26. Explain why water remains cool in the earthern pots. Ans- In summer water gets cooled in earthern pots.The reason is that the water seeps out  through the pores in the  pot and it evaporates.The latent heat required for evaporation is taken from water inside the  pot which therefore  gets cooled. 27.A patient suffering from high fever is advised to put a wet clothstrips on his  forehead . why? Ans- Doctor advice to put the strips of wet cloth on the forehead of a patient having high fever. The reason is that  water of the strips evaporates.During evaporation  water takes latent heat from the body of  the patient and thus the  temperature of his body decreases. 28.What do you mean by sublimation? give one example of the substance .  Ans -Sublimation is the process by which a solid when heated directly changes into its vapour  without first changing into the solid for example  camphor which on heating directly changes to the  vapour. Example - Napthalene balls, Camphor, etc. 29. Why does the size of naphthalene balls  decreases When left open ? Ans- Naphthalene balls which we used to protect  the woolen clothes from insects directly changes  into vapour and with time they become smaller in  size because this type of sublimation also takes  place without heating. 30. Describe an experiment to demonstrate the process of sublimation? Ans- Take some camphor for Ammonium Chloride  powder keep the Powder in a China dish and cover it with an inverted funnel then close the end of a final with a piece of cotton now place the dish  on a wire mesh kept on a tripod stand to heat it  by burner. .  You will notice that the fumes of Ammonium  Chloride are seen in the funnel above the dish.  These vapours when rises gets cooled and changes to solid Ammonium Chloride when gets  deposited on the walls of funnel. Thus ammonium chloride on heating changes directly from solid to vapour and these vapours on  cooling directly changes to solid state.