Chapter Review:

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  There are many changes around us which is continuous process.
• Some changes occur naturally and some changes are made by us. 
• Some such changes are: Raw egg to boiled egg, milk to curd, flour to bread, bud to flower, cow dung to biogas, rusting of iron, freezing water into ice, burning of coal, woods into furnitures, fibres into fabric, Changing of season, changing shape of moon etc.
• Some changes are reversible and some are non-reversible. 
• A change may occur by heating a substance or by mixing it with some
  other.
• There are different types of changes, i.e physical changes, chemical changes, reversible, non-reversible periodic changes and aperiodic changes. 
• The changes which can be reversed to their original forms are called reversible changes. Like water changes into ice, 
• The changes which can not come into their original forms are called non-reversible or irreversible changes. Like changing milk into curd, rusting of iron, burning of coal or papers etc. 
• Changes which are repeated after regular intervals of time are called periodic changes. like (i) Rising of sun. (ii) Motion of pendulum. (iii) Occurring of seasons one after the other.
• There are changes that do not take place regularly. These are called aperiodic change.
• When an object is heated it becomes larger in size and on cooling it contracts this is known as contraction. 
• When an object expands or melts on heating this is known as expansion. 
• Changing of water into gas on heating is called evaporation. 

 

Exercise Question:

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Que1: To walk through a waterlogged area, you usually shorten the length of your dress byb folding it. Can this change be reversed?

Ans: yes, by unfolding the dress this  change can be reversed.

Que2: You accidentally dropped your favourite toy and broke it. this is a change you did not want. Can this change be reversed?

Ans: No, it cannot be reversed, so it is an irreversible change.

Que3: Some change are listed in the following tables. For each change, write in the blank column, whether the change can be reversed or not.

S.No.                  Change                                 Can be reversed (Yes/No)    

1.          The sawing of piece of wood    
2.            The melting of ice candly
3.            Dissolving sugar in water
4.              The cooking of food
5.            The ripening of a mango
6.               Souring of milk

Ans: 

 S.No.                  Change                                 Can be reversed (Yes/No)    

1.          The sawing of piece of wood                          No
2.            The melting of ice candly                             Yes
3.            Dissolving sugar in water                             Yes
4.              The cooking of food                                    No
5.            The ripening of a mango                               No
6.               Souring of milk                                           No

Que4: A drawing sheet changes when you draw a picture onit. Can you reverse this changed?

Ans: It is all depands on situations: 

1. By erasing the drawing we can undo the change. for example, if we draw with the help of pencil, we can erase the drawing by using an eraser. Then, it is a reversible change.
2. If we draw using a pen, we cannot erase the drawing.  Then this changed cannot be reversed, so it is irreversible change.

Que5: Give example to examples the difference between changed that can or cannot be reversed.

Ans: 

Examples of Reversible changes:

1. Melting of Ice into water. By freeing the water we can obtain ice again.
2. Folding a paper: By unfolding it, we can undo the change.
3. Hot milk to cold milk: By Boiling milk, we can make it warm.

Example of Irreversible changes:

1. Buring of candle.
2. Bursting of crackers.
3. Cutting of trees.

Que6: A thick coating of a paste of plaster pf paris(POP) is applied over the bandage on a fractured bone. it becomes hard on dying to keep the fractured bone immobilised. Can the change in POP be reversed?

Ans: When water is added to POP, it undergoes a chemical changes. its composition changes and is converted into another substance. since it is a chemical change, so it cannot be reversed?

Que7: A bag of cement lying in the open gets wet due to rain during the night. The next day the sun shines brightly. Do you think the changes, which have occurred in the cement. could be reversed?

Ans: Dueto water, cement hardens ans its composition changes. since it is a chemical change which canot be reversed.

 

 

 

 

Important Question:

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Que: Give two examples of irreversible changes.

Ans:

(a) Milk to cheese

(b) Cooking of food.

Que: Give two examples of slow changes.

Ans:

(i) Growing of plants

(ii) Ripening of fruits

Qur: Give one example of change in size.

Ans: Stretching of a spring.

Que: What changes the direction of a body?

Ans: Force.

Que: What kind of change is breaking of wood - reversible or irreversible?

Ans: Irreversible.

Que: Give an example of reversible change.

Ans: Melting is an example of reversible change.

Que: Growing of nails is an example of which type of change?

Ans: Slow change.   

Que: Name the process in which solid directly changes into vapour.

Ans: Sublimation.

Que: Water turns into which state when it becomes ice?

Ans: Solid state.

Que: Give an example of natural change.

Ans: Growth of a baby.

Que: Rusting of an iron is which type of change?

Ans: Irreversible change.

Que: When ice changes into water, which type of change occurs in it?

Ans: Change of state.

Que: Give an example of physical change.

Ans: Cooling of molten wax.

Que: Give an example of chemical change.

Ans: Burning of wax.

Que: What is change?

Ans: An alteration in the physical or chemical properties of a matter due to effect of some kind of energy is called change.

Que: What are the different forms of changes noticed?

Ans: The various type of changes noticed are:

1. Change in shape

2.Change in size

3. Change in colour

4. Change in state

5. Change in direction

Que: Define reversible change.

Ans.The changes in which it is possible to get back to the original product again is called reversible change.

Que: State the list of changes caused due to heating.

Ans: The changes caused due to heating are:

• Expansion
• Change in state
• Combustion

Que: What is chemical change?

Ans: The result which leads to the formation of new substance is called chemical change.

Que: What do you mean by combustion?

Ans: The property of a substance by which substance start burning on heating is called combustion.

Que: Define evaporation.

Ans: The process of conversion of liquids into its vapour state by heating is called evaporation.

Que: What do you mean by expansion?

Ans: Solids, liquids and gases occupy more space when they are heated. This is called expansion.

Que: Define fast changes with two examples.

Ans: The type of change which takes short time to complete is known as fast change. e.g.,

(i) Burning of forest.

(ii) Striking of a matchstick.

Que:List the characteristics of physical change?
Ans:

Following are the characteristics of physical change:

1. No new substances are formed.
2. Properties of a substance doesn’t change.
3. These changes are reversible.

Que:List the characteristics of chemical change?

Ans:

Following are the characteristics of chemical change:

1. Properties of products are different from reactants.
2. Most of the chemical changes are irreversible.
3. Loss or gain of energy occurs in a chemical change.

Que:Define melting?

Ans:The process by which solid substances melt and turn into liquid is known as melting.

Que:Define contraction?

Ans:It is the phenomenon in which a substance shrinks or changes its shape.

Que:Why the stretching of a rubber band is reversible change?

Ans:The stretching of a rubber band is a reversible change because after we leave the rubber, it gets back to its original position automatically.

Que:Why the burning of paper is an irreversible change?

Ans:
Burning of paper is an irreversible change because after burning, the paper, it is not possible to get back the same paper which was burnt.

Important Question:

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Que: Give two examples of irreversible changes.

Ans:

(a) Milk to cheese

(b) Cooking of food.

Que: Give two examples of slow changes.

Ans:

(i) Growing of plants

(ii) Ripening of fruits

Qur: Give one example of change in size.

Ans: Stretching of a spring.

Que: What changes the direction of a body?

Ans: Force.

Que: What kind of change is breaking of wood - reversible or irreversible?

Ans: Irreversible.

Que: Give an example of reversible change.

Ans: Melting is an example of reversible change.

Que: Growing of nails is an example of which type of change?

Ans: Slow change.   

Que: Name the process in which solid directly changes into vapour.

Ans: Sublimation.

Que: Water turns into which state when it becomes ice?

Ans: Solid state.

Que: Give an example of natural change.

Ans: Growth of a baby.

Que: Rusting of an iron is which type of change?

Ans: Irreversible change.

Que: When ice changes into water, which type of change occurs in it?

Ans: Change of state.

Que: Give an example of physical change.

Ans: Cooling of molten wax.

Que: Give an example of chemical change.

Ans: Burning of wax.

Que: What is change?

Ans: An alteration in the physical or chemical properties of a matter due to effect of some kind of energy is called change.

Que: What are the different forms of changes noticed?

Ans: The various type of changes noticed are:

1. Change in shape

2.Change in size

3. Change in colour

4. Change in state

5. Change in direction

Que: Define reversible change.

Ans.The changes in which it is possible to get back to the original product again is called reversible change.

Que: State the list of changes caused due to heating.

Ans: The changes caused due to heating are:

• Expansion
• Change in state
• Combustion

Que: What is chemical change?

Ans: The result which leads to the formation of new substance is called chemical change.

Que: What do you mean by combustion?

Ans: The property of a substance by which substance start burning on heating is called combustion.

Que: Define evaporation.

Ans: The process of conversion of liquids into its vapour state by heating is called evaporation.

Que: What do you mean by expansion?

Ans: Solids, liquids and gases occupy more space when they are heated. This is called expansion.

Que: Define fast changes with two examples.

Ans: The type of change which takes short time to complete is known as fast change. e.g.,

(i) Burning of forest.

(ii) Striking of a matchstick.

Que:List the characteristics of physical change?
Ans:

Following are the characteristics of physical change:

1. No new substances are formed.
2. Properties of a substance doesn’t change.
3. These changes are reversible.

Que:List the characteristics of chemical change?

Ans:

Following are the characteristics of chemical change:

1. Properties of products are different from reactants.
2. Most of the chemical changes are irreversible.
3. Loss or gain of energy occurs in a chemical change.

Que:Define melting?

Ans:The process by which solid substances melt and turn into liquid is known as melting.

Que:Define contraction?

Ans:It is the phenomenon in which a substance shrinks or changes its shape.

Que:Why the stretching of a rubber band is reversible change?

Ans:The stretching of a rubber band is a reversible change because after we leave the rubber, it gets back to its original position automatically.

Que:Why the burning of paper is an irreversible change?

Ans:
Burning of paper is an irreversible change because after burning, the paper, it is not possible to get back the same paper which was burnt.