7.13MHCERT 7th General Science 13. Changes - physical and chemical

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MHCERT - General Science -

 chapter 13. Changes Physical and Chemical



1. Distinguish between the following.


(a) Physical change and Chemical change 

Physical Change

  1. The properties of the original substance remain the same. 
  2. Their compositions remain unchanged. 
  3. No new substance is formed. 
  4. E.g. melting, boiling, evaporating 




Chemical Change

  1.  New and different properties are seen in the new substance.
  2.  Their original composition changes.
  3.  The new substance is formed.
  4.  E.g. burning and corrosion.


(b) Periodic change and non-periodicchange

Periodic change

  1. The changes that occur again and again after a definite interval of time are called periodic changes.
  2.  There is consistency in time.
  3.  Interval is fixed.
  4. E.g. Sunrise, sunse3t, moments of the hands of the clock.

Non-periodic change

  1. The changes that may occur again but cannot be said for sure when they will occur again are called non-periodic changes.
  2. There is no consistency in time.
  3. Interval time is not fixed.
  4. E.g.Heavy rainfall, floods, storms etc.



(c) Natural change and man-made change

Natural Change 

  1. Changes that occur naturally are called natural changes
  2. They are natural events.
  3. E.g. ripening of fruits. spoiling of milk etc.

Man-made changes

  1. The changes that do not occur naturally but are created by a man deliberately are called man-made changes.
  2. These changes occur when a man wishes to have some changes to fulfil his need.
  3. E. g. cooking food. sharpening a pencil, making a table etc.


2. Under which different types can the following changes be classified?

(a) Conversion of milk into yoghurt - 

useful, man-made, slow, irreversible, chemical 

(b) Bursting of a cracker -

Man-made, quick, irreversible, non-periodic, chemical


(c) Occurrence of an earthquake.

 Natural, harmful, non-periodic

(d) Revolution of the earth around the sun.

Natural, useful, periodic

(e) Stretching of a spring.

Man-made,  reversible,  physical


3. Give reasons.

(a) While purchasing canned food, its expiry date should be checked.



 Ans.

  1. Canned food has a specific time limit to be used within. 
  2. It is given on the can. 
  3. After the expiry date, there may be some chemical or irreversible changes in it that are harmful to our health. 
  4. So, while purchasing canned food, its expiry date should be checked.

(b) An iron article should be given a coat of paint.
Ans.

  1. Iron corrodes due to the oxygen, moisture and vapours of chemicals in the air. 
  2. This makes it weak, delicate and short-lived. 
  3. To avoid this, an iron article should be given a coat of paint.


(c) A wooden article should be polished.
Ans.

  1. Wood is attacked by fungus and termites. 
  2. This makes the wooden article weak and short-lived.  
  3. To avoid this irreversible natural damage, a wooden article should be polished.

(d) Copper and brass utensils should be tinned.
Ans.

  1. Copper and brass corrode due to the oxygen, moisture and chemical vapour in the air. 
  2. A greenish layer is seen to form on such articles. 
  3. This is harmful to health. During the tinning process, a thin coating of tin metal is given which prevents corrosion
  4. So, they are tinned.

(e) A dry handkerchief gets wet at once on dipping in water, but it takes longer for a wet handkerchief to dry.

Ans.

  1. Handkerchief getting wet. 
  2. It is a fast change, as it quickly absorbs water. 
  3. But the evaporation of such water takes time. 
  4. That is drying a wet handkerchief is a slow change. 
  5. So, a dry handkerchief gets wet at once on dipping in water but it takes longer for a wet handkerchief to dry.

4. What will you take into account while identifying the following? 

(a) A physical change in a substance. 

I will take into account whether the original substance remains the same, whether the properties of the original substance are not changed and whether its composition remains unchanged. If it is so, it is a physical change.

(b) A chemical change in a substance.

I will observe whether the substance is transformed into another substance or not and whether the new substance has new and different properties or not. If it is so, it is a chemical change.

5. Read the paragraph and note down the various types of changes it mentions.


It was nearing six o'clock in the evening. The sun was setting. A breeze was blowing. Leaves on the tree were shaking. Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys. Then he felt hungry and went into the house. Mother made a dough from wheat flour and fried purees. While eating hot purees, his attention was drawn outside the window. It had started raining. There was lightning, too. Sahil was enjoying his dinner in the dim light.


It was nearing six o'clock in the evening.
Periodic change, 
 

The sun was setting. 
Natural change, Periodic change

A breeze was blowing. 
Natural change,  non-periodic change

Leaves on the tree were shaking. 
Natural change, physical change, non-periodic change

Sahil was sitting in the courtyard, rolling balls of wet soil and shaping them into various toys.
Man-made change, useful change,  reversible change,  physical change
 

Then he felt hungry and went into the house. 
Natural change,  periodic change

Mother made a dough from wheat flour
Man-made change, useful change, reversible change, physical change

 and mother fried purees. 
Man-made change, irreversible change, useful change, chemical change

While eating hot purees, his attention was drawn outside the window.
 It had started raining. 
Natural change, physical change, non-periodic change

There was lightning, too.
Natural change, physical change, fast change, non-periodic change

 Sahil was enjoying his dinner in the dim light.


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11. Cell Structure and Micro- organisms✍️

https://eeducationplusall.blogspot.com/2022/02/cell-structure-and-micro-organisms.html

12. Muscular system and digestive system in human beings ✍️

https://eeducationplusall.blogspot.com/2022/02/mhcert-7th-science-12-muscular-system.html

13. Changes- Physical and Chemical

https://eeducationplusall.blogspot.com/2022/02/mhcert-science-13-changes-physical-and.html

14. Elements, Compounds and Mixtures

https://eeducationplusall.blogspot.com/2022/02/mhcert-science-14-elements-compounds.html

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