Science textbook solutions for Class 7 science Chapter 13 Changes – Physical and Chemical


Important points to remember : 
  1. Changes like  the  ripening of a fruit, spoiling of milk occur naturally. These are called  natural changes. 
  2. Many changes such as sharpening a pencil, baking bread, cooking food are useful to us and these are, therefore, called  useful changes.  
  3. The changes that are not useful  or changes that  do us harm are called harmful changes. 
  4. The changes that can occur in a forward and reverse direction again and again are called  reversible changes. some changes occur again and again after  a definite  interval  of time.  Such changes are called  periodic changes. 
  5. time interval is  not  fixed. Such  changes are called  non-periodic changes. 
  6. The change due to which one substance is transformed into another substance having new  and  different properties is  called a chemical change.
  7. properties of  the original substances remain the same, that is, their composition remains unchanged. No  new substance is formed. Such a change is called a  physical change.


Question 1: Distinguish between the following.
a) Physical change and chemical change
Answer :Physical changes refer to such changes in the material when the mass of the substance and the internal arrangement of the molecules and atoms remains same, even after the reaction. On the contrary, when changes involve the formation of the new substance from the original substance is called a chemical change.

b) Periodic change and non-periodic change
Answer : Periodic changes are changes that occur periodically at regular intervals and non-periodic changes are changes that do not occur periodically at regular intervals

c) Natural change and man-made change
Answer : natural changes are changes that occur naturally and man has no part to play in these changes. Manmade changes are changes that occur due to man and man plays a role in these changes.


Question 2: Under which different types can the following changes be classified?
a) Conversion of milk into yoghurt- Chemical change

b) Bursting of a cracker- Chemical change

c) Occurence of an earthquake- Natural change

d) Revolution of the earth around the sun- Periodic change

e) Stretching of a spring- Physical change


Question 3: Give reasons.
a) While purchasing canned food, its expiry date should be checked.
Answer :Customers should always check the expiry date before buying canned food because eating spoilt or rotten food can lead to various health problems such as food poisoning as well as many digestion diseases. Death can also occur.

b) An iron article should be given a coat of paint
Answer : Answer: Iron articles are painted to prevent them from rusting. After painting iron surface does not come in contact with moisture and air which cause rusting.

c) A wooden article should be polished.
Answer : A wooden article should be polished because it protects the wooden article from sunlight and all other external conditions and also it reduces the probability of wooden mites damaging the furniture.

d) Copper and brass utensils should be tinned.
Answer: Tin coating is the process of applying a protective tin coating to copper or brass to prevent corrosion. Copper is somewhat poisonous in nature if taken in excess. So copper & brass utensils are coated with tin so as to prevent direct contact to food

e) A dry handkerchief gets wet at once on dipping in water, but it takes long for a wet handkerchief to dry.
Answer : Answer: A dry handkercheif gets wet at once because it quickly absorbs the water in which it is dipped. However, it takes longer for a wet handkerchief to dry because the water needs to evapourate from the handkerchief.


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

a) A physical change in a substance.
Answer : physical change involves a change in physical properties. Examples of physical properties include melting, transition to a gas, change of strength, change of durability, changes to crystal form, textural change, shape, size, color, volume and density.

b) A chemical change in a substance.
Answer : The signs of chemical change include changes in color, temperature, the production of light, changes in odor, and the formation of gases.


Question 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.

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


#let's recall 
1.  What are the causes of the changes occurring in our surroundings ? 
Answer : Due to heavy rainfall, excessive water flows through the lands which changes the surrounding. The unexpected movement of tectonic plates called Earthquake, destroys most of the buildings if its height is high. Soil erosion erodes most of the soil which causes land sliding around us.

2. What is meant by man-made changes ? Which are these changes ?
Answer : Man - made changes are those changes which are created by humans and it does not occur naturally . For example : pollution of air is created by humans by burning fossil fuels and smoke coming out from vehilcles and factories.


#Use your brain power 
1. What kind of change is the falling of a tree  in a storm ?
Answer.  :  Changes that occur due to environmental factors are called natural changes. e.g. Storms, drought, falling tree, forest-fire, etc. - Changes that occur to human intervention are called manmade changes

2. What kind of change  is the  conversion of milk   into yoghurt ?
Answer : chemical change : Milk is converted into curd or yogurt by the process of fermentation. Milk consists of globular proteins called casein. The curd forms because of the chemical reaction between the lactic acid bacteria and casein.

3. Give some examples of fast and slow changes that occur in your surroundings.
Answer : Examples for fast changes: Burning of paper, Burning of a candle. Examples for slow changes: Rusting of iron, Germination of seed, curdling of milk etc.

4. What is meant by irreversible change ? Give some examples.
Answer : The changes that can occur in a forward and reverse direction again and again are called  reversible changes. A  ripe mango, however, cannot be transformed  back into  a raw mango. Wood cannot  be obtained from the  ash formed on burning the wood.

5. Which type of  change is  the change  of seasons from summer to rains to winter ? 
Answer : Summer is a warm season because the days are longer and the Sun is high in the sky, giving direct light to the ground. Winter is a cold season because the days are shorter and the Sun is low in the sky, giving indirect light to the ground.

6. Which  hands of  a clock  show periodic  changes from 6.00 am to 6.00 pm ? How many times ?
Answer : As the clock's hands move throughout the day from 6.00 am to 6.00 pm, the minute and hour hands are pointing in the same direction 11 times.

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