Std 7 Science Chapter 13 Changes – Physical and Chemical – question and answers

Exercise

1. Distinguish between the following.

(a) Physical change and chemical change

Physical Change	Chemical Change
1. The properties of the original substances remain the same.	1. The properties of the original substances does not remain the same.
2. Their chemical composition remains unchanged.	2. Their chemical composition does not remain the same.
3. No new substance is formed.	3. New substances are formed.
4. Change is reversible.	4. Change is irreversible.
5. Change temporary.	5. Change is permanent.
6. e.g. water freezing to ice	6. milk turning to curds

(b) Periodic change and non-periodic change

Periodic change	Non-periodic change
1. Changes that occur again and again after a definite interval of time.	1. It cannot be said for sure when certain changes will recur after one occurrence.
2. e.g. day-night	2. e.g. bird flying from a tree.

(c) Natural change and man-made change

Natural Change	Man-made Change
1. Changes that take place by nature.	1. Changes that are made by man.
2. occurrence of day & night, change of seasons, ripening fruits, etc	2. Making furniture, baking bread, sharpening pencil, etc.

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) Occurrence of an earthquake.	Natural Change
(d) Revolution of the earth around the sun.	Periodic Change
(e) Stretching of a spring.	Physical Change

3. Give reasons.

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

Ans. While purchasing canned food, its expiry date should be checked because all canned food can be consumed within a particular period. Micro-organisms may start growing in the canned food after the date of expiry. If we consume such food we may fall sick.

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

Ans. Iron articles can get rusted when exposed to air. This is called corrosion. Corrosion is caused by oxygen, moisture, vapours of chemicals in the air. Iron articles are given a thin coat of zinc to prevent corrosion.

(c) A wooden article should be polished.

Ans. Wood is a natural material. Wooden articles can be affected by heat, moisture or termites. In order to protect the wooden article from termites and for it to last long, it is polished.

(d) Copper and brass utensils should be tinned.

Ans. A greenish layer is seen to form on a copper and brass article when exposed to air. This is corrosion. In order to prevent the copper and brass utensils from corrosion they are coated with tin.

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

Ans. A dry handkerchief gets wet because it quickly absorbs the water. On the other hand, it takes long for it to dry because the water has to evaporate which is a slow process.

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

(a) A physical change in a substance.

Ans. While identifying whether there is physical change in a substance, I will consider the following:

1. The properties of the original substances remain the same.

2. Their chemical composition remains unchanged.

3. No new substance is formed.

4. Change is reversible.

5. Change temporary.

 (b) A chemical change in a substance.

Ans. While identifying whether there is chemical change in a substance, I will consider the following:

1. The properties of the original substances does not remain the same.

2. Their chemical composition does not remain the same.

3. New substances are formed.

4. Change is irreversible.

5. Change is permanent.

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 (Time-Irreversible, Periodic).
• The sun was setting (Periodic change).
• A breeze was blowing (Natural, non-periodic 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, periodic change) and went into the house.
• Mother made a dough from wheat flour and fried purees (Physical Irreversible change).
• While eating hot purees, his attention was drawn outside the window. It had started raining (Natural change).
• There was lightning (Natural, Physical, Chemical change)

Project : Visit a place where work like powder coating, spray painting is done. Make a note of the information you obtain.

Let’s recall.

1. What are the causes of the changes occurring in our surroundings?

Ans. Changes occurring in our surrounding may be natural changes or man-made changes.

2. What is meant by man-made changes ? Which are these changes ?

Ans. Changes that are made by man are man-made changes. For example making furniture, baking bread, sharpening a pencil, etc.

3. What processes occur during the change of state of matter ?

Ans. During the change of state of matter the processes of evaporation and condensation occur.

4. What happens when water is taken in a bowl and heated ?

Ans. When water is taken in a bowl and heated, it will change to water vapour.

5. We have studied some processes that are a part of the water-cycle. Which are these processes ?

Ans. Evaporation and condensation are the processes that occur during the water cycle.

6. Do the original properties of water change during those processes ?

Ans. No the original properties of water do not change during the process of the water-cycle.

Observe and discuss.

What changes have taken place in the materials seen in the following pictures ? What kind of changes are they ?

Which points will you consider while classifying the following changes into two groups – a fruit falling from a tree, rusting of iron, raining, lighting an electric bulb, cutting vegetables ?

Ans.

Physical Change	Chemical Change
Fruit falling from a tree, raining, cutting vegetables	rusting of iron, lighting an electric bulb

Which of the above changes have occurred of their own accord or naturally ?

Natural Change	Man-made Change
fruit falling from a tree, raining, rusting of iron	cutting vegetables, lighting an electric bulb

1. Which of the changes shown in the picture alongside are temporary ?

Ans. Separating iron fillings and ice melting into water are temporary changes.

2. Which of the changes are permanent ?

Ans. Raw mango changing to ripe mango and the bud changing into a flower.

3. In which of the changes did the original matter undergo a change ?

Ans. When a raw mango changed into a ripe mango the original matter underwent a change.

4. In which, did the original matter remain unchanged ?

Ans. Separation of iron filing and ice changing into water, in these to changes the original matter remained unchanged.

5. In which of the changes was a new substance with a new property formed ?

Ans. In the change from raw mango to ripe mango the original substance formed into a new property.

Use your brain power !

1. What kind of change is the falling of a tree in a storm ?

Ans. Falling of a tree in a storm is a natural, physical but harmful change.

2. What kind of change is the conversion of milk into yoghurt ?

Ans. Milk turning into yogurt is a natural, chemical and a useful change.

3. Which of the following are physical changes and which are chemical – making a table from wood, burning wood, breaking of a glass object, ripening of a tomato, rusting of iron ?

Physical Change	Chemical Change
making a table from wood, breaking of a glass object,	burning wood, ripening of a tomato, rusting of iron

Give some examples of fast and slow changes that occur in your surroundings.

Fast Change	Slow Change
bursting of balloon, tearing of paper, cutting an apple, etc.	ripening of a fruit, milk turning into curds, preparing food, etc.

3. What is meant by irreversible change ? Give some examples.

Changes that occur only once and cannot be brought back to its original form is called irreversible change. for examples ripening of a fruit, cutting of vegetable, bursting a balloon, etc.

4. Which type of change is the change of seasons from summer to rains to winter ?

Ans. The change of season is a periodic change.

7. Which hands of a clock show periodic changes from 6.00 am to 6.00 pm? How many times ?

Ans. Both the hour hand and the minute hand of the clock show periodic change. This change occurs twice every day.

Can you tell ?

1. You made a link from the glass piece. How will you change the shape of the link to give it its original shape ?

Ans. We will heat each piece carefully and try to straighten the glass pieces which we have bent earlier to form a link.

2. How will you restore the original candle after melting one ?

Ans. We can restore the original candle by heating the melted pieces of candle and putting them in a candle mould and let it solidify.

3. What comes after daytime ?

Ans. Night time follows daytime.

4. What is the next stage after sunrise ?

Ans. The next stage after sunrise is sunset.

5. What comes after a high tide in the sea ?

Ans. After high tide come low tide in the sea.