Maharashtra Board Class 7 Science Chapter 13 Changes Physical and Chemical PDF Download

Changes – Physical and Chemical

Let's recall.

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

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

Observe and discuss.

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

13.1 Various Changes

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?

Can you tell?

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

We have previously studied some examples of changes. Changes like the ripening of a fruit, spoiling of milk occur naturally. These are called natural changes. Can you think of other examples of such natural changes?

What changes have you seen taking place in the materials in your surrounding? Let us learn about changes in greater detail in this lesson.

Can you tell?

We see many man-made materials in our day-to-day life. For what purposes are they produced?

Many changes such as sharpening a pencil, baking bread, cooking food are useful to us and these are, therefore, called useful changes. The changes that are not useful or changes that do us harm are called harmful changes.

Use your brain power!

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

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

Teacher's Note

Changes happen all around us. Like when we cook rice, it becomes soft. This is a useful change. When iron gets rust, it becomes weak. This is a harmful change.

Exam Trick

Remember: Natural changes happen on their own. Man-made changes need our help. In your exam, think – did nature do it alone, or did a person do it?

Points to Remember

Natural changes happen without our help.
Man-made changes happen because we do them.
Useful changes help us in our life.
Harmful changes damage things or hurt us.

Think About It

Can you now classify the natural and man-made changes you have listed as useful and harmful changes? So far, we have studied certain types of changes. What can you tell about two of these, namely, bursting of a balloon and ripening of a fruit, from the point of view of their duration i.e. the time they take to happen?

The duration of the bursting of a balloon is far shorter than that of the ripening of a fruit. Changes that take place in a short period of time are called fast/quick changes. While changes that take place over a long period of time are called slow changes.

Use your brain power!

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

Have some fun!

Apparatus: Pieces of glass bangles, a candle, a matchbox, etc.

Procedure: Hold a piece of glass bangle in the candle flame supporting it with your fingers. Take care to heat the piece in the middle keeping its two ends outside the flame. Observe it as the middle gets heated and becomes soft. It bends due to the finger-pressure on its ends. In this way, bring the two ends together and let the piece cool.

A toran can be made by linking such pieces with each other. Make such torans of different patterns and colours and decorate your class and home.

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?

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

Melting some wax and obtaining the original solid wax again is something that we can do repeatedly. 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.

Can you tell?

Use your brain power!

What is meant by irreversible change? Give some examples.

Can you tell?

1. What comes after daytime?

2. What is the next stage after sunrise?

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

4. A bird sitting on a tree flies away.

5. Flooding

6. A meteor streaking across the sky.

On considering the above examples, we find that some changes occur again and again after a definite interval of time. Such changes are called periodic changes. On the other hand, it cannot be said for sure when certain changes will recur after one occurrence. Even if they recur, the time interval is not fixed. Such changes are called non-periodic changes.

Teacher's Note

Reversible changes can be undone. Like melting ice turns to water, but freezing water gives ice again. Irreversible changes cannot be undone. Like burning paper to ash cannot be reversed.

Exam Trick

Remember: Reversible = can go back. Irreversible = cannot go back. Like you can fold a paper many times, but once you burn it, you cannot get it back.

Points to Remember

Fast changes happen very quickly.
Slow changes take a long time.
Reversible changes can be undone.
Irreversible changes cannot be undone.
Periodic changes happen again and again at the same time.

Use your brain power!

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

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

Observe.

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

2. Which of the changes are permanent?

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

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

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

In some of the above examples of changes, the 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.

The change due to which one substance is transformed into another substance having new and different properties is called a chemical change.

13.2 Various Chemical and Physical Changes

Let's recall.

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

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

The process of formation of vapour from a liquid is called evaporation. Drying of clothes, formation of salt from seawater are possible due to evaporation. We have studied some processes that are a part of the water-cycle. Which are these processes? Do the original properties of water change during those processes? Previously, we have learnt about the processes of dissolving, boiling, melting. They are all examples of physical change.

Use your brain power!

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?

Try this.

Apparatus: Evaporating dish, sugar, burner, tripod, etc.

Procedure: Take some sugar in an evaporating dish. Place the dish on a tripod and heat it. Observe the changes taking place in the sugar. Stop heating when a blackish substance is seen at the bottom of the evaporating dish. What kind of change does the above process bring about?

Teacher's Note

Physical changes do not make new things. Chemical changes make new things. Like boiling water is still water, but burning wood makes ash which is a new thing.

Exam Trick

Remember: Physical = looks different but same thing inside. Chemical = becomes a completely new thing. Like melting chocolate is physical, but baking a cake is chemical.

Points to Remember

Physical changes do not create new substances.
Chemical changes create new substances.
Dissolving sugar in water is a physical change.
Burning wood is a chemical change.
Rusting of iron is a chemical change.

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