Notes-Class 7-Science-Chapter-13-Changes-Physical & Chemical-Maharashtra Board

Introduction : When there is a change in the structure and state of the substances that surround us, then there are changes caused in the surroundings. There are physical, chemical and biological changes occurring around us. Some of the changes are natural while some are man-made changes.

• We need many different things in our day-to-day life. These things are obtained through different types of processes. To satisfy our needs these processes are done and various things are manufactured.
• When man changes the substances around and produce things for his own use, then such changes made by him are called man-made changes. These changes can be physical or chemical changes. E.g. Lighting an electric bulb and cutting vegetables are man-made changes

Types of Changes :

Man-made change : The changes induced by man are called man-made changes. Man-made changes are due to man’s wish to satisfy his needs. e.g. cutting down the trees.

Natural changes : The changes that occur naturally are called natural changes. The falling of a tree in a storm is a natural change

Useful changes: The changes that produce useful things and help in our needs are useful changes. e.g. Sharpening a pencil, cooking food, baking bread etc.

Harmful changes : The changes that are not useful for us and that cause harm to us are harmful changes. e.g. Rusting of iron, spoiling of food etc.

Fast changes : The changes occurring rapidly are called fast changes. They happen in a short period of time.

Slow changes : Changes that take place over a long period of time are slow changes.

Reversible changes : The changes that can occur in a forward and reverse directions repeatedly again and again are reversible changes.

Irreversible changes : The changes that can never occur back in a reverse direction again are irreversible changes.

Periodic changes : Periodic changes are changes that occur again and again after a definite interval of time.

e.g., High and low tide of the sea movement of hands of clock.

Non-periodic changes : The non-periodic changes are the changes in which the time interval is not fixed. The change can occur any time or will not occur at all. e.g. Breaking of storm.

Physical change : When the substance changes from one form into another, without the change in basic characteristics, then it is called a physical change. Physical change does not form a substance with new properties. e.g., Processes of dissolving, boiling, melting etc.

Chemical change : The change in which the properties of the substance change and form entirely new substance is called chemical change. e.g. Photosynthesis, corrosion.

Corrosion : Corrosion means rusting. Corrosion is caused by oxygen, moisture, vapours of chemicals in the air. Metals show corroding. e.g., Iron article rusts and get a reddish brown layer on it. On the copper articles there is a greenish layer. Due to corrosion, things become weak.

Tinning and galvanization : A thin coat of zinc is given on the iron articles to prevent corrosion. This is called galvanization. A thin coat of tin is given on copper and brass articles to prevent rusting. This is called tinning.

A new process called powder coating has been developed. In which coats of various colours are applied on metals like iron and aluminium to prevent corrosion

Identify changes in below figure :

Identify changes in below figure :

Figure	Changes
	Milk is turned into yoghurt. This is permanent, chemical and irreversible change.
	Ice is melting forming water. This is a physical change.
	Seed is sprouting. This is a natural change
	The water is boiling. This is a physical change.
	The fire crackers are burning. This is man-made change which is fast, chemical and irreversible in nature.
	The wood is on fire resulting into ashes. This is a chemical change.
	The fire crackers are exploding. This is man-made, fast, chemical and irreversible change.

[collapse]