Ch 2 Physical Changes and Chemical changes class 7th chemistry ICSE ‎

What is a change?
Change can be defined as alteration in physical and chemical properties of matter which give a new shape and properties to it.
Changes may involved different kinds of alterations in shape, size, colour, state and composition
Various changes have been classified into the following types as:
Natural changes : Changes which takes place by nature are known as Natural changes. For example: change in weather, flowering of plant, etc.


Man made changes: The changes that takes place due to activities of man are called man made changes.
For example: burning of fuels, making of chapattis, formation of curd, etc.

Periodic changes: The changes that occur after a fixed interval of time are called periodic changes.
For example: phenomenon of day and night

Non periodic changes : Other changes that may take place at any time are called non periodic changes.
For example : rusting of iron, breaking of glass, etc.
Desirable changes: The changes which are useful to us are called desirable changes. For example: ripening of fruits, drying of clothes, curdling of milk,etc.

Undesirable changes : The changes which are harmful to us are called undesirable changes.
For example: spoiling of food stuff, breaking of a glass,etc.

Fast changes : The changes which occur in a short duration of time are called fast changes. For example: burning of a paper, burning of matchstick.

Slow changes : The changes which take longer duration of time for their completion are called slow changes. For example: formation of curd from milk, rusting of iron, etc.
Fast and slow are relative terms for example the Heartbeat becomes fast when one is excited and slow when one is at rest

Reversible changes : A change is reversible if the substance can be brought back to their original form after the occurrence of the change.
A reversible change is indicated by double arrow. For example; evaporation and sublimation is an example of a reversible change
Irreversible changes : A change is Irreversible if the substance cannot be brought backwards original state.
For example; burning of matchstick, souring of milk, breaking of Glass tumbler, germination of seed all are examples of Irreversible changes.
Physical changes : 1. A physical change is the one in which the substance undergoing the change is not destroyed.
2. No new element or compound is formed.
3. Only the colour shape or size of the substance may change.
For example : 1. Conversion of water into ice and conversion of water into steam are physical changes because ice and steam can be converted back into water.
2. Breaking of chalk stick.
3. Glowing of an electric bulb.
4. Magnetising an iron nail .
5. Powdering a sugar.
6. Drying of clothes.
7. Breaking of clay pot is a physical change
Chemical changes: A chemical change is the one in which :
1. new substances are formed.
2. properties of the substance before and after the change are different.
3. original properties disappear and new properties are observed.
4. changes brought permanently and change is irreversible.
For example;
burning of paper Take a small piece of paper burn it with the matchstick when paper burns carbon dioxide water vapour and as are produced none of these has the property of paper hence this the chemical change.
2. Burning of wood: During burning of wood new substances like ash and smoke are produced which have properties of different from that of wood. Thus, burning of wood is a reversible change because we cannot get back wood from smoke and ash.
3. Rusting of iron : Rusting converts the grey solid iron into a reddish brown powder solid which cannot be magnetized. Since the properties of rust are different from the iron. Therefore, rusting is a chemical change.
4. Burning of magnesium : When a piece of magnesium ribbon is heated in a flame, it burns with a dazzling light and produces White powder of magnesium oxide.
5. Souring of milk: Milk is a sweet liquid where is curd is sour in taste and is in solid state .
Souring of milk is an irreversible change because we cannot get milk from curd.
Burning of a candle : When candle burns it also melts. Melting is a alone a physical change because on cooling solid wax is formed but burning produces carbon dioxide and moisture which pass into the atmosphere.
Therefore, burning of a candle is an irreversible chemical change because we cannot get back candle from the gas and moisture formed during the burning process.
Cooking of food : When food is cooked many Complex carbohydrates are broken down into simpler Sugars, many water soluble vitamins and fat soluble vitamins are destroyed.
The texture of foods of change which cannot be reversed.
Changes involve energy
The change during which the heat is absorbed are called endothermic changes.
Endo means in and thermic means heat and endothermic change is the one in which the cooling is observed.
For example: if you put a small amount of glucose on your tongue it is alls and your tongue feel cool.
Dissolution: