Physical and Chemical Changes | Class 7 Science for NMMS 2024-25

 Physical Changes

Physical Properties of a substance are those characteristics of a substance that describes its physical nature.

Examples of physical properties are colour, density, shape, size and volume are some physical properties.

Physical Change is a change which occurs when there is an alteration in the physical properties of a substance.

The physical change does not result in the formation of any new substance but can alter the shape and size of the existing substance.

Physical Properties:

Melting of ice is a physical change

The physical change is generally reversible

Chemical Change

The chemical property of a substance are that characteristic of a substance that describes its chemical nature. For Example toxicity of a substance or how a substance reacts with other substances is its chemical property.

A chemical change or chemical reaction is any change in the chemical properties of a substance. Whenever a substance undergoes a chemical change, a new substance is formed.

Examples of Chemical Changes:

• Rusting of iron occurs when iron gets in contact with moisture
• When a magnesium ribbon is burnt it radiates white light and converts into ashes
• Extracting of iron from the iron ore
• Formation of plastic

Chemical Properties

A chemical change is always accompanied by any one or all of the following way:

• Radiation or absorption of heat
• Production of sound
• Change in the colour of the substanc
• Change in the smell of the substance
• Formation of a gas
• Formation of a solid as residue

The characteristics of chemical changes are as follows:

1. One or more new substances are formed in a chemical change.
2. A chemical change is a permanent change.
3. Sound may be produced in a chemical change.
4. A permanent change in colour may take place.
5. A gas may be formed during the chemical change.

Magnesium (Mg) + Oxygen (O2) $\to$ Magnesium oxide (MgO)

Copper Sulphate Solution (blue) + Iron $\Rightarrow$ Iron Sulphate Solution (green) + Copper (brown deposit)

Physical change	Chemical change
Physical changes are mostly reversible.	Chemical changes are not reversible.
No new substances are formed.	One or more new substances are formed.
The substance retain its chemical properties.	The new substances formed have different properties from the original substance.

A change or reaction in which heat is released is called an exothermic reaction.
Carbon + Oxygen $\Rightarrow$ Carbon dioxide + Heat

A change or reaction in which heat is absorbed from the surrounding is called an endothermic reaction.
Carbon + Sulphur + Heat $\Rightarrow$ Carbon disulphide

Rusting of Iron

When the iron comes in contact with oxygen and water, reacts and forms a red colored substance over it. It is called Rust.

The rusting of iron is a slow continuous process which makes the object useless.
Iron (Fe) + Oxygen (O2, from the air) + water (H2O) $⟶$ rust (iron oxide Fe2O3)

The rusting of iron is a chemical change because iron oxide is formed as a new product.
 

The following conditions are essential for rusting:

1. Presence of oxygen.
2. Presence of water or water vapour.

How to prevent rusting of iron

• By applying paint on iron objects so that they cannot come in contact with oxygen and moisture in the environment
• Galvanization of iron which means applying a layer of zinc or chromium metals on the iron.

Crystallisation

Crystallisation: The process of cooling a hot concentrated solution of a substance to obtain crystals is called crystallization.

Formation of crystals of copper sulphate:

1. Take a cupful of water in a beaker and add a few drops of dilute sulphuric acid.
2. Heat the water.
3. When it starts boiling add copper sulphate powder slowly while stirring continuously.
4. Continue adding copper sulphate powder till no more powder can be dissolved.
5. Filter the solution and allow it to cool.
6. Look at the solution after some time to visualize the crystals.
   

The process of evaporation is not a good technique of separation because of following reasons:

1. The soluble impurities do not get removed in the process of evaporation of a salt solution.
2. The crystals of salts obtained by the process of evaporation are small and the shape of crystals cannot be seen clearly.