Chapter 6 Physical and Chemical Changes Class 7 Notes

Physical and Chemical Changes Class 7 Notes

In this chapter we are going study about the changes that we encounter or see in our everyday life. We see these changes happening in our surroundings. For example- our mother making curd from milk, students cutting paper, burning of paper etc.

The question arises how these changes are occurring and what type of changes are these. We will study about the science behind these changes in higher classes but in this chapter we will study about the types of changes.

All these changes are put into two categories namely- Physical Change and Chemical Change.

Physical and Chemical Changes Class 7 Overview

Let us look at the topics to be discussed in this chapter 6 Physical and Chemical Changes Class 7.

• Physical and Chemical Properties
• Physical Changes and their examples
• Chemical Changes and their examples
• Rusting of Iron
• Galvanization
• Crystallization

 Physical Properties and Chemical Properties

What are physical and chemical properties?

Physical Properties: These are the characteristics of a substance that describe the physical nature of the substance.

Some examples of physical properties are- color. density, shape and size, etc.

Chemical Properties: These are the properties or characteristics of the substance that describe its chemical properties.

Example- the toxicity of a substance, reaction of a substance with other substances, etc.

Physical Change and Chemical Change

Let us define the term physical change and chemical change. We will also look at the difference between physical change and chemical change.

What is a Physical Change?

A physical change is a change when there is a change in the physical properties, as described above, of the substances. In this type of change no new substances are formed.

Some examples of Physical changes are-

Here are some 5 examples of physical change.

• Cutting of paper
• Melting of wax
• freezing of water
• melting of ice
• crushing chalk into dust

What is a Chemical Change?

A chemical change is a change when there is a change in the chemical properties, as described above, of the substances. In this type of change new substances with new properties are formed.

A chemical change is also called a chemical reaction.

Also Read: Chapter 5 Class 7 Acids Bases and Salts Notes

A chemical change is always accompanied by one of the followings-

• Giving off or absorption of heat, light or any other radiation
• change in color
• production of sound
• release of gas
• change in smell
• formation of solid as residue

Some examples of chemical changes are-

Here are some 5 examples of chemical change.

• burning of paper
• rusting of iron
• explosion of fireworks
• Spoiling of food
• Digestion of food

Difference between Physical Change and Chemical Change

Reaction between Vinegar and Baking Soda

When baking soda (Sodium bicarbonate) reacts with vinegar (acetic acid), a hissing sound with bubbles is observed.

• This hissing sound is due to the formation or release of carbon di oxide.

To check if the gas formed is CO₂ or not, it is passed through lime water (Ca(OH)₂).

On passing CO₂ the lime water turns milky due to the formation of Calcium Carbonate (CaCO₃).

CO₂+ Ca(OH)₂ → CaCO₃ + H₂0

Rusting of Iron

Define Rust or rusting of iron.

When iron or substances made up of iron come in contact with moist air (oxygen + water), it reacts and a red colored substance is formed over it. This called Rust and the process is called rusting.

Iron + Moist Air → Rust (Iron oxide + water)

Fe + O₂ + xH₂0 → Fe₂O₃ . xH₂0

Note: Fe₂O₃ . xH₂0 is called Rust.

The rust eats away or damages the iron. So this needs to be prevented.

Methods of Preventing Rusting

As we discussed rusting eats away the materials, so here are some methods to be used for preventing the rusting of iron or materials made up of iron.

Stainless Steel does not rust because it is made up of iron mixed with carbon and metals like chromium, nickel and manganese.

• By applying paint
• By applying oil or grease
• By galvanization

Galvanization

Define Galvanization.

Galvanisation or Galvanization is a process by which a layer of zinc or chromium is applied on iron or substances made up of iron.

Crystallization

Crystallization is a process by which crystals of pure substances are obtained from its solution.

We can obtain crystals of copper sulphate by adding powder of copper sulphate to boiling water and then letting it cool.

Note: Crystallization is a physical change.

We hope you enjoyed these chapter 6 Physical and Chemical Changes Class 7 Notes. These Physical and Chemical Changes Class 7 Notes will surely help you fetch good marks in your class 7 science exams.