Class 9 Physical and chemical changes, Types of pure substances

Topics to be covered

`=>` Physical and chemical changes
`=>` Types of pure substances

𝐏𝐑𝐲𝐬𝐒𝐜𝐚π₯ 𝐚𝐧𝐝 π‚π‘πžπ¦π’πœπšπ₯ π‚π‘πšπ§π πžπ¬

`color{green}(β€’)` The interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance.

`color{green}(β€’)` For eg: ice, water and water vapour are chemically the same.

`color{green}(β€’)` Burning is a chemical change. During this process one substance reacts with another to undergo a change in chemical composition.

`color{green}(β€’)` Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a `color{red}("𝐜𝐑𝐞𝐦𝐒𝐜𝐚π₯ 𝐫𝐞𝐚𝐜𝐭𝐒𝐨𝐧.")`


On the basis of their chemical composition, substances can be classified either as elements or compounds.

`color{green}("𝟏 π„π‹π„πŒπ„ππ“π’ :")` Pure substances that are made up of only one kind of atoms are known as elements.

Elements can be further divided into metals,non-metals and metalloids.

`color{green}("ππ‘πŽππ„π‘π“πˆπ„π’ πŽπ… πŒπ„π“π€π‹π’:")`

`color{green}(β€’)` They have a lustre (shine).

`color{green}(β€’)` They have silvery-grey or golden-yellow colour.

`color{green}(β€’)` They conduct heat and electricity.

`color{green}(β€’)` They are ductile (can be drawn into wires).

`color{green}(β€’)` They are malleable (can be hammered into thin sheets).

`color{green}(β€’)` They are sonorous (make a ringing sound when hit).

Examples of metals are gold, silver, copper, iron, sodium, potassium etc. Mercury is the only metal that is liquid at room temperature.

`color{green}("ππ‘πŽππ„π‘π“πˆπ„π’ πŽπ… 𝐍𝐎𝐍-πŒπ„π“π€π‹π’:")`

`color{green}(β€’)`They display a variety of colours.

`color{green}(β€’)` They are poor conductors of heat and electricity.

`color{green}(β€’)` They are not lustrous, sonorous or malleable.

Examples of non-metals are hydrogen, oxygen, iodine, carbon (coal, coke), bromine, chlorine etc.

Some elements have intermediate properties between those of metals and non-metals, they are called metalloids; examples are boron, silicon, germanium etc.

`color{green}("𝟐 π‚πŽπŒππŽπ”ππƒπ’")`

A compound is a substance composed of two or more elements, chemically combined with one another in a fixed proportion.