Class 9 Physical and chemical changes, Types of pure substances

### Topics to be covered

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

### 𝐏𝐡𝐲𝐬𝐢𝐜𝐚𝐥 𝐚𝐧𝐝 𝐂𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐂𝐡𝐚𝐧𝐠𝐞𝐬

color{green}(•) 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}(•) For eg: ice, water and water vapour are chemically the same.

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

color{green}(•) Chemical change brings change in the chemical properties of matter and we get new substances. A chemical change is also called a color{red}("𝐜𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐫𝐞𝐚𝐜𝐭𝐢𝐨𝐧.")

### TYPES OF PURE SUBSTANCES

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

color{green}("𝟏 𝐄𝐋𝐄𝐌𝐄𝐍𝐓𝐒 :") 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}("𝐏𝐑𝐎𝐏𝐄𝐑𝐓𝐈𝐄𝐒 𝐎𝐅 𝐌𝐄𝐓𝐀𝐋𝐒:")

color{green}(•) They have a lustre (shine).

color{green}(•) They have silvery-grey or golden-yellow colour.

color{green}(•) They conduct heat and electricity.

color{green}(•) They are ductile (can be drawn into wires).

color{green}(•) They are malleable (can be hammered into thin sheets).

color{green}(•) 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}("𝐏𝐑𝐎𝐏𝐄𝐑𝐓𝐈𝐄𝐒 𝐎𝐅 𝐍𝐎𝐍-𝐌𝐄𝐓𝐀𝐋𝐒:")

color{green}(•)They display a variety of colours.

color{green}(•) They are poor conductors of heat and electricity.

color{green}(•) 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 𝐂𝐎𝐌𝐏𝐎𝐔𝐍𝐃𝐒")

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