Page 30 - Understanding NCERT Science 09
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2.3 Physical and Chemical • burning of paper and wood.
2. Try segregating the things
Changes around you as pure substances
or mixtures.
In the previous chapter, we have learnt about
a few physical properties of matter. The 2.4 What are the Types of Pure
properties that can be observed and specified
like colour, hardness, rigidity, fluidity, Substances?
density, melting point, boiling point etc. are
the physical properites. On the basis of their chemical composition,
The interconversion of states is a physical substances can be classified either as elements
change because these changes occur without or compounds.
a change in composition and no change in the
chemical nature of the substance. Although 2.4.1 ELEMENTS
ice, water and water vapour all look different
and display different physical properties, they Robert Boyle was the first scientist to use the
are chemically the same. term element in 1661. Antoine Laurent
Both water and cooking oil are liquid but Lavoisier (1743–94), a French chemist, was the
their chemical characteristics are different. first to establish an experimentally useful
They differ in odour and inflammability. We definition of an element. He defined an element
know that oil burns in air whereas water as a basic form of matter that cannot be broken
extinguishes fire. It is this chemical property down into simpler substances by chemical
of oil that makes it different from water. reactions.
Burning is a chemical change. During this Elements can be normally divided into
process one substance reacts with another to metals, non-metals and metalloids.
undergo a change in chemical composition. Metals usually show some or all of the
Chemical change brings change in the following properties:
chemical properties of matter and we get new • They have a lustre (shine).
substances. A chemical change is also called • They have silvery-grey or golden-yellow
a chemical reaction. colour.
During burning of a candle, both physical • They conduct heat and electricity.
and chemical changes take place. Can you • They are ductile (can be drawn into
distinguish these? wires).
• They are malleable (can be hammered
into thin sheets).
Q 1. Classify the following as chemical copper, iron, sodium, potassium etc. Mercury
uestions
They are sonorous (make a ringing
•
sound when hit).
Examples of metals are gold, silver,
or physical changes:
• cutting of trees,
is the only metal that is liquid at room
• melting of butter in a pan,
• rusting of almirah,
Non-metals usually show some or all of the
• boiling of water to form steam, temperature.
• passing of electric current, following properties:
through water and the water • They display a variety of colours.
breaking down into hydrogen • They are poor conductors of heat and
and oxygen gases, electricity.
• dissolving common salt in • They are not lustrous, sonorous
water, or malleable.
• making a fruit salad with raw Examples of non-metals are hydrogen,
fruits, and oxygen, iodine, carbon (coal, coke), bromine,
IS MATTER AROUND US PURE? 19
Rationalised 2023-24
