Page 42 - Chemistry ICSE Class IX

2 Chemical Changes and

Reactions

Everything around us changes with time. Some changes can be noticed
CONTENTS immediately, some others after some time, while some others go unnoticed.
Some common changes which occur around us are,
ͻ WŚLJƐŝĐĂů ĐŚĂŶŐĞƐ
ͻ ŚĞŵŝĐĂů ĐŚĂŶŐĞƐ ƒ Evaporation of water ƒ Cooking of rice
ͻ ŚĂŶŐĞƐ ŝŶǀŽůǀŝŶŐ ďŽƚŚ ĐŚĞŵŝĐĂů ƒ Germination of seeds ƒ Growing of plants
ĂŶĚ ƉŚLJƐŝĐĂů ĐŚĂŶŐĞƐ ƒ Ripening of fruits ƒ Burning of wood
ͻ ŽŶĚŝƟŽŶƐ ŶĞĐĞƐƐĂƌLJ ĨŽƌ Ă ĐŚĞŵŝĐĂů
ƌĞĂĐƟŽŶ ƚŽ ƚĂŬĞ ƉůĂĐĞ ƒ Melting of ice ƒ Change of day and night
ͻ dLJƉĞƐ ŽĨ ĐŚĞŵŝĐĂů ƌĞĂĐƟŽŶƐ ;Žƌ ƒ Flowering of plants ƒ Burning of reworks
ĐŚĞŵŝĐĂů ĐŚĂŶŐĞƐͿ ƒ Formation of curd from milk ƒ Growing of a baby
ͻ ŶĞƌŐLJ ĐŚĂŶŐĞƐ ŝŶ ĐŚĞŵŝĐĂů
ƌĞĂĐƟŽŶƐ These changes become noticeable due to one or more of the following
ͻ ZĞĂĐƟŽŶƐ ŝŶ ǁŚŝĐŚ ŚĞĂƚ ŝƐ ĞŝƚŚĞƌ changes:
ĂďƐŽƌďĞĚ Žƌ ĞǀŽůǀĞĚ ƒ Change in position, shape and size ƒ Change in colour
ͻ ZĞĂĐƟŽŶƐ ŝŶ ǁŚŝĐŚ ĞůĞĐƚƌŝĐĂů ĞŶĞƌŐLJ ƒ Change in physical state ƒ Change in temperature
ŝƐ ĞŝƚŚĞƌ ĂďƐŽƌďĞĚ Žƌ ĞǀŽůǀĞĚ Ͷ
ĞůĞĐƚƌŽĐŚĞŵŝĐĂů ƌĞĂĐƟŽŶƐ ƒ Change in composition ƒ Change in structure
ͻ ZĞĂĐƟŽŶƐ ŝŶ ǁŚŝĐŚ ůŝŐŚƚ ĞŶĞƌŐLJ There is always a cause for a change.
ŝƐ ĞŝƚŚĞƌ ĂďƐŽƌďĞĚ Žƌ ĞǀŽůǀĞĚ Ͷ In this chapter, you will learn something about Physical and Chemical
WŚŽƚŽĐŚĞŵŝĐĂů ƌĞĂĐƟŽŶƐ
ͻ ŽŵďƵƐƟŽŶ changes and their causes.
ͻ ŚĂŶŐĞ ŝŶ ŵĂƐƐ ŽŶ ďƵƌŶŝŶŐ
ͻ ,Žǁ ŝƐ ƚŚĞ ďĂůĂŶĐĞ ŽĨ ĐĂƌďŽŶ Physical Changes
ĚŝŽdžŝĚĞ ĂŶĚ ŽdžLJŐĞŶ ŵĂŝŶƚĂŝŶĞĚ ŝŶ
ŶĂƚƵƌĞ What is a physical change
ͻ KdžLJŐĞŶ ĐLJĐůĞ
ͻ ĂƌďŽŶ ĚŝŽdžŝĚĞ ĐLJĐůĞ A physical change may be de ned as follows:
A change in which only the physical properties of substance get
changed, while its chemical composition remains unchanged, is called a
physical change.
During a physical change, physical properties, such as colour, volume,
size, shape, and magnetic properties, of the substances may change.
Some typical physical changes are
ͻഩ ǀĂƉŽƌĂƟŽŶͬĐŽŶĚĞŶƐĂƟŽŶ ŽĨ ǁĂƚĞƌ For example, when sugar is added to water, it disappears to form
;Žƌ ĂŶLJ ŽƚŚĞƌ ůŝƋƵŝĚͿ a solution. In the solution, both water and sugar retain their properties.
ͻഩ ŝƐƐŽůƵƟŽŶ ŽĨ ĐŽŵŵŽŶ ƐĂůƚ Žƌ Sugar can be recovered from its solution by evaporating water. Therefore,
ƐƵŐĂƌ ŝŶ ǁĂƚĞƌ dissolution of sugar in water is a physical change.
ͻഩ ƌLJƐƚĂůůŝƐĂƟŽŶ ŽĨ Ă ƐƵďƐƚĂŶĐĞ ĨƌŽŵ
ŝƚƐ ƐŽůƵƟŽŶ What are the characteristics of a physical change
ͻഩ DĂŐŶĞƟƐĂƟŽŶ ŽĨ ĂŶ ŝƌŽŶ ƉŝĞĐĞ ĂŶĚ A physical change is characterised by the following properties:
ĚĞŵĂŐŶĞƟƐĂƟŽŶ ŽĨ Ă ŵĂŐŶĞƚ
ͻഩ ^ƵďůŝŵĂƟŽŶ ŽĨ ŝŽĚŝŶĞ͕ ĐĂŵƉŚŽƌ͕ ĞƚĐ ƒ In a physical change, only the physical properties of the substances
ͻഩ 'ůŽǁŝŶŐ ŽĨ ĂŶ ĞůĞĐƚƌŝĐ ďƵůď undergo change. Their chemical properties remain unchanged.
ͻഩ &ŽƌŵĂƟŽŶ ŽĨ ĚĞǁ ƒ In a physical change, no new substance is formed.
ͻഩ ƌĞĂŬŝŶŐ ŽĨ Ă ŐůĂƐƐ ĐŽŶƚĂŝŶĞƌ ƒ A physical change can be reversed easily, i.e., the original form of the
ͻഩ ĞŶĚŝŶŐ ŽĨ Ă ŐůĂƐƐ ƚƵďĞ substance can be obtained easily by simple methods.
ͻഩ DĞůƟŶŐ ŽĨ Ă ƐŽůŝĚ
ͻഩ ƌLJŝŶŐ ŽĨ ĐůŽƚŚĞƐ ƒ Physical changes are temporary and can be reversed. That is, physical
changes are reversible.