Page 43 - Chemistry ICSE Class IX
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Chemical Changes and Reactions 31
Usually no or very small amount of energy change is involved in
physical changes.
Chemical Changes
What is a chemical change
A chemical change may be de ned as follows:
A change in which chemical composition and chemical properties
of the reacting substances undergo a change is called a chemical change. Rusting of iron
Thus, due to a chemical change, the nature of the reacting substance is
completely changed and a new substance is formed. The new substance so Some common chemical changes
formed is called the product. Properties of the product are different from are
those of the original substance. For example, when carbon is burnt, carbon ͻഩ ƵƌŶŝŶŐ ŽĨ ĐŽĂů ;Žƌ ĐĂƌďŽŶͿ͕ ǁŽŽĚ͕
dioxide is produced. Carbon dioxide has properties entirely different from ƉĂƉĞƌ͕ ĞƚĐ͘
those of carbon. Thus, the burning of carbon (or coal) is a chemical change. ͻഩ ůŽƫŶŐ ŽĨ ďůŽŽĚ
Carbon (s) + Oxygen (g) o Carbon dioxide (g) ͻഩ ƵƌŶŝŶŐ ŽĨ Ă ĐĂŶĚůĞ
usually black solid, from air colourless gas which ͻഩ ^ŽƵƌŝŶŐ Žƌ ĐƵƌĚůŝŶŐ ŽĨ ŵŝůŬ
combustible material turns limewater milky ͻഩ ƵƌŶŝŶŐ ŽĨ Ă ŵĂƚĐŚƐƟĐŬ
(noncombustible) ͻഩ ƵƌŶŝŶŐ ŽĨ ŵĂŐŶĞƐŝƵŵ ŝŶ Ăŝƌ
What are the characteristics of a chemical change ͻഩ ZŝƉĞŶŝŶŐ ŽĨ ĨƌƵŝƚƐ ĂŶĚ ŐƌŽǁŝŶŐ ŽĨ
Ă ƉůĂŶƚ ŝŶƚŽ Ă ƚƌĞĞ
A chemical change is characterised by the following properties: ͻഩ ŝŐĞƐƟŽŶ ŽĨ ĨŽŽĚ
In a chemical change, the chemical composition and chemical ͻഩ ZƵƐƟŶŐ ŽĨ ŝƌŽŶ
properties of the reacting substance are completely changed. For ͻഩ ůĞĐƚƌŽůLJƐŝƐ ŽĨ ǁĂƚĞƌ
example, during the burning of coal, the dark coloured solid coal ͻഩ ĞĐĂLJŝŶŐ ŽĨ ŽƌŐĂŶŝĐ ŵĂƩĞƌ
gets converted into a gaseous substance called carbon dioxide. The ͻഩ ƵƌƐƟŶŐ ŽĨ Ă ĐƌĂĐŬĞƌ
composition and properties of carbon dioxide are entirely different ͻഩ ƵƩĞƌ ƚƵƌŶŝŶŐ ƌĂŶĐŝĚ
from those of coal.
Whenever a chemical change takes place, one or more new substances
are formed. For example, during the burning of coal, the new product
formed is carbon dioxide.
A chemical change cannot be reversed easily, i.e., a substance
undergoing chemical change cannot be regained by simple methods.
For example, it is not possible to get back milk after it is converted into
curd.
A chemical change is a permanent change. For example, a grown-up
tree cannot be converted back into a small plant.
Burning of coal
During a chemical change, a certain amount of energy in the form
of heat, light or sound, is either absorbed or evolved. For example,
when a cracker bursts, heat, light and sound energies are evolved.
How does a physical change differ from a chemical change
The main points of difference between a physical and a chemical change are
given in Table 2.1.
Table 2.1 Differences between physical and chemical changes
Physical change Chemical change
1. In a physical change, only physical properties, such as 1. In a chemical change, the chemical composition and
colour, physical state and volume, change. Chemical chemical properties of the reactants undergo a change.
properties remain unchanged.
2. No new substance is formed. 2. A new substance is formed. It has properties different
from those of the reacting substances.
