Page 56 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

6. Physical and Chemical Changes

Chapter in a Nutshell

v Changes around us are of two types—physical changes & chemical changes.
v A physical change is a change which occurs when there is an alteration or change in the
physical properties of a substance. This change does not result in the formation of a new

substance. For example, when we cut a banana, no new substance is formed.
v Most of the physical changes are reversible. Heat may or may not evolve or absorb during a

physical change.
v Changes in which new substances are formed are called chemical changes. A new substance

is formed after a chemical change whose properties are entirely different from that of the
original substances. These changes are also called chemical reactions. For example, baking

of bread, etc.
v A chemical change is a permanent change. It cannot be reversed by simple physical processes.
Energy in any form may be released or absorbed during a chemical change.

v In a chemical reaction, the substances which take part in bringing about the change are
called reactants whereas the substances produced as a result of the reaction are called

products.
v Based on the energy released in a chemical reaction, they can be of two types—exothermic
reaction & endothermic reaction.

v An exothermic reaction is a reaction in which energy is released in the form of heat or light.
For example, the reaction between water and calcium chloride to form hydrochloric acid

and calcium hydroxide.
v An endothermic reaction is a reaction that absorbs heat from the environment. The

absorbed heat energy provides energy for the reaction to take place. For example, melting
of ice, mixing of water and potassium chloride, etc.

v Objects made of iron when exposed to moist air, acquire a dull reddish-brown coating called
rust, after sometime. This process is called rusting.
Fe + H O + O  Fe O .H O
2 2 2 3 2
Iron Water Oxygen Hydrated iron oxide
v Rusting of iron can only take place in the presence of moisture (water vapour) & air (oxygen).

Rusting of iron cannot take place in absence of these two.
v Rusting of iron is a permanent chemical change because we cannot get back the iron from

its oxide.

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