Page 46 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

v Just as atoms, ions can exist independently or in groups. Such ions, made up of two or more

atoms are called polyatomic ions. For example, a hyroxide ion is made up of hydrogen (H)
and oxygen (O) and carries a negative charge. It is represented as OH .
–
v When oppositely charged ions are attracted towards each other by electrostactic forces of
attraction, they form compounds called ionic compounds.

v When ionic compounds are dissolved in water, they dissociate into their constituent ions.
For example, when sodium chloride (NaCl) is dissolved in water, it dissociates into Na and
+
Cl ions.
–
v When two or more chemical substances interact with each other to form new chemical

substances, it is called a chemical reation.
v Substances that react with each other in a chemical reaction are called reactants while the

new substances formed in a reaction are called products.
v A representation of a chemical reaction using symbols of elements and compounds is called

a chemical equation.
v Various compounds in nature are broadly classified as acids, bases and salts.

v The chemical substances which produce a particular colour change and help detect the
nature of the various substances are called indicators.

v Acids are chemical substances that are sour in taste and acidic in nature. Substances such as
lemon, orange, tamarind, amla, grapes and curd are acidic.

v Acids found naturally in plant and animal products are called natural or organic acids. These

acids are edible. For example, oxalic acid, malic acid, etc.
v Acids made in laboratory from the minerals found in the earth’s crust are called mineral

acids. For example, sulphuric acid (H SO ), hydrochloric acid (HCl), etc.
2 4
v Acids that dissociate fully in water to produce the maximum number of H ions are known
+
as strong acids. Higher the concentration of H ions, stronger will be the acid. For example,
+
sulphuric acid (H SO ) & nitric acid (HNO ). These are highly corrosive.
2 4 3
v Acids that do not dissociate fully in water and release less number of H ions are called weak
+
acids. Organic acids are weak acids. For example, acetic acid (CH COOH).
3
v Concentrated acids are pure form of acids. They contain a very litte quantity of water They
can cause severe burns.

v Dilute acids are not pure form of acids. They are formed by mixing a small amount of acid

with water. They too can cause burns if not handled with care.
v Higher the percentage of water, lower is the damage caused by the acid.

v Acids turn blue litmus to red, are sour in taste, corrosive in nature so stored in glass
containers.

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