Page 47 - Ripples SCIENCE 7 - TEJPUR Edition 2024 Answer Key

v Acids react with metals to form salt & hydrogen gas.

v Acids react with bases to form salt and water.
v Gaseous pollutants, such as sulphur dioxide and oxides of nitrogen present in the air react

with water and oxygen in the atmosphere to form sulphuric acid and nitric acid, respec-
tively. These acids dissolve in rainwater and come down as acid rain. Acid rain is unusually

acidic and corrosive in nature.

v Bases are chemical substances that have a bitter taste and a soapy texture. They are metal-
lic oxides or hydroxides. For example, sodium oxide (Na O), potassium hydroxide (KOH), etc.
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v A base which dissolves in water is known as alkali. For example, sodium hydroxide (NaOH).
v Bases are bitter in taste, are soapy to touch, may or may not dissolve in water and turn red

litmus to blue.
v An indicator is a dye that changes colour when it is put into an acid or an alkali. Indicators

are of two types–natural and artifical indicators.

v Natural indicators are prepared from natural substances. For example, turmeric, litmus and
China rose petals.

v Litmus is a natural purple-coloured dye extracted from lichens. It turns red in acidic solu-
tions and blue in basic solutions. It shows no change in colour in neutral substances.

v Turmeric gives brownish-red colour in a basic medium and yellow colour in an acidic me-
dium.

v China rose indicator is prepared from the extract of the petals of its flowers. A solution of

china rose turns greenish in a basic solution and bright pink in an acidic solution.
v Indicators prepared from artificial substances are known as artificial or synthetic indicators.

For example, phenolphthalein, crystal violet, and methyl orange.
v pH scale helps know the strength of an acid or a base. It ranges from 1-14.

v The solutions with pH value less than 7 are acids and those with pH value more than 7 are
bases. Solutions with pH value 7 are called neutral solutions.

v pH value can be measured using the universal indicator. A universal indicator is composed

of a solution of several compounds and it gives colours for most of the pH values. It can be
used as liquid or as a strip of paper called the pH strip.

v When an acid reacts with a base, they nullify or neutralise the effect of each other forming
a salt, which is neither acidic or basic, and water. Such reaction is known as neutralisation

reaction. It is also known as an exotheric reaction.
v Acids & bases play a very important role in our everyday life.

i. Formic acid is injected into our body when an ant bites, which causes pain and inflam-
mation. This can be neutralised by rubbing baking soda over the affected area.

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