Page 41 - Science Class 07

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Classification of Substances—
Acid and Base
• Classifica on of Substances • Acidic Substances
• Basic Substances • Prepara on of Alkalies
A substance is a sample of ma er whose
physical and chemical proper es are the same
throughout the sample because the ma er has
a constant composi on. It is common to see
substances change states from one state of
ma er to another. To differen ate the states of
ma er at least at a par cle level, we look at
the behaviour of the par cles within the
substance. When substances change state, it is
because the spacing between the par cles of
the substances is changing due to the gain or
loss of energy. For example, we all have
probably observed that water can exist in three
forms with different characteris c ways of
behaving–the solid state (ice), liquid state
(water) and gaseous state (water vapour and
steam). Due to water’s prevalence, we use it to
exemplify and describe the three different
states of ma er. As ice is heated and the
par cles of ma er that make up water gain
energy, eventually the ice melts into water that
eventually boils and turns into steam.
CLASSIFICATION OF SUBSTANCES
We see a large number of substances in our life.
Science-7
• Prepara on of Acids
• Neutral Substances
They all possess different kinds of proper es.
However, we can classify them on the basis of
some common proper es that they externally
possess.
On the basis of these common proper es, we
can classify the substances in the following
groups:
v Acidic Substances
v Basic Substances
v Neutral Substances
ACIDIC SUBSTANCES
An acid is a substance that donates hydrogen
ions. Because of this, when an acid is dissolved
in water, the balance between hydrogen ions
and hydroxide ions is shi ed. Now, there are
more hydrogen ions than hydroxide ions in the
solu on. This kind of solu on is acidic. The
world acid is derived from the La n word
‘Acidus’ which means sour. Acids are present in
abundance in nature. Sour taste of fruits and
vegetables say that acid is present in them, e.g.,
lemon and oranges contain citric acid hence,
called citrus fruits. Raw apples contain malic
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