Page 42 - Science Class 07

acid grapes and tamarind contain tartaric acid, v Carbon is a non-metal. When carbon reacts
vinegar contains ace c acid. Milk turns sour with oxygen, carbon dioxide is formed
a er some me because of conversion of which is acidic in nature.
lactose into the lac c acid. The above
C + O CO 2
2
men oned acids are weak acids. All the
Carbon Oxygen Carbon dioxide
naturally occuring acids are weak acids because
(Non-metal) (Air) (Acidic)
of their biological origin. These acids are also
called organic acids. v When carbon dioxide is dissolved in water,
carbonic acid is formed.
Let us read about some weak acids and their

common sources: CO + H O H CO 3
2
2
2
(Carbon (Water) (Carbonic
Acid Occurrence
dioxide) acid)
Ace c acid Vinegar
v Sulphur is a non-metal. It reacts with
Malic acid Apples
oxygen to form sulphur dioxide.)
Lac c acid Sour milk
S + O SO
2 2
Oxalic acid Tomatoes
(Sulphur) (Oxygen) (Sulphur
Ascorbic acid Guava, Amla, Chillies dioxide)
Tartaric acid Tamarind, Grapes v When sulphur dioxide is dissolved in water,
Citric acid Orange, Lemon sulphurous acid is formed.

SO + H O H SO
Some acids are very strong. They are called 2 2 2 4
inorganic acids because they are not found in (Sulphur (Water) (Sulphurous
plants and animals (except hydrochloric acid dioxide) acid)
which is found in our stomach). They are Test for Acids
prepared in laboratory from naturally occurring
To test the nature of a substance whether it is
inorganic compounds. These acids are used in acidic or basic, we use indicators.
industries.
Indicators
Some common strong acids are as follows:
Indicators are substances that are used to test
Acid Formula whether a substance is acidic or basic or
Hydrochloric HCl neutral in nature. They change their colours

Nitric acid HNO 3 when added to a solu on containing an acidic
or a basic substance. Turmeric, which is a
Sulphuric acid H SO
2 4
commonly used spice, is used as indicator.
Phosphuric acid H PO
3 4 However, the most commonly used indicators
are litmus-paper, methyl orange and phenolph-
PREPARATION OF ACIDS
thalein. When an indicator changes its colour,
Acids are produced when non-metallic oxides
when brought in to the contact of a substance,
(residues obtained a er burning non-metals in
we get to know the nature of the substance.
the presence of oxygen) are dissolved in water.

42 Science-7