This document discusses acids, bases, and indicators. It defines acids as sour substances and bases as bitter substances. It explains that litmus paper and other natural indicators like turmeric can be used to test whether a substance is acidic or basic by changing color in acid or base. Neutralization occurs when an acid and base are mixed, forming a salt and neutral solution. Examples of neutralization in everyday life include treating indigestion with antacids, reducing bee stings with baking soda, adjusting soil pH, and treating factory waste before disposal.

1. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
ACID, BASE AND SALT
Introduction
Substances can be divided into three types – Acid, Base and Salt.
Acid: Taste of acid is sour. There are many substances that contain acid and so taste
sour. For example – lemon, curd, pickles, orange juice, vinegar, etc.
Substances that taste sour are called acidic. The chemical nature of such substance is
known as ACIDIC.
The word acid comes from Latin ‘ACERE’ which means sour.
Base: Taste of base is bitter. Substances that contain base taste bitter. For example;
soap or soap solution, baking soda, washing soda, etc.
The chemical nature of substances that contain base is known as BASIC.
Indicator: It is not always possible to know the acidic or basic nature of substances by
tasting them. Tasting a substance in laboratory is not also advisable because it may be
harmful. Thus, to test the chemical nature (acidic or basic nature) of substance a special
kind of substance is used. This special kind of substance which is used to test the acidic
or basic nature of anything is known as INDICATOR.
Indicator is a substance that shows the acidic or basic nature of a substance by change
in its color.
Types of Indicator: Indicators can be divided into two types.
Natural Indicator: Indicators that are obtained from naturally occurring substance are
called NATURAL INDICATORS. For example; litmus, turmeric, china rose, etc.
Synthetic Indicator: Indicators that are made in laboratory are called SYNTHETIC
INDICATORS. For example; phenolphthalein, methyl orange, etc.
Natural Indicators
Litmus: Litmus is extracted from Lichens. Lichen is a composite organism. Lichens
consist of fungi and algae living in symbiotic relationship.
Litmus is a purple coloured liquid in distilled water. Litmus comes in the form of strips of
two colours. One is called blue litmus paper and another is called red litmus paper.
Litmus liquid and litmus paper are used to detect the acidic or basic nature of a
substance.
Colour of litmus paper in acid: Blue litmus paper turns into red when dipped in acidic
solution.
Colour of litmus paper in base: Red litmus paper turns into blue when dipped in basic
solution.
Turmeric: Turmeric is used as another natural indicator. Turmeric is of yellow colour.
Turmeric paper turns into red when it is dipped into basic solution. Turmeric paper does
not change its colour with acid.
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2. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
ACID, BASE AND SALT
China Rose: China rose is another natural indicator. China rose solution gives dark
pink (magenta) colour with acid and green colour with base.
Acid Rain: Carbon dioxide, sulphur dioxide and nitrogen dioxide which are released
from vehicles and chimneys mix with droplets of rain and turn the rain water acidic.
When this acidic rain water falls over earth, it is known as acid rain. Acid rain damages
the buildings and is harmful for plants and animals.
Taj Mahal; which is made of marble is in threat because of acid rain. Many parts of Taj
Mahal and other many historical buildings and monuments have got damaged due to
acid rain.
Characteristics of Acid
 Sour in taste.
 Turns blue litmus paper red.
 Turns the solution of China rose to dark pink colour (magenta).
Characteristics of Base
 Bitter in taste.
 Turns red litmus paper blue.
 Turns solution of China rose to green.
 Turns turmeric paper to red.
Neutralisation
When solution of acid is mixed with the solution of base, both of them neutralize each
other and a third substance; called salt; is formed. Such phenomenon is called
neutralization or neutralization reaction. The solution formed because of mixing of
solution of acid and base is neither acidic nor basic in nature. Such solution is known as
neutral solution.
Salt formed because of neutralization reaction may be acidic or basic in nature. Acidic
or Basic nature of salt depends upon the strength of acid and base. In neutralization
reaction heat is evolved. Reactions in which heat is evolved are known as exothermic
reactions. Thus, neutralization reaction is an exothermic reaction.
Sodium hydroxide is a base and hydrochloric acid is an acid. When solution of sodium
hydroxide is mixed with the solution of hydrochloric acid, both neutralize each other and
common salt (Sodium chloride) is formed. Since, it is an exothermic reaction, so
reaction mixture becomes slightly hot. The reaction involved in this can be written as
follows:
Sodium chloride is the chemical name of common salt which is used in household.
Neutralization Reaction in Everyday Life
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3. VIDYA BHARTI EDUCATIONAL INSTITUTIONS
SCIENCE NOTES CLASS 7TH
ACID, BASE AND SALT
There are many uses of neutralization reaction in everyday life.
Indigestion: Our stomach releases hydrochloric acid to kill bacteria; if any; present in
food. Hydrochloric acid released in our stomach also helps in the digestion of food.
Sometime our stomach produces more hydrochloric acid than required. Production of
more hydrochloric acid in the stomach manifests as indigestion. This condition can be
painful and causes pain in the stomach. To get rid of such symptoms, a medicine made
of milk of magnesia (a base) is taken orally. Milk of magnesia, being a base neutralizes
the hydrochloric acid and gives relief from pain because of indigestion.
Ant sting: Ant sting or bee sting contains methanoic acid. Methanoic acid is also known
as formic acid. Bee or ant injects formic acid into our skin, while biting. Injection of acid
by ant or bee results in pain at the place of bite. Rubbing baking soda over the skin
gives relief from pain due to ant or bee sting. Baking soda, which is a base, neutralizes
the effect of acid injected by bee or ant. Another base, such as zinc carbonate (Calamite
solution) is also used in the case of ant or bee sting.
Soil Treatment: Sometimes soil becomes acidic or basic due to excess use of fertilizers
or wrong method of harvesting. Acidic or basic nature of soil affects the yield as plants
do not grow properly on such soil. Acidic soil is treated with slaked lime or quick lime.
Slaked lime (Calcium hydroxide) and quick lime (Calcium oxide) are bases. Use of
slaked lime or quick lime neutralizes the acidic nature of soil. Basic soil is treated with
organic matter. Organic matter releases acid and neutralizes the basic nature of soil.
Factory Waste: Wastes of most of the factories are acidic. If such acidic wastes are
flushed into rivers, acid present in them kills the aquatic organisms and pollutes the
water. Thus, factory wastes are treated with basic substances to neutralize the acid
present in them before being flushed in the river.
CONTACT: 9654089891, 9811083910, 9953898004
HEAD OFFICE: 131/12 KRISHNA COLONY, STREET NO 4, NEAR SHIV MANDIR, GURGAON