7-Science-Acids Bases and Salts-Notes&VL.pdf

notes

1. Class VII Science
Title of the book: Science Textbook for Class VII
Chapter 5: Acids, Bases and Salts
General Instructions
Refer to Youtube link for explanation of the chapter
https://www.youtube.com/watch?v=i2x4foEuRcI
Question answer to be done in science notebook

Introduction
Acids, bases and salts are the three important groups of chemical compounds which are useful to
us in many ways. For example, our stomach makes an acid which is necessary for the digestion
of food: baking soda used in baking bread is a base whereas common salt used in cooking food is
a salt.
Explanation
Acids and bases
Substances that contain acids such as curd, lemon juice, orange juice tastes sour whereas
substances that contain bases such as baking soda tastes bitter and feel soapy on touching.
There are yet another class of substances which is neither acidic nor basic. Such substances are
said to be neutral. Example: water.

2. Indicators are special type of chemicals which are used to test whether a given substance is
acidic or basic. The indicators change their colour when added to a solution containing an acidic
or a basic substance. Turmeric, Litmus, China rose petals etc., are some of the naturally
occurring indicators.
Natural Indicators
1. Litmus: A natural dye
Litmus is a natural indicator. Litmus solution is a purple colored dye which is extracted from a
type of a plant called ‘lichen’. When litmus is added to an acidic solution, it turns red. And when
added to a basic solution, it turns blue. Litmus is made into two types: Blue litmus and Red
litmus. It can be used in the form of strips of litmus paper or in the form of solution.
i) Acids turns blue litmus to red.
ii) Bases turn red litmus to blue.
ii) Bases turn red litmus to blue.









Fig - Lichen

2. Turmeric is another natural indicator
Turmeric paper is used as an indicator. Turmeric paper is yellow in color.
i) Turmeric paper is yellow in acid solution.
ii) Bases turn the yellow turmeric paper to red.
Turmeric paper can be prepared as follows:
Take a tablespoonful of turmeric powder, add a little water and make a paste. Deposit the
turmeric paste on a blotting paper and dry it. The yellow paper thus obtained is turmeric paper.
Cut thin strips and use these strips of turmeric paper as indicator.

3. 3. China Rose as an indicator
Collect some China rose petals and place them in a beaker. Add some warm water. Keep the
mixture for sometime till water becomes colored. Use the coloured water as an indicator.
i) China rose indicator turns acidic solutions to dark pink (magenta).
ii) China rose indicator turns basic solutions to green.
4. Phenolphthalein Indicator
Phenolphthalein is a synthetic indicator (man-made) acid-base indicator.
i) It is colourless in acidic solution.
ii) It gives pink colour in basic solution.

Acid Rain
As the name indicates the rain containing excess of acids is called acid rain. The rain becomes
acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide which are released into the
air as pollutants dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid
respectively. Acid rain can cause damage to the buildings, Historical monuments, plants and
animals.
Neutralisation
The reaction between an acid and a base is known as neutralization. Salt and water are produced
in this process with the evolution of heat.
Acid + Base → Salt + Water (Heat is evolved)
Hydrocholoric acid +Sodium hydroxide → Sodium chloride +Water +Heat

Neutralisation in Everyday Life
1. Indigestion: Too much of an acid in the stomach causes indigestion. To relieve
indigestion, we take an antacid such as milk of magnesia, which contains magnesium
hydroxide (base).It neutralizes the effect of excessive acid.
2. Ant sting: The sting of an ant contains formic acid. When an ant bites, it injects the acidic
liquid into the skin. The effect of the sting can be neutralized by rubbing moist baking
soda (sodium hydrogen carbonate) or calamine solution , which contains zinc carbonate.
3. Soil treatment: Excessive use of chemical fertilizers makes the soil acidic. Plants do not
grow well when the soil is either too acidic or too basic. When the soil is too acidic, it is
treated with bases like quick lime or slaked lime. If the soil is basic, organic matter is
added to it. Organic matter releases acids which neutralizes the basic nature of the soil.
4. Factory wastes: The wastes of many factories contain acids. If they are allowed to flow
in water bodies they will kill the fish and other organisms. Therefore, it’s neutralized by
adding basic substances.

5. Question and Answers
Q1.State differences between acids and bases.
Ans.
Differences between acids and bases
Acids Bases
Taste Sour Bitter
Litmus Change blue litmus red Change red litmus blue
China Rose Turns dark pink Turns green

Q2. Ammonia is found in many household products such as window cleaners. It turns red litmus
blue .What is its nature?
Ans. The turning of red litmus to blue closely indicates the basic nature of ammonia
Q3. Name the source from which litmus soluton is obtained. What is the use of the solution?
Ans. Litmus solution is obtained from lichens. It is used as an indicator to identify acids and
bases.
Q4.Is the distilled water acidic/basic/neutral?
Ans. Distilled water is neutral .It does not change the colour of blue or red litmus paper.
Q5. Describe the process of neutralization with the help of an example.
Ans. Neutralization is the reaction of an acid with a base to form water and salt. When acid
reacts with bases, neutralization takes place giving salt and water. Take some dilute hydrochloric
acid in a test tube .Blue litmus turns red.
Take some sodium hydroxide in a test tube. Add a few drops of phenolphthalein to it. The
solution turns pink. Now add dilute hydrochloric acid to it slowly. The solution turns colourless.
This indicates that neutralization has taken place.
Q6. Dorji has a few bottles of soft drinks in his restaurant. But unfortunately these are not
labelled. He has to serve the drinks on the demand of customers. One wants acidic drink, other
wants basic and third wants neutral drink. How will dorji decide which drink is to be served to
whom?
Ans. Dorji can identify the drinks with litmus paper.
An acidic drink turns blue litmus red.
A basic drink turns red litmus blue.
A neutral solution does not show any change with blue or red litmus paper.
Now he can serve drinks accordingly on demand

6. Q8. Explain why?
a) An antacid tablet is taken when you suffer from acidity.
b) Calamine solution is applied on the skin when an ant bites.
c) Factory waste is neutralized before disposing it into the water bodies.
Ans a) An antacid helps to neutralize excess acid in the stomach when you suffer from acidity.
b) Ant bite contains formic acid. Calamine is basic in nature and thus it helps to neutralize
the effect of acid.
c) Factory wastes are acidic in nature, these wastes make the water acidic. Acidic water is
harmful for aquatic life, hence factory wastes are neutralized before it is disposed into water.

Q9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide, and
third is a sugar solution. How will you identify them? You have only turmeric indicator.
Ans. Turmeric turns red in a basic solution and remains yellow in an acidic and neutral solution.
Add turmeric to all these.
The solution turns red in sodium hydroxide. The other two remain yellow. Now divide the red
solution into two. Add the other two solutions slowly to the red solution. The solution which
turns into red colour is hydrochloric acid and the one that does not is a sugar solution.
Q10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the
solution? Explain.
Ans. The solution in which blue litmus paper remains blue can be basic or neutral. Add two
drops of acid to the solution. If the solution still remains blue it is neutral. Bases do not show any
change with blue litmus but it changes red litmus blue.