NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts

NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts is an important chapter. This chapter focuses on Acids, Bases and Salts and many other important topics related to them such as crystallization. Here we are going to give a brief summary on these topics and all your doubts will be covered. We have also covered the questions and answers of this chapter.

Read more: NCERT Solutions For Class 7 Science Chapter 1 Nutrition In Plants

Class 7 NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts: Introduction

You must have noticed that substances like lemon, tamarind, common salt, sugar and vinegar that we use in our daily life do not taste the same. Their different tastes are given below:-

Substance	Taste (sour/bitter/ any other)
Lemon juice	sour
Orange juice	sour
Vinegar	sour
Curd	sour
Tamarind (imli)	sour
Sugar	sweet
Common salt	salty
Amla	sweet and sour
Baking soda	bitter
Grapes	sour
Unripe mango	sour
Cucumber	bitter

We find that some of these substances taste sour, some taste bitter, some taste sweet and some taste salty.

NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts

Acids

Substances that tastes sour for eg. curd, lemon juice, orange juice and vinegar contain acids. The acids give them a sour taste and the chemical nature of these substances is acidic. The word  acid comes from the Latin word acere which means sour. Natural acids are present in these substances.

Bases

Substances that tastes bitter and feel soapy when we rub between our fingers for eg. baking soda are known as bases generally. The nature of such substances is said to be basic. 

Indicators

Special types of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. When we add an indicator to a solution containing acid or base, the indicators change their color. 

Some naturally occurring indicators are –

1. Turmeric
2. Litmus
3. China rose petals (Gudhal)

SOME COMMONLY OCCURING ACIDS AND BASES AND THEIR SOURCES.

Name of acid	Found in
Acetic acid	Vinegar
Formic acid	Ant’s sting
Citric acid	Citrus fruits such as oranges, lemons, etc.
Lactic acid	Curd
Oxalic acid	Spinach
Ascorbic acid (Vitamin C)	Amla, Citrus fruits
Tartaric acid	Tamarind, grapes, unripe mangoes, etc.
Name of base	Found in
Calcium hydroxide	Lime water
Ammonium hydroxide	Window cleaner
Sodium hydroxide/ Potassium hydroxide	Soap
Magnesium hydroxide	Milk of magnesia

NOTE: We should not taste unknown substances because they could harm us. 

NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts: Natural Indicators Around Us

Litmus: A natural dye

It is the most commonly used natural indicator. It is extracted from lichens. Litmus has a mauve (purple) colour in distilled water. When litmus is added to an acidic solution, it turns red. When litmus is added to a basic solution, it turns blue.

Litmus is available in the form of solution or in form of strips of paper.

Generally, it is available as red and blue litmus paper.

Activity 1

• We have mixed some distilled water with the following substances given in the table in a plastic cup/tumbler/test tube. 
• Then we put a drop of the solution on a strip of the red one with the help of a dropper and repeat the same exercise with the blue litmus paper. 
• We note down if there is any change in colour. 

Substance	Change in colour :-
	Red Litmus	Blue Litmus
Lemon juice	No	Yes
Tap water	No	Yes
Detergent solution	Yes	No
Aerated drink	No	Yes
Soap solution	Yes	No
Shampoo	Yes	No
Common salt solution	No	No
Sugar solution	No	Yes
Vinegar	No	Yes
Baking soda solution	Yes	No
Milk of magnesia	Yes	No
Washing soda solution	Yes	No
Lime water	Yes	No

We see that some solutions do not change the colour of either red or blue litmus. They are known as neutral solutions. These substances do not contain any acidic or basic substances in them. 

Turmeric as an indicator

Activity 2

• We take a tablespoonful of turmeric powder. And add a little water and make a paste. 
• We make turmeric paper by depositing turmeric paste on blotting paper/filter paper and drying it. Then cut thin strips of the yellow paper obtained. 
• Put a drop of soap solution on the strip of turmeric paper. 

We observe that:

S. No.	Test solution	Effect on turmeric solution	Remarks
1.	Lemon juice	Blue	Acidic
2.	Orange juice	Blue	Acidic
3.	Vinegar	Blue	Acidic
4.	Milk of magnesia	Red	Basic
5.	Baking soda	Red	Basic
6.	Lime water	Red	Basic
7.	Sugar	No change	Neutral
8.	Common salt	No change	Neutral

China Rose as Indicator

Activity 3

• We collect some China rose (Gudhal) petals and place them in a beaker. 
• We add some warm water and keep the mixture for some time till water becomes coloured. 
• Then we use the coloured water as an indicator and add five drops of the indicator to each of the solutions given in Table below. 
• We note down the effect of the indicator on acidic, basic and neutral solutions. 

China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

S. No.	Test solution	Final colour	Remarks
1	Shampoo (dilute solution)	green	basic
2	Lemon juice	pink	acidic
3	Soda water	pink	acidic
4	Sodium hydrogen carbonate solution	green	basic
5	Vinegar	pink	acidic
6	Sugar solution	neutral	neutral
7	Common salt solution	neutral	neutral

NOTE: If you do not get the same result when using solid baking soda on dry litmus paper, make a solution of baking soda and then try.

In the table below are given some common chemicals’ tests of litmus paper, turmeric paper and china rose solution which are found in a school laboratory.

S. No.	Name of acid	Effect on litmus paper	Effect on turmeric paper	Effect on China rose solution
1	hydrochloric acid	blue litmus red	no change	Pink
2	sulphuric acid	blue litmus red	no change	Pink
3	nitric acid	blue litmus red	no change	pink
4	acetic acid	blue litmus red	no change	pink
5	sodium hydroxide	red litmus blue	red	green
6	ammonium hydroxide	red litmus blue	red	green
7	calcium hydroxide (lime water)	red litmus blue	red	green

Acid Rain

Rain containing excess acid is called an 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 buildings, historical monuments, plants and animals.

NCERT Solutions for Class 7 Science Chapter 5 Acids, Bases and Salts: Neutralisation( Neutralization reaction class 7 )

We fill one fourth of a test tube with dilute hydrochloric acid and add 2-3 drops of phenolphthalein to the solution. To the acidic solution we add drops of sodium hydroxide solution by a dropper and stir it gently till the pink colour just appears.

Then we add one more drop of dilute hydrochloric acid. We observe that the solution again becomes colourless.

Again when we add one drop of sodium hydroxide solution the solution again becomes pink in colour. Since phenolphthalein gives a pink colour we come to know that the solution is basic. 

But when the solution is acidic, it remains colourless.

When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed. The resulting solution is neither acidic nor basic, i.e. it is neutral. 

When we touch a test tube just after neutralisation the temperature of the test tube will be high. Because in a neutralisation reaction, heat is always produced, or evolved. The evolved heat raises the temperature of the reaction mixture.

Thus a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature.

Neutralisation is thus defined as,

‘The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat..’

Acid+Base → Salt+Water (Heat is evolved)

For eg. 

Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)

Neutralisation in everyday life

Indigestion

Indigestion is caused when too much acid is produced in our stomach. It may be painful.  To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid and gives us relief.

Ant bite

When an ant bites, it injects the acidic liquid (formic acid) into the skin which causes pain. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate. 

Soil treatment

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 quicklime (calcium oxide) or slaked lime (calcium hydroxide). 

If the soil is basic, organic matter (compost) is added to it. Organic matter releases acids which neutralises the basic nature of the soil.

Factory wastes

The wastes of many factories contain acids. When they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances.

Exercises from textbook

More Questions

6. Mark ‘T’ if the statement is true and ‘F’ if it is false: 

8. Explain why: