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Acids, Bases and Salts Class 7 Notes Science Chapter 2

Acids and Bases

Many substances that you encounter in your daily life have different properties. Substances have either acidic, basic or neutral properties.

Acids

Acids are substances that taste sour (though tasting them is not safe) and they are acidic in nature. Substances like curd, vinegar, lemon, orange juice, etc. contain natural acids.
The word acid comes from the Latin word 'ACERE' which means sour. 

Bases

Bases are bitter to the taste and slippery to the touch and include substances like baking soda milk of magnesia, etc. The chemical nature of substances that contain a base is basic.

Indicators

There are special substances that are used to test whether something is acidic or basic. These substances are called indicators. Indicators change their colour when added to an acidic or basic solution. Some natural indicators are litmus, turmeric and china rose petals.

Natural IndicatorsNatural Indicators

Question for Chapter Notes: Acids, Bases & Salts
Try yourself:Which of the following is true about acids?
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Types of Indicators

Indicators are of two types- natural indicators and synthetic indicators

Natural Indicators - These indicators are obtained from naturally occurring substances.Example: litmus paper, Chinese rose, turmeric

Synthetic Indicators - These indicators are made in a laboratory. Example: methyl orange, phenolphthalein.

Natural Indicators Around Us

(i) Litmus: A Natural Dye

Litmus is a common natural indicator extracted from lichens. Litmus has a purple colour in a neutral medium, red colour in an acidic medium, and blue in a basic medium.

LichensLichens

  • When a drop of freshly prepared lemon juice is put on red litmus, it remains red, indicating the acidic nature of lemon juice.
  • If blue litmus is used instead, then it turns red, again indicating the acidic nature of lemon juice.
  • A solution of litmus turns red if it is acidic, and blue if it is basic.

Reaction of litmus paper with acid and baseReaction of litmus paper with acid and base

S. No

Name of acid /base

Effect on litmus paper

Effect on turmeric paper

1

Hydrochloric acid

Blue litmus turns red

Remains yellow

2

Sulphuric acid

Blue litmus turns red

Remains yellow

3

Sodium hydroxide

Red litmus turns blue

Turns reddish brown

4

Potassium hydroxide

Red litmus turns blue

Turns reddish brown


Question for Chapter Notes: Acids, Bases & Salts
Try yourself:Which of the following is a natural indicator?
View Solution

(ii) Turmeric 

Turmeric changes colour to indicate acidic and basic substances.

Reaction of turmeric with neutral, acidic and basic solutionsReaction of turmeric with neutral, acidic and basic solutions

  • Turmeric paper changes colour in a basic medium.
  • In an acidic medium, it remains yellow, while in a basic medium, it turns reddish-brown.
  • When a drop of soap solution is put on turmeric paper, it turns red. This indicates the basic nature of soap solution.

S. No

Test solution

Effect on turmeric paper

1

Lemon juice

Yellow

2

Orange juice

Yellow

3

Lime water

Reddish-brown

4

Milk of magnesia

Reddish-brown


(iii) China Rose 

China rose(Gudhal petals) is also a natural indicator extracted from China rose petals. A solution of china rose turns green in a basic solution and bright pink or magenta in an acidic solution.

Reaction of china rose solution with acid and baseReaction of china rose solution with acid and base

S.No.

Name of the acid

Effect on China rose

1

Hydrochloric acid

Dark red

2

Sulphuric acid

Dark red

3

Sodium hydroxide

Green

4

Potassium hydroxide

Green


Question for Chapter Notes: Acids, Bases & Salts
Try yourself:What color does turmeric paper turn when exposed to a basic solution?
View Solution

Acid Rain

Acid rain is rain that becomes more acidic than normal due to pollutants in the air. This acid is created when pollutants in the air, such as carbon dioxide, sulphur dioxide and nitrogen dioxide, dissolve in rainwater, and form acids like carbonic acid, sulphuric acid and nitric acid.

Acid RainAcid Rain

This rain causes a lot of damage to buildings, plants and animals. The acidity in the rain can erode structures, damage plant life, and affect aquatic ecosystems. This is one important reason why we must avoid polluting our atmosphere.

Question for Chapter Notes: Acids, Bases & Salts
Try yourself:What causes acid rain?
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Neutralisation

Neutralisation is the reaction between an acid and a base to form salt and water. During neutralisation, salt and water are formed, along with the release of some heat.

Following are some examples of neutralisation reactions:

Eg: H2SO4 + 2NaOH--------------->Na2SO4 + 2H2O + Heat

Neutralisation reaction Neutralisation reaction The neutralisation reaction between an acid and a base can be identified by an indicator. 

For example, for the reaction between hydrochloric acid and sodium hydroxide, phenolphthalein is added as an indicator.
Chemical Reaction: HCl + NaOH------------>NaCl + H2O + Heat

Acids, Bases and Salts Class 7 Notes Science Chapter 2

Phenolphthalein is pink in sodium hydroxide. When hydrochloric acid is added to it, the colour slowly fades to pale pink and then colourless. This change in colour indicates neutralisation.

Question for Chapter Notes: Acids, Bases & Salts
Try yourself:
What is the purpose of using phenolphthalein as an indicator in a neutralisation reaction?
View Solution

Neutralisation in Everyday Life

Indigestion

The stomach contains hydrochloric acid, which helps in the digestion of food. However, when this acid is produced in excess, it causes indigestion, which is painful.
An antacid such as milk of magnesia neutralises the excessive acid in the stomach and provides relief from the pain due to indigestion.
Milk of magnesiaMilk of magnesia

Ant Bite


Ant bite contains formic acid. When an ant bites, it injects this formic acid into the skin. This causes pain. It can be neutralised by rubbing the ant bite with moist baking soda, which contains sodium hydrogen carbonate (NaHCO3), or with a solution of calamine, which contains zinc carbonate (ZnCO3).
Formic Acid (HCOOH)Formic Acid (HCOOH)

Soil Treatment

Excessive use of fertilisers makes the soil acidic. As a result, plants cannot grow properly, and the yield decreases. Adding a base like quick lime(calcium oxide) or slaked lime (calcium hydroxide) neutralises the soil and makes it suitable for plants.
Slacked lime and Quick limeSlacked lime and Quick limeSimilarly, when soil is basic in nature, organic matter is added to release acid and make it neutral, and thus, suitable for plants.

Factory Waste


Factory waste contains acids. This waste must be treated with bases for neutralisation before it is released into a water source. Otherwise, it can cause damage to living organisms in the water source.
Industrial Wastewater Treatment PlantIndustrial Wastewater Treatment Plant

Question for Chapter Notes: Acids, Bases & Salts
Try yourself:
How can indigestion be relieved?
View Solution

Important Points

  • Acid is sour in taste.
  • Acid turns blue litmus paper to red.
  • Acid turns China rose solution to dark pink.
  • Base is bitter in taste.
  • Base turns red litmus paper to blue.
  • Base turns turmeric paper or solution to red.
  • Base turns China rose solution green.
  • Indicator is a special substance which shows the acidic or basic character of a substance by a change in colour.
  • Litmus, turmeric and the solution of China rose are natural indicators.
  • Litmus is obtained from lichens.
  • Solutions of acid and base neutralize each other when mixed in proper ratio. This is called neutralization or neutralization reaction.
  • A neutralization reaction is a reaction between the solutions of acid and base, in which a new substance called salt is formed.
  • Salt formed because of neutralization reaction may be acidic or basic in nature. The acidic or basic nature of salt depends upon the strength of acid and base used for neutralization reaction.
The document Acids, Bases and Salts Class 7 Notes Science Chapter 2 is a part of the Class 7 Course Science Class 7.
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FAQs on Acids, Bases and Salts Class 7 Notes Science Chapter 2

1. What are some common natural indicators for acids and bases?
Ans. Common natural indicators for acids and bases include red cabbage, turmeric, and litmus.
2. How does neutralisation occur between an acid and a base?
Ans. Neutralisation occurs when an acid reacts with a base to form salt and water, with the acidic and basic properties being neutralized.
3. Can you provide an example of neutralisation in everyday life?
Ans. An example of neutralisation in everyday life is when we use antacids to treat acidity in the stomach, where the antacid neutralizes the excess acid.
4. What are some important points to remember about acids, bases, and salts?
Ans. Some important points to remember are the definitions of acids and bases, the pH scale, common indicators, and the process of neutralisation.
5. How can indicators help identify if a substance is acidic or basic?
Ans. Indicators change color depending on the pH of a substance, helping to identify if it is acidic (turns red) or basic (turns blue).
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