Notes-NCERT-Class-7-Science-Curiosity-Chapter-2-Exploring Substances: Acidic, Basic, and Neutral-CBSE

Nature — Our Science Laboratory :

In our daily life, we use many edible substances like lemon, baking soda, tamarind, common salt, sugar, curd, and vinegar. These substances have different tastes — some are sour, some are bitter, some are sweet, and some are salty.

Classification of Edible Substance :

Some edible substances and their tastes are listed in the table shown below

Based on their taste, substances can be classified into three important groups of chemical substance, i.e. acids, bases and salts.

(i) Acids :

• The word acid comes from the Latin word ‘acere’, which means sour.
• Substances that taste sour are called acids.
• Examples : lemon juice, orange juice, unripe mango, curd, and vinegar.
• These substances contain natural acids, and their chemical nature is acidic.

Acids in Edible Substances :

(ii) Bases :

• Those substances which taste bitter and feel soapy or slippery to touch are known as bases, e.g. baking soda, soap solution, lime water, washing powder, etc. The chemical nature of such substances is said to be basic.
• But, everything that tastes bitter may not contain a base. e.g. coffee, bitter gourd, etc.

Acid-Base Indicators :

Some special substances that are used to identify acidic or basic nature of a substance are known as indicators.

Natural Indicators Around Us :

• Some naturally occurring indicators are litmus, turmeric, red rose petals, purple cabbage, Indian blackberry (jamun), red hibiscus (gudhal) flowers, beetroot, etc.
• They are used to test whether a substance is acidic or basic in nature and thus act as acid-base indicators.

Note : Synthetic indicators are made by humans in the laboratory, e.g. phenolphthalein.

(i) Litmus as an indicator :

Source: A natural substance obtained from lichens, which are a symbiotic association of a fungus and an alga.

Forms: Available as a solution or in the form of paper strips (blue and red).

Function:

• Acidic solutions turn blue litmus paper/solution to red.
• Basic solutions turn red litmus paper/solution to blue.

(ii) Red rose as an indicator :

Red Rose Extract Preparation: An extract is prepared by crushing fallen red rose petals and immersing them in hot water for 5-10 minutes until the water becomes colored. The liquid is then filtered.

Function:

• Acidic solutions turn the red rose extract to a shade of red.
• Basic solutions turn the extract to a shade of green.
• Neutral solutions do not cause a color change.

(iii) Turmeric as an indicator :

Preparation: Turmeric paper is made by creating a paste of turmeric powder and water, applying it to filter paper, and drying it. The paper is then cut into strips.

Function:

• Basic solutions turn the yellow turmeric paper to red. e.g. Turmeric stain on clothes turn red when it is washed with soap. It is because the soap solution is basic in nature.
• It does not change color in acidic or neutral solutions.

Limitation: While turmeric paper is an effective indicator for basic substances, it cannot be used to differentiate between acidic and neutral substances.

(iv) Olfactory Indicators :

These are substances whose odor changes in an acidic or basic medium.

Example: Finely chopped onions stored overnight with strips of cotton cloth.

• When an acid (tamarind water) is applied to a strip, the onion odor persists.
• When a base (baking soda solution) is applied, the onion odor is altered or disappears.

Other Natural Indicators :

The document notes that extracts from several other plants and vegetables can serve as acid-base indicators, including:

• Red hibiscus (gudhal)
• Purple cabbage
• Beetroot
• Indian blackberry (jamun)
• Hydrangea: A plant whose flower color is dependent on the soil's nature. It produces blue flowers in acidic soil and pink or red flowers in basic soil.

What Happens When Acidic Substances Mix with Basic Substances? :

Neutralisation : The reaction in which an acid reacts with a base to form salt and water is called neutralisation.

• Acids and bases are chemically opposite substances. So, when an acid is mixed
• with a base, they neutralise (or cancel) the effect of each other.
• In this process, both the acidic nature of acid and basic nature of base are destroyed.
• The solution formed is neutral, but salt produced during the reaction can be acidic, basic or neutral.
• Some heat is always evolved, during the reaction, which raises the temperature of the mixture.

The general reaction takes place is given as

Acid + Base -> Salt + Water + Heat

Neutralisation in Daily Life :

The principle of neutralization is used to manage various everyday problems.

(i) Ant Bites: When a red ant bites, it injects an acidic liquid (formic acid) into the skin, causing pain and redness. Applying a base, such as moist baking soda, neutralizes the acid and provides relief.

(ii) Soil Treatment:

• Acidic Soil: If soil becomes too acidic (e.g., from excessive use of chemical fertilizers), it can harm plant growth. It is treated with bases like lime (calcium oxide) to neutralize the acidity.
• Basic Soil: If soil is too basic, organic matter (manure, composted leaves) is added. The organic matter releases acids, which neutralize the basic nature of the soil.

(iii) Industrial Waste: Factory waste is often acidic. Releasing it directly into lakes and rivers can harm or kill aquatic life, such as fish. To prevent this, the acidic waste must be treated with basic substances to neutralize it before discharge.

Key Figures and Ancillary Concepts :

Acharya Prafulla Chandra Ray :  Known as the "Father of Modern Indian Chemistry."

Contributions:

• Earned a doctorate in chemistry from the United Kingdom.
• Pioneered scientific research in India upon his return.
• Established India’s first pharmaceutical company in 1901.
• Authored writings on the history of chemistry in India, highlighting the achievements of ancient Indian scientists.

Advocacy: As a social reformer, he advocated for the use of the mother tongue as the medium of instruction in educational institutions.

Turmeric as a "Golden Spice"

• Turmeric, which belongs to the ginger family, is known not only as a spice but also for its cultural and medicinal importance.
• In Ayurveda, it is believed to have many health benefits and is often used in traditional home remedies.

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