Imagine visiting a science fair where simple everyday substances reveal surprising properties! This chapter delves into how we can identify substances as acidic, basic, or neutral, just like Ashwin and Keerthi discovered at the fair entrance when invisible writing appeared on paper.

Many substances around us can be categorised based on their chemical nature. To do this, we often use special substances called indicators. Indicators are substances that change colour or odour when they come into contact with an acidic or a basic substance. This change helps us determine the nature of the unknown substance.

Nature—Our Science Laboratory

Nature itself provides us with many substances that can act as indicators.

Litmus As An Indicator

Litmus is a very common natural indicator. It is extracted from organisms called lichens.

Litmus is available both as a solution and, more commonly, as paper strips: red litmus paper and blue litmus paper.

Activity 2.1: Let Us Explore

By testing various everyday substances with both red and blue litmus paper strips, we can classify them:

• Substances that turn blue litmus paper red are considered acidic.
• Substances that turn red litmus paper blue are considered basic.
• Substances that cause no change in the colour of either red or blue litmus paper are considered neutral.

Common samples tested include lemon juice, soap solution, vinegar, baking soda solution, tap water, sugar solution, and salt solution. Lime water, a solution of calcium hydroxide, is prepared by adding lime (calcium oxide, or 'chuna') to water, letting it settle, and filtering the liquid.

After performing tests with various samples, we can group them based on their effect on litmus paper:

Group A (Turns blue litmus red)	Group B (Turns red litmus blue)	Group C (No change on either litmus)
Lemon juice	Soap solution	Tap water
Amla juice	Baking soda solution	Sugar solution
Tamarind water	Lime water	Salt solution
Vinegar	Washing powder solution

This classification based on litmus tests confirms the acidic, basic, or neutral nature of the tested substances.

Activity 2.2: Let Us Relate And Explore

Many edible acidic substances share a common characteristic: they taste sour. For instance, lemon juice, amla juice, tamarind water, and vinegar are all sour and were found to be acidic using litmus.

Some common edible substances and the acids they contain:

• Lemon: Citric acid
• Vinegar: Acetic acid
• Amla: Ascorbic acid (Vitamin C), Citric acid
• Tamarind: Tartaric acid

Important Caution: While sour taste is often associated with acids, you must never taste unknown substances to determine their nature. Some acids are highly corrosive and dangerous.

Basic substances, on the other hand, often feel slippery or soapy to the touch. Rubbing a baking soda solution between your fingers, for example, gives this feeling.

Bases generally taste bitter, but similar to acids, not everything that tastes bitter is necessarily a base (e.g., bitter gourd). Also, do not taste unknown substances.

Red Rose As An Indicator

Many coloured plant parts, like flower petals, can also serve as natural indicators.

Activity 2.3: Let Us Prepare

A simple indicator can be prepared from red rose petals by crushing them and soaking them in hot water. After some time, the coloured water extract can be filtered and used.

Activity 2.4: Let Us Find Out

Testing substances with red rose extract reveals different colour changes compared to litmus:

• When an acidic substance (like lemon juice) is added to red rose extract, the colour typically turns a shade of red or pink.
• When a basic substance (like soap solution) is added to red rose extract, the colour typically turns a shade of green.
• Neutral substances generally cause no significant colour change to the red rose extract.

This confirms that red rose extract is another viable natural acid-base indicator.

Other plant materials like beetroot, purple cabbage, Indian blackberry (jamun), and red hibiscus (gudhal) flowers can also function as natural indicators.

Fascinating Fact: The colour of Hydrangea flowers can change depending on the soil's acidity! Blue flowers grow in acidic soil, while pink or red flowers grow in basic soil. This shows how the chemical nature of the environment affects plants.

Turmeric As An Indicator

Turmeric (haldi), a common spice, is also a natural indicator.

Activity 2.5: Let Us Prepare

Turmeric paste can be made by mixing turmeric powder with a little water. Filter paper or cotton cloth can be dipped in this paste and dried to create "turmeric paper strips".

Activity 2.6: Let Us Investigate

When substances are tested using turmeric paper:

• Turmeric paper's yellow colour turns red when a basic substance (like soap solution or baking soda solution) is added.
• Turmeric paper's yellow colour remains unchanged when an acidic substance (like lemon juice) or a neutral substance (like sugar solution) is added.

This means turmeric paper is effective at identifying basic substances but cannot distinguish between acidic and neutral ones.

This property explains why a yellow curry stain (turmeric) on white fabric often turns reddish-brown when washed with soap (which is basic). The cleaning liquid used in the opening science fair demonstration could have been turmeric solution, and the writing made with a basic solution like soap or lime water.

Fascinating Fact: Turmeric, known as the 'Golden' spice, is widely used in cooking and traditional medicine for its colour, taste, and purported health benefits, demonstrating a link between traditional knowledge and scientific exploration.

Besides colour change, some substances, called olfactory indicators, change their odour in acidic or basic mediums. Onion is an example of an olfactory indicator. When cotton strips are soaked in finely chopped onion and then tested:

• Acids generally do not destroy the characteristic onion smell.
• Bases generally destroy or reduce the characteristic onion smell.

Investigating other substances with onion-treated strips would reveal how their odour changes in acidic or basic conditions, providing another way to identify their nature, especially useful for someone who cannot see colour changes.

KNOW A SCIENTIST: Acharya Prafulla Chandra Ray, considered the 'Father of Modern Indian Chemistry', made significant contributions to chemistry research in India, highlighting ancient Indian scientific achievements and establishing India's first pharmaceutical company.

What Happens When Acidic Substances Mix With Basic Substances?

When acidic substances and basic substances are mixed, they react with each other in a process called neutralisation.

Activity 2.7: Let Us Experiment

Mixing an acidic solution (like dilute lemon juice with blue litmus, turning it red) with a basic solution (like lime water), and carefully adding the base drop by drop while swirling, shows a gradual change. Initially red (acidic), the solution's colour will eventually change to blue as more base is added, indicating it has become basic. If enough base is added to precisely counteract the acid, the solution becomes neutral, meaning it is neither acidic nor basic.

This experiment demonstrates that acids and bases effectively cancel out or neutralise each other's properties.

A neutralisation reaction is represented by the following general equation:

$ \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} + \text{Heat (Evolution)} $

During neutralisation, a new substance called a salt is formed along with water. Importantly, this reaction typically releases energy in the form of heat, causing the temperature of the mixture to rise.

Neutralisation In Daily Life

Neutralisation is not just a laboratory concept; it happens and is applied in many everyday situations:

Situation 1: Ant Bites: When a red ant bites, it injects an acidic liquid, specifically formic acid, into the skin, causing pain and irritation. Applying a mild base, such as moist baking soda (sodium bicarbonate), to the affected area can neutralise the formic acid, providing relief from the sting.

Situation 2: Soil Treatment: Plants require soil with a suitable pH (acidity or basicity) level to grow well. If soil becomes too acidic, often due to excessive use of chemical fertilisers, it hinders plant growth. This acidic soil can be treated with basic substances like quicklime (calcium oxide) or slaked lime (calcium hydroxide) to neutralise the excess acid.

Conversely, if the soil is too basic, it can be treated with organic matter like manure or compost. These organic materials decompose and release acids, which help neutralise the basic nature of the soil, bringing it closer to a neutral state suitable for plant growth.

Situation 3: Factory Waste Treatment: Industrial waste often contains highly acidic or basic substances. Releasing untreated waste directly into water bodies like lakes and rivers can harm aquatic life, including fish. Before disposal, factory waste is typically treated with neutralizing agents (acids treated with bases, and bases treated with acids) to make it neutral and less harmful to the environment.

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In a Nutshell:

• Substances are primarily classified as acidic, basic, or neutral.
• Acid-base indicators are substances that change colour or odour in response to acidic or basic conditions, helping us identify the nature of other substances. Examples include litmus (from lichens), red rose extract, red hibiscus, purple cabbage, and turmeric. Olfactory indicators (like onion) use changes in odour.
• Acids turn blue litmus red, and typically turn red rose extract red/pink.
• Bases turn red litmus blue, typically turn red rose extract green, and turn yellow turmeric paper red.
• Neutral substances generally do not affect the common indicators like litmus, red rose extract, or turmeric paper.
• Neutralisation is the reaction between an acid and a base, producing salt and water, and generating heat. ($ \text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} + \text{Heat} $)
• The principle of neutralisation is applied in daily life, such as treating ant bites, correcting soil acidity or basicity for agriculture, and managing industrial waste.

The opening mystery of the science fair banner can be explained by the principles learned: writing with a basic solution (like soap or lime water) on paper treated with a natural indicator (like turmeric solution) would cause the indicator to change colour where the basic solution was applied, making the writing visible.

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