Acids And Bases

In our daily lives, we encounter many substances with different tastes, such as lemon, tamarind, common salt, sugar, and vinegar. Some substances taste **sour**, some **bitter**, some **sweet**, and some **salty**.

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Substances that taste **sour** often contain **acids**. The chemical nature of such substances is described as **acidic**. The term 'acid' originates from the Latin word 'acere', meaning sour. Examples of naturally occurring substances that taste sour due to the presence of acids include:

• Curd (contains Lactic acid)
• Lemon juice (contains Citric acid)
• Orange juice (contains Citric acid)
• Vinegar (contains Acetic acid)
• Tamarind, grapes, unripe mangoes (contain Tartaric acid)
• Amla, Citrus fruits (contain Ascorbic acid or Vitamin C)
• Ant's sting (contains Formic acid)
• Spinach (contains Oxalic acid)

Substances that taste **bitter** and feel **soapy** or slippery when rubbed in solution between fingers are generally known as **bases**. The chemical nature of these substances is said to be **basic** or alkaline (when soluble in water).

Example: Baking soda tastes bitter and feels soapy when rubbed in water. It is a base (Sodium hydrogencarbonate).

Other examples of bases and where they are found:

• Lime water (contains Calcium hydroxide)
• Window cleaner (contains Ammonium hydroxide)
• Soap (may contain Sodium hydroxide or Potassium hydroxide)
• Milk of magnesia (contains Magnesium hydroxide)

Caution: It is important **not to taste or touch unknown substances** to determine if they are acidic or basic, as many acids and bases, especially in concentrated forms, can be corrosive and harmful.

Natural Indicators Around Us

Since tasting or touching is not a safe way to identify acids and bases, special substances called **indicators** are used. Indicators change their colour when they are added to solutions containing acidic or basic substances. They help us differentiate between acidic, basic, and neutral substances.

Some common indicators occur naturally.

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Litmus: A Natural Dye

**Litmus** is the most widely used natural indicator. It is extracted from organisms called **lichens**.

In pure or distilled water, litmus has a mauve (purple) colour. Litmus reacts differently to acids and bases:

• When added to an **acidic solution**, litmus turns **red**.
• When added to a **basic solution**, litmus turns **blue**.

Litmus is available as a solution or more commonly as thin strips of paper called **litmus paper**. It is typically available as **red litmus paper** and **blue litmus paper**.

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Activity 5.1

Testing various solutions using red and blue litmus paper:

Procedure: Take a small amount of the test solution. Use a dropper to put a drop of the solution on a strip of red litmus paper. Observe any colour change. Repeat the process with a strip of blue litmus paper.

Solutions to be tested: Lemon juice, tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water.

Observation & Inference:

Table 5.2: Effect on litmus paper (Example Observations):

S. No.	Test solution	Effect on red litmus paper	Effect on blue litmus paper	Inference (Acidic/Basic/Neutral)
1.	Lemon juice	No change (remains red)	Turns red	Acidic
2.	Tap water	No change (remains red)	No change (remains blue)	Neutral (usually, can vary)
3.	Detergent solution	Turns blue	No change (remains blue)	Basic
4.	Baking soda solution	Turns blue	No change (remains blue)	Basic
5.	Sugar solution	No change (remains red)	No change (remains blue)	Neutral
6.	Vinegar	No change (remains red)	Turns red	Acidic
...	...	...	...	...

Solutions that **do not change the colour** of either red or blue litmus paper are called **neutral solutions**. These substances are neither acidic nor basic.

Note: To get clear results, especially with baking soda, it's important to use a solution (dissolved in water) rather than the dry solid on dry litmus paper.

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Turmeric Is Another Natural Indicator

**Turmeric** powder is commonly used as a spice, but it also acts as a natural indicator. Its colour changes in the presence of bases.

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Activity 5.2

Making turmeric indicator paper and testing solutions:

Procedure: Make a paste of turmeric powder with a little water. Spread this paste on blotting paper or filter paper and let it dry to create yellow turmeric paper strips. Put a drop of a test solution onto a strip of this turmeric paper.

Observation & Inference: Turmeric paper is typically **yellow**. When a basic solution (like soap solution) is dropped on it, the colour changes to **red or reddish-brown**. Acidic or neutral solutions do not cause this colour change; the paper remains yellow.

This explains why a yellow turmeric stain on white clothes turns red when washed with soap (because soap solutions are basic).

Table 5.3: Effect on turmeric indicator (Example Observations):

S. No.	Test solution	Effect on turmeric solution/paper	Remarks (Acidic/Basic/Neutral)
1.	Lemon juice	No change (Yellow)	Acidic
2.	Orange juice	No change (Yellow)	Acidic
3.	Vinegar	No change (Yellow)	Acidic
4.	Milk of magnesia	Turns red/reddish-brown	Basic
5.	Baking soda	Turns red/reddish-brown	Basic
6.	Lime water	Turns red/reddish-brown	Basic
7.	Sugar	No change (Yellow)	Neutral
8.	Common salt	No change (Yellow)	Neutral

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China Rose As Indicator

**China rose** (Gudhal) petals can also be used to prepare a natural indicator. The coloured water obtained by soaking China rose petals in warm water serves as the indicator solution.

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Activity 5.3

Preparing China rose indicator and testing solutions:

Procedure: Collect China rose petals, place them in a beaker, and add warm water. Let the mixture stand until the water becomes coloured. This coloured water is the China rose indicator. Add a few drops of this indicator solution to different test solutions (shampoo, lemon juice, soda water, baking soda solution, vinegar, sugar solution, common salt solution).

Observation & Inference: The China rose indicator shows different colours in acidic, basic, and neutral solutions:

• In **acidic solutions**, the indicator turns **dark pink** or **magenta**.
• In **basic solutions**, the indicator turns **green**.
• In **neutral solutions**, the indicator's colour usually remains unchanged or shows a subtle different colour (e.g., light pink depending on the initial colour).

Table 5.4: Effect on China rose indicator (Example Observations):

S. No.	Test solution	Initial colour of solution	Final colour after adding indicator	Inference (Acidic/Basic/Neutral)
1.	Shampoo (dilute)	Clear/light coloured	Green	Basic
2.	Lemon juice	Clear	Dark pink/Magenta	Acidic
3.	Soda water (carbonated water)	Clear	Dark pink/Magenta	Acidic (due to dissolved $CO_2$)
4.	Sodium hydrogencarbonate solution (Baking soda)	Clear	Green	Basic
5.	Vinegar	Clear	Dark pink/Magenta	Acidic
6.	Sugar solution	Clear	No significant change (remains light pink)	Neutral
7.	Common salt solution	Clear	No significant change (remains light pink)	Neutral

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Activity 5.4

This activity involves testing common laboratory acids (hydrochloric acid, sulfuric acid, nitric acid, acetic acid) and bases (sodium hydroxide, ammonium hydroxide, calcium hydroxide) using the three natural indicators studied (litmus, turmeric, China rose) and potentially a synthetic indicator like phenolphthalein if introduced by the teacher. This reinforces the characteristic colour changes of the indicators in acidic and basic environments using standard chemical solutions.

Table 5.5: Effect of laboratory acids/bases on indicators (Example Observations):

S. No.	Name of acid/base	Effect on litmus paper (Red/Blue)	Effect on turmeric paper (Yellow to...)	Effect on China rose solution (Light pink to...)	Effect on Phenolphthalein solution (Colourless to...)
1.	Dilute hydrochloric acid (Acid)	Blue to Red	No change (remains Yellow)	Dark pink/Magenta	No change (remains Colourless)
2.	Dilute sulfuric acid (Acid)	Blue to Red	No change (remains Yellow)	Dark pink/Magenta	No change (remains Colourless)
3.	Dilute sodium hydroxide (Base)	Red to Blue	Red/Reddish-brown	Green	Pink
4.	Dilute calcium hydroxide (Base)	Red to Blue	Red/Reddish-brown	Green	Pink

Caution: Handle laboratory acids and bases with extreme care. They are corrosive and can irritate or harm the skin. Wear appropriate safety gear if working with them.

Acid Rain: Rainwater that contains excess acids is called acid rain. These acids ($Carbonic acid, Sulfuric acid, Nitric acid$) are formed when pollutants like carbon dioxide, sulfur dioxide, and nitrogen dioxide in the air dissolve in rainwater. Acid rain can damage buildings (especially historical monuments like marble structures), plants, and harm aquatic life.

Neutralisation

**Neutralisation** is a chemical reaction that occurs when an **acid** is mixed with a **base**. Acids and bases have opposite chemical properties, and when combined in appropriate amounts, they counteract or neutralise each other's effects.

In a neutralisation reaction, the acidic nature of the acid and the basic nature of the base are destroyed, resulting in a solution that is neither acidic nor basic (i.e., it becomes neutral).

The products of a neutralisation reaction are typically a **salt** and **water**. The reaction is usually accompanied by the evolution of **heat**, which causes the temperature of the reaction mixture to rise.

The general equation for neutralisation is:

$\textsf{Acid} + \textsf{Base} \rightarrow \textsf{Salt} + \textsf{Water} + \textsf{Heat}$

A specific example is the reaction between Hydrochloric acid (an acid) and Sodium hydroxide (a base):

$\textsf{HCl} + \textsf{NaOH} \rightarrow \textsf{NaCl} + \textsf{H}_2\textsf{O} + \textsf{Heat}$

(Sodium chloride or NaCl is common table salt).

The resulting salt itself can be acidic, basic, or neutral depending on the specific acid and base that reacted.

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Activity 5.5

This activity demonstrates neutralisation using a synthetic indicator, **phenolphthalein**. Phenolphthalein is a common indicator that is **colourless in acidic and neutral solutions** but turns **pink in basic solutions**.

Procedure: Start with a test tube containing dilute hydrochloric acid (acidic, colourless). Add a few drops of phenolphthalein indicator (solution remains colourless). Then, add sodium hydroxide solution (a base) drop by drop while stirring gently. Initially, the solution remains colourless as the acid is still in excess. As more base is added, the acid gets neutralised. The point where the solution just turns light pink indicates that all the acid has been neutralised and the solution has become slightly basic. Adding one more drop of acid at this point will turn the solution colourless again (acidic). Adding one more drop of base will turn it pink again (basic).

By carefully adding the base until the pink colour just appears, one can determine the point of neutralisation where the acid and base have reacted in suitable amounts.

Touching the test tube immediately after neutralisation reveals that the temperature has increased, confirming that heat is evolved during the reaction.

Neutralisation In Everyday Life

Neutralisation reactions are important and useful in various everyday situations:

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Indigestion

Our stomach naturally produces **hydrochloric acid** to help digest food. However, sometimes the stomach produces **too much acid**, leading to indigestion which can be painful. To get relief, people take **antacids**. Antacids contain bases (like magnesium hydroxide in Milk of magnesia). These bases neutralise the effect of the excess acid in the stomach, reducing acidity and relieving indigestion.

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Ant Bite

When an ant bites, it injects an acidic liquid (specifically **formic acid**) into the skin. This acid causes irritation and pain. Applying a basic substance can neutralise this acid. Common remedies include rubbing moist **baking soda** (sodium hydrogencarbonate) or using **calamine solution** (which contains zinc carbonate) on the affected area to neutralise the acidic sting.

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Soil Treatment

Plants grow best in soil that is neither too acidic nor too basic. Excessive use of chemical fertilisers can make the soil too acidic. If the soil is too acidic, farmers treat it with **bases** like quick lime (calcium oxide) or slaked lime (calcium hydroxide) to neutralise the excess acid. If the soil is too basic (alkaline), **organic matter** (like compost) is added. Organic matter decomposes and releases acids that help neutralise the basic nature of the soil, making it more suitable for plant growth.

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Factory Wastes

Wastes discharged from many factories are often **acidic**. Releasing these acidic wastes directly into water bodies (rivers, lakes) can harm or kill aquatic life (fish, etc.). Therefore, factory wastes are typically treated by adding **basic substances** to them to neutralise the acids before they are released into water bodies, protecting the environment.

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