Acids, Bases and Salts

1. Physical Properties and Acid–Base Indicators

Basic Characteristics

• Acids are generally sour in taste and turn blue litmus paper red.
• Bases are generally bitter, feel soapy to touch, and turn red litmus paper blue.
• Excess stomach acid caused by overeating or indigestion can be neutralised using a mild base such as baking soda or an antacid.

Types of Indicators

Natural Indicators

• Litmus: A purple dye extracted from lichens. It remains purple in a neutral medium, turns red in acids, and blue in bases.
• Turmeric: Yellow turmeric turns reddish-brown in a basic medium, such as soap solution. It becomes yellow again when the base is diluted with water.
• Other plant indicators: Red cabbage, coloured flower petals such as those of hydrangea, petunia and geranium, and beetroot extract can show colour changes in acidic or basic solutions.

Synthetic Indicators

• Phenolphthalein: Colourless in acidic and neutral solutions; pink in basic solutions.
• Methyl orange: A laboratory indicator used to distinguish acidic and basic media through colour changes.

Olfactory Indicators

Olfactory indicators are substances whose characteristic smell changes in acidic or basic media.

• Onion: Its smell remains in an acidic medium but is reduced or altered in a basic medium such as sodium hydroxide.
• Vanilla essence and clove oil: Their smell is retained in dilute hydrochloric acid but changes or disappears in dilute sodium hydroxide.

2. Chemical Properties of Acids and Bases

2.1 Reaction of Metals with Acids

Active metals react with acids to form a salt and release hydrogen gas.

For example, zinc reacts with dilute sulphuric acid:

• Effervescence is observed because hydrogen gas is released.
• When the gas is collected in soap bubbles and a burning candle is brought near them, it burns with a characteristic pop sound.
• Similar reactions may occur with hydrochloric acid and acetic acid. Nitric acid generally behaves differently because it is a strong oxidising agent.

Reaction of Bases with Metals

Some metals, such as zinc, react with strong bases to produce hydrogen gas and a salt.

The salt formed is sodium zincate. This reaction does not occur with every metal.

2.2 Metal Carbonates and Hydrogencarbonates with Acids

Metal carbonates and metal hydrogencarbonates react with acids to produce a salt, carbon dioxide and water.

Test for Carbon Dioxide

Carbon dioxide turns lime water milky because insoluble calcium carbonate is formed.

On passing excess carbon dioxide, the milkiness disappears because soluble calcium hydrogencarbonate is formed.

Limestone, chalk and marble are different natural forms of calcium carbonate.

2.3 Neutralisation Reaction

When hydrochloric acid is added to sodium hydroxide containing phenolphthalein, the pink colour disappears. Adding excess sodium hydroxide makes the solution pink again.

2.4 Reactions of Metallic and Non-metallic Oxides

3. Scientific Concepts Behind Acids and Bases

Electrical Conductivity and Ions

• Aqueous solutions of acids conduct electricity because they contain mobile ions.
• Hydrochloric acid produces H⁺ and Cl⁻ ions; nitric acid produces H⁺ and NO₃⁻ ions; sulphuric acid produces H⁺ and SO₄²⁻ ions.
• Glucose and alcohol contain hydrogen but do not conduct electricity because they do not ionise to produce free hydrogen ions.

Role of Water in Ionisation

Acids show acidic behaviour only in the presence of water. Dry hydrogen chloride gas does not change dry blue litmus paper, but it turns moist blue litmus paper red.

Hydrogen ions do not exist freely in water; they combine with water molecules to form hydronium ions (H₃O⁺).

Water-Soluble Bases: Alkalis

Bases that dissolve in water are called alkalis.

Alkalis are generally bitter, soapy and may be corrosive.

Dilution and Heat

• Dissolving concentrated acids or bases in water is highly exothermic.
• Always add acid slowly to water while stirring continuously.
• Never add water directly to concentrated acid because the sudden heat may cause splashing, burns or cracking of the container.
• Dilution decreases the concentration of hydronium or hydroxide ions per unit volume.

4. The pH Scale and Its Applications

Universal Indicator and pH

A universal indicator is a mixture of indicators that displays different colours at different hydrogen-ion concentrations.

• pH = 7: Neutral solution
• pH below 7: Acidic solution
• pH above 7: Basic or alkaline solution
• A lower pH indicates a higher concentration of hydronium ions.
• The term pH is associated with the idea of the “power of hydrogen”; the letter p is derived from the German word potenz.

Strong and Weak Acids

Approximate pH Values

Importance of pH in Everyday Life

1. Human body: Most body processes function within a narrow pH range, approximately 7.0–7.8.
2. Acid rain: Rainwater with pH below 5.6 is considered acidic and can harm aquatic organisms.
3. Venus: Its atmosphere contains thick clouds of sulphuric acid.
4. Soil: Plants require a suitable soil pH. Excessively acidic soil can be treated with quicklime, slaked lime or chalk.
5. Indigestion: Excess hydrochloric acid in the stomach causes irritation. Antacids such as milk of magnesia neutralise it.
6. Tooth decay: Tooth decay begins when mouth pH falls below about 5.5. Bacteria convert sugars into acids that attack enamel. Basic toothpaste helps neutralise these acids.
7. Bee and nettle stings: Acidic stings may be relieved by suitable mild bases. Nettle hairs inject methanoic acid, while a basic plant such as dock may provide relief.

Common Naturally Occurring Acids

5. Chemistry and Industrial Uses of Salts

Families and pH of Salts

Salts containing the same positive or negative ion are said to belong to the same family.

5.1 Sodium Hydroxide

Sodium hydroxide is manufactured by the chlor-alkali process, which involves electrolysis of brine.

• Anode: Chlorine gas, used in water treatment, disinfectants, PVC and chemical industries.
• Cathode: Hydrogen gas, used in fuel applications, ammonia manufacture and hydrogenation.
• NaOH solution: Used in soaps, detergents, paper, artificial fibres and metal degreasing.

5.2 Bleaching Powder

Bleaching powder is calcium oxychloride, commonly represented as Ca(ClO)₂ in school-level chemistry.

• Bleaches cotton, linen, paper pulp and washed clothes.
• Acts as an oxidising agent.
• Disinfects drinking water.

5.3 Baking Soda

Baking soda is sodium hydrogencarbonate (NaHCO₃).

On heating, it decomposes:

• In baking powder, it reacts with a mild edible acid and releases carbon dioxide, making cakes and bread soft and spongy.
• Used as a mild antacid.
• Used in soda-acid fire extinguishers.

5.4 Washing Soda

Washing soda is hydrated sodium carbonate: Na₂CO₃·10H₂O.

• Used in glass, soap and paper industries.
• Used to manufacture sodium compounds such as borax.
• Acts as a household cleaning agent.
• Removes permanent hardness of water.

5.5 Water of Crystallisation and Plaster of Paris

Water of crystallisation is the fixed number of water molecules chemically associated with one formula unit of a salt.

• Blue copper sulphate crystals are CuSO₄·5H₂O. On heating, they lose water and become white anhydrous CuSO₄. Adding water restores the blue colour.
• Gypsum is CaSO₄·2H₂O.

Plaster of Paris

Plaster of Paris is calcium sulphate hemihydrate: CaSO₄·½H₂O.

It is prepared by heating gypsum at approximately 373 K.

When mixed with water, Plaster of Paris sets into hard gypsum.

• Used for setting fractured bones.
• Used for making toys, decorative articles and moulds.
• Used for smoothing walls and ceilings.

6. Important Conceptual Questions

Identifying Acid, Base and Distilled Water Using Red Litmus

1. Dip red litmus paper into all three test tubes. The solution that turns it blue is the base.
2. Use the newly formed blue litmus paper in the remaining two test tubes. The solution that turns it red is the acid.
3. The remaining solution, which causes no colour change, is distilled water.

Compound Producing Effervescence with Dilute HCl

The compound is calcium carbonate.

The released carbon dioxide extinguishes a burning candle.

Why Rainwater Conducts Electricity but Distilled Water Does Not

Distilled water contains very few ions and conducts electricity poorly. Rainwater dissolves gases such as carbon dioxide from the atmosphere, producing ions in solution and allowing it to conduct electricity.

pH Change When Milk Turns into Curd

Fresh milk has a mildly acidic pH of about 6. During fermentation, bacteria produce lactic acid. The concentration of hydrogen ions increases, so the pH decreases.

Why Baking Soda Is Added to Milk

Baking soda makes the milk slightly alkaline. The lactic acid produced during fermentation first neutralises the baking soda, delaying the development of sufficient acidity needed for curd formation.

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