Chapter 2: Acids, Bases and Salts
2.1 Understanding the Chemical Properties of Acids and Bases
2.1.1 Acids and Bases in the Laboratory
- Acids taste sour and turn blue litmus paper red, whereas bases taste bitter and turn red litmus paper blue.
- Litmus is a natural indicator extracted from lichen. Other natural indicators include turmeric, red cabbage leaves, and colored petals of Hydrangea, Petunia, and Geranium.
- Synthetic indicators like methyl orange and phenolphthalein can also be used to test for acids and bases.
- Olfactory indicators are substances whose odor changes in acidic or basic media, such as onion, vanilla, and clove oil.
2.1.2 How do Acids and Bases React with Metals?
- An acid reacts with a metal to form a salt and release hydrogen gas (Acid + Metal → Salt + Hydrogen gas).
- Bases can also react with certain metals to form salts and hydrogen gas. For example, sodium hydroxide reacts with zinc to form sodium zincate (Na2ZnO2).
2.1.3 How do Metal Carbonates and Metal Hydrogencarbonates React with Acids?
- Both metal carbonates and metal hydrogencarbonates react with acids to form a corresponding salt, carbon dioxide, and water.
- The evolved CO2 gas turns lime water (calcium hydroxide solution) milky due to the formation of a white precipitate of calcium carbonate (CaCO3).
2.1.4 How do Acids and Bases React with each other?
- The reaction between an acid and a base to produce salt and water is known as a neutralisation reaction (Base + Acid → Salt + Water).
2.1.5 Reaction of Metallic Oxides with Acids
- Metallic oxides react with acids to form salt and water, meaning metallic oxides are basic in nature (basic oxides).
2.1.6 Reaction of a Non-metallic Oxide with Base
- Non-metallic oxides react with bases to produce salt and water, showing that non-metallic oxides are acidic in nature.
2.2 What do All Acids and All Bases Have in Common?
2.2.1 What Happens to an Acid or a Base in a Water Solution?
- Acids generate hydrogen ions (H+ or hydronium ions, H3O+) in the presence of water, which conduct electricity in solution.
- Bases generate hydroxide ions (OH-) in water. Water-soluble bases are called alkalis.
- The process of dissolving an acid or a base in water is highly exothermic. Dilution involves adding acid slowly to water with constant stirring to decrease ion concentration per unit volume.
2.3 How Strong are Acid or Base Solutions?
2.3.0 The pH Scale
- The strength of acids and bases can be measured using the pH scale, which ranges from 0 (highly acidic) to 14 (highly alkaline), with 7 being neutral.
- A universal indicator, a mixture of several indicators, displays different colors at different H+ ion concentrations.
- Strong acids produce more H+ ions, whereas weak acids produce fewer H+ ions.
2.3.1 Importance of pH in Everyday Life
- Our body functions within a narrow range of pH 7.0 to 7.8. Rainwater with a pH below 5.6 is called acid rain.
- Stomach pH is regulated by hydrochloric acid. Excess acid causes indigestion, which is neutralized using antacids like magnesium hydroxide (Milk of magnesia).
- Tooth decay begins when mouth pH falls below 5.5, causing corrosion of calcium hydroxyapatite (tooth enamel).
- Plants and animals use chemical self-defence, such as bee stings releasing acid or nettle leaves injecting methanoic acid.
2.4 More About Salts
2.4.1 and 2.4.2 Family and pH of Salts
- Salts sharing common positive or negative radicals belong to the same family of salts.
- Salts of a strong acid and a strong base are neutral (pH 7). Salts of a strong acid and weak base are acidic (pH < 7), while salts of a strong base and weak acid are basic (pH > 7).
2.4.3 Chemicals from Common Salt
- Common salt (sodium chloride, NaCl) is an important raw material for producing daily-use chemicals.
- Sodium hydroxide (NaOH) is produced via the chlor-alkali process by electrolyzing brine.
- Bleaching powder (Ca(ClO)2) is manufactured by reacting chlorine gas with dry slaked lime.
- Baking soda (sodium hydrogencarbonate, NaHCO3) is a mild non-corrosive base used in baking, antacids, and fire extinguishers.
- Washing soda (Na2CO3.10H2O) is obtained by recrystallizing sodium carbonate and is used as a domestic cleaning agent and for water softening.
2.4.4 Are the Crystals of Salts really Dry?
- Salts like copper sulphate contain water of crystallisation, which is a fixed number of water molecules in one formula unit (e.g., CuSO4.5H2O).
- Gypsum (CaSO4.2H2O) loses water when heated to 373 K to become Plaster of Paris (calcium sulphate hemihydrate, CaSO4.1/2H2O), used for casting fractured bones and making decorations.