Class 10 Science · Chapter 2 NotesAcids, Bases and Salts

Acids and bases are identified by their distinct physical and chemical properties. Acids are generally sour in taste and turn blue litmus paper red, while bases are bitter and turn red litmus paper blue. Indicators are substances that change color or odor in the presence of an acid or a base. Natural indicators include litmus (extracted from lichens) and turmeric. Synthetic indicators like methyl orange and phenolphthalein are also used in laboratories. Olfactory indicators, such as onion, vanilla, and clove, change their smell depending on whether they are in an acidic or basic medium, providing an alternative way to test substances.

• AcidSour taste, turns blue litmus red
• BaseBitter taste, soapy touch, turns red litmus blue
• Olfactory IndicatorsSubstances whose odor changes in acidic or basic media

Acids react with metals to produce a salt and hydrogen gas. This is demonstrated when zinc granules react with dilute sulphuric acid, releasing bubbles of hydrogen that burn with a pop sound. Bases also react with certain metals, like zinc, to form complex salts and hydrogen gas. Furthermore, acids react with metal carbonates and metal hydrogencarbonates to yield a salt, carbon dioxide, and water. The evolved carbon dioxide can be tested by passing it through lime water, which turns milky due to the formation of calcium carbonate precipitate.

• Acid + Metal → Salt + Hydrogen gas
• Acid + Metal Carbonate → Salt + Carbon dioxide + Water
• Lime water testCa(OH)₂ + CO₂ → CaCO₃ + H₂O

When an acid and a base react, they nullify each other's effects to produce salt and water. This is called a neutralization reaction. Similarly, metallic oxides react with acids to produce salt and water, which proves that metallic oxides are basic in nature. Conversely, non-metallic oxides react with bases to produce salt and water, indicating that non-metallic oxides are acidic in nature. These reactions are fundamental to understanding how different chemical species balance each other in various environments.

• NeutralizationBase + Acid → Salt + Water
• Metallic oxides are basic oxides
• Non-metallic oxides are acidic oxides

Acids and bases only show their characteristic properties in the presence of water. Acids produce hydrogen ions (H⁺) or hydronium ions (H₃O⁺) in aqueous solutions, while bases produce hydroxide ions (OH⁻). The strength of these solutions is measured on the pH scale, which ranges from 0 to 14. A pH of 7 is neutral, values below 7 are acidic, and values above 7 are basic. The scale is based on the concentration of hydrogen ions; the higher the concentration, the lower the pH value. Diluting an acid or base involves adding it to water slowly, which decreases the ion concentration per unit volume.

• pH < 7Acidic; pH = 7: Neutral; pH > 7: Basic
• Strong acids produce more H⁺ ions; weak acids produce fewer
• DilutionAlways add acid to water, never water to acid, as it is highly exothermic

pH levels influence many biological and environmental processes. The human body functions within a narrow pH range of 7.0 to 7.8. In the digestive system, the stomach produces hydrochloric acid (HCl) to help digest food; overproduction leads to acidity, which is treated with antacids like Milk of Magnesia. Tooth decay begins when the pH in the mouth falls below 5.5, causing the enamel to corrode. Plants also require specific soil pH for healthy growth. Even self-defense mechanisms in nature, like bee stings or nettle hair, involve the injection of acids (methanoic acid) that cause pain and irritation.

• Acid RainRainwater with a pH less than 5.6
• AntacidsMild bases used to neutralize excess stomach acid
• Tooth EnamelMade of calcium hydroxyapatite, corrodes at pH < 5.5

Sodium chloride (common salt) is a vital raw material for many chemicals. Through the chlor-alkali process, electricity is passed through brine (aqueous NaCl) to produce sodium hydroxide, chlorine gas, and hydrogen gas. Other important derivatives include bleaching powder (CaOCl₂), used for disinfection and textile bleaching; baking soda (NaHCO₃), used in cooking and fire extinguishers; and washing soda (Na₂CO₃.10H₂O), used in glass and soap industries and for removing water hardness. Each of these substances plays a critical role in domestic and industrial applications.

• Chlor-alkali process2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂
• Baking SodaSodium hydrogencarbonate, used to make bread soft and spongy
• Washing SodaSodium carbonate decahydrate, used to remove permanent hardness of water

Many salt crystals that appear dry actually contain a fixed number of water molecules within their structure, known as water of crystallization. For example, copper sulphate crystals are blue because they contain five water molecules (CuSO₄.5H₂O); heating them removes the water and turns the salt white. Gypsum (CaSO₄.2H₂O) is another such salt. When gypsum is heated to 373 K, it loses water to become Plaster of Paris (calcium sulphate hemihydrate). When mixed with water, Plaster of Paris sets into a hard mass of gypsum, making it useful for medical plasters and decorative materials.

• Water of CrystallizationFixed water molecules in one formula unit of salt
• Plaster of ParisCaSO₄.½H₂O
• GypsumCaSO₄.2H₂O