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Acids, Bases, and Salts

Definitions and Nature

• Antacids are bases that neutralise excess acid in the stomach.
• A solution that turns red litmus paper blue is basic.
• Soda water (water with dissolved CO₂) is acidic in nature.
• Ammonia gas (NH₃) is basic, dissolves quickly in water, and produces white fumes with hydrogen chloride.
• Salts and Hydrolysis: A salt, like copper sulphate, forms an acidic aqueous solution due to hydrolysis.

pH Scale and Concentration

• The pH value is a numeric value that indicates the acidic or basic nature of a solution.
• A pH of 7 indicates a neutral solution (e.g., pure water).
• An acidic solution has a pH value less than 7 (e.g., 6 or 5).
• A basic solution has a pH value greater than 7 (e.g., 8 or 9).
• Concentration Change: When pH increases from 3 to 6, the H⁺ ion concentration decreases by 1000 times (each pH unit represents a 10-fold change).

Types of Acids

• Lewis Acids are electron-pair acceptors (e.g., AlCl₃, BF₃, FeCl₃).
• Ammonia (NH₃) is not a Lewis acid; it is a Lewis base.
• Classification by Composition:
  • Oxyacids contain oxygen (e.g., Nitric Acid – HNO₃, Sulphuric Acid – H₂SO₄).
  • Hydracids do not contain oxygen (e.g., Hydrochloric Acid – HCl).

Important Acids and Their Uses

• Sulphuric Acid (H₂SO₄) is considered a basic chemical in industry.
• Aqua regia, a mixture of nitric acid and hydrochloric acid, is used to dissolve gold.
• Safety Precaution: Concentrated sulphuric acid must be diluted by adding acid to water (not water to acid) because the reaction is highly exothermic.

Common Chemical Compounds

Common Names and Chemical Identities

• Bleaching Powder: Calcium oxychloride.
• Baking Soda: Sodium bicarbonate (NaHCO₃).
• Washing Soda: Sodium carbonate (Na₂CO₃).
• Plaster of Paris: Calcium sulphate hemihydrate.
• Blue Vitriol / Neela Thotha: Copper Sulphate (CuSO₄).
• Caustic Soda: Sodium Hydroxide (NaOH).
• Hypo (used in photography): Sodium Thiosulphate.
• Epsom Salt: Magnesium Sulphate (MgSO₄).

Properties and Specific Uses

• Bleaching Powder (Calcium Oxychloride):
  • It is an oxidising agent (oxidant).
  • It reacts with dilute acids to release chlorine gas.
  • Untrue Statements: It is not highly soluble in water (only partially soluble). Its colour is white or pale yellow, not a distinct light yellow.
• Copper Sulphate (CuSO₄):
  • Used for destroying fungi in water tanks.
• Baking Soda (Sodium Bicarbonate, NaHCO₃):
  • Used in fire extinguishers.
• Sodium Thiosulphate (Hypo):
  • Used in photography as a “fixer” to dissolve unexposed silver bromide.
  • Also used as an antichlor (removes excess chlorine).

Chemical Processes and Applications

Chemical Tests and Reactions

• Gas Identification: Carbon Dioxide (CO₂) gas turns limewater milky.
• Eno Salt Reaction: When poured into water, the bubbles formed are due to Carbon Dioxide (CO₂) gas.
• Ionic Compounds:
  • Generally insoluble in alcohol.
  • In the solid state, they are poor conductors of electricity (they conduct only in molten or aqueous solution state).

Salt Formation and Setting

• Table Salt: Sodium Chloride (NaCl) is a product of a strong acid (HCl) and a strong base (NaOH).
• Cements and Settling:
  • The presence of NaCl increases the rate of setting of Plaster of Paris.
  • Gypsum is added to cement to slow down its rate of setting.

Industrial and Household Applications

• Food and Agriculture:
  • Ethylene is used for the artificial ripening of fruits.
  • Carbon Dioxide is used in soft drinks.
• Pottery and Glass:
  • Zinc oxide is used in glazing pottery.
  • Cobalt oxide imparts a deep blue colour to glass.
• Photography:
  • The useful element is Silver Bromide (coated on photographic plates).

Environmental Chemistry

Acid Rain

• The main components of acid rain are Nitric Acid (HNO₃) and Sulphuric Acid (H₂SO₄).
• Sulphuric Acid (H₂SO₄) is present in the maximum amount in acid rain.

Minerals and Composition

Silicates

• Quartz crystals are made of Silicon Dioxide (SiO₂).
• Sodium Silicate is a related compound.

Alkaline Earth Metals

• All alkaline earth metals form hydrated salts.
• Barium and Strontium are not found free in nature (always found in compounds).

Table Salt (Iodisation)

• Iodised salt contains Potassium Iodide (KI) to provide iodine.
• Common forms of salt include sea salt, rock salt, and processed common salt. (The data implies one should not be used as table salt, but the specific answer is not provided in the excerpts).
• The predominant salt in the ocean is Sodium Chloride.

Know More About Acid, Base and Salt:

• Acid, Base and Salt – Old Year Questions
• Acid, Base and Salt One Liner Questions & Answers