Questions
5-6 questions per board exam paper
Difficulty
Medium
Importance
Core — never skip
Overview
Acids, Bases, and Salts form the foundational chemistry of aqueous solutions and reactivity. Mastery of this topic is essential for board exams and competitive technical assessments, focusing on chemical behavior, ion concentration, and ionic equilibrium.
The pH Scale
The pH scale quantifies the acidity or alkalinity of a solution based on the concentration of hydronium ions. It is a logarithmic scale where each unit change represents a ten-fold change in H+ concentration.
- pH = -log[H+]
- pH < 7 indicates acidic, pH > 7 basic, pH = 7 neutral
- At 298K, pH + pOH = 14
- Universal indicator shows a spectrum of colors based on pH range
- Strong acids/bases dissociate completely in water
Neutralisation Reactions
A neutralisation reaction occurs when an acid reacts with a base to form salt and water, releasing heat in the process. These are essentially exothermic reactions driven by the combination of hydrogen and hydroxide ions to form water.
- Acid + Base -> Salt + Water
- General equation: HCl + NaOH -> NaCl + H2O
- Heat of neutralisation for strong acid-strong base is constant (-57.1 kJ/mol)
- Acts as the fundamental principle for volumetric titrations
- Involves formation of an ionic compound (salt)
Common Salts
Salts are ionic compounds formed by the neutralization of acids and bases, often acting as precursors for industrial chemicals. Understanding the parent acid and base of common salts is critical for predicting their hydrolysis behavior.
- Sodium Chloride (NaCl): From HCl and NaOH
- Sodium Hydroxide (NaOH): Chlor-alkali process
- Bleaching Powder (CaOCl2): Calcium oxychloride
- Baking Soda (NaHCO3): Sodium hydrogen carbonate
- Washing Soda (Na2CO3.10H2O): Sodium carbonate decahydrate
- Plaster of Paris (CaSO4.1/2H2O): Calcium sulphate hemihydrate
Formula Sheet
pH = -log[H+]
Kw = [H+][OH-] = 10^-14
Acid + Metal -> Salt + Hydrogen
Metal Carbonate + Acid -> Salt + Water + CO2
Exam Tip
Always identify the parent acid and base of a given salt to instantly determine whether the resulting solution will be acidic, basic, or neutral.
Common Mistakes
- Confusing the acidic nature of non-metallic oxides with metallic oxides.
- Ignoring the water of crystallization in formulas like Plaster of Paris versus Gypsum.
- Applying the pH formula incorrectly by forgetting that it is a logarithmic scale, not linear.
More Revision Notes
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