Acids are substances that produce H+ (hydrogen ions) when dissolved in water. Bases are substances that produce OH- (hydroxide ions) in water. Salts are ionic compounds formed from the neutralisation of acids and bases.
Key Concepts
- Arrhenius Definition:
- Acid: Produces H+ ions in water (e.g., HCl → H+ + Cl-)
- Base: Produces OH- ions in water (e.g., NaOH → Na+ + OH-)
- Indicators: Substances that change colour in acidic or basic solutions.
- Litmus: Red in acid, blue in base
- Phenolphthalein: Colourless in acid, pink in base
- Methyl orange: Red in acid, yellow in base
- Turmeric: Remains yellow in acid, turns red in base
- pH Scale: A scale from 0 to 14 measuring how acidic or basic a solution is.
- pH < 7: Acidic
- pH = 7: Neutral
- pH > 7: Basic/Alkaline
Neutralisation Reaction: Acid + Base → Salt + Water (exothermic)
Strong vs Weak Acids: Strong acids (HCl, H2SO4, HNO3) fully dissociate in water. Weak acids (CH3COOH, H2CO3) partially dissociate.
Important Salts: NaCl (common salt), NaHCO3 (baking soda), Na2CO3 (washing soda), CaOCl2 (bleaching powder), NaOH (caustic soda), Ca(OH)2 (slaked lime).
---
How does litmus paper help identify acids and bases?
- Dip blue litmus into an unknown solution. If it turns red, the solution is acidic.
- Dip red litmus into the solution. If it turns blue, the solution is basic.
- If neither changes colour, the solution may be neutral.
What happens when an acid reacts with a metal?
- Acid + Metal → Salt + Hydrogen gas
- 2HCl + Zn → ZnCl2 + H2 (upward arrow)
- The H2 produced can be tested by bringing a burning splint near the gas — it burns with a pop sound.
What happens when HCl reacts with NaOH?
- HCl + NaOH → NaCl + H2O (neutralisation)
- The salt formed is sodium chloride (common salt). The reaction is exothermic.
Explain how baking soda (NaHCO3) is used in baking.
- NaHCO3 + heat → Na2CO3 + H2O + CO2
- CO2 makes the dough rise and become soft and spongy.
- Tartaric acid in baking powder neutralises the bitter Na2CO3 taste.
What is the action of water on calcium oxide (quicklime)?
- CaO + H2O → Ca(OH)2 + heat (large amount)
- This is a combination reaction and is exothermic.
- Ca(OH)2 (slaked lime) is used in whitewashing.
Sodium hydroxide solution is electrolysed. What products are obtained?
- 2NaOH(aq) → 2Na+ + 2OH-
- At cathode: H2 gas, At anode: Cl2 gas (if NaCl is used) or O2
- This is the chlor-alkali process producing NaOH, Cl2, and H2.
Why does tooth decay occur when pH of mouth falls below 5.5?
- Bacteria in mouth break down sugars → produce acids → pH drops below 5.5.
- Enamel (mainly calcium phosphate) dissolves in acid when pH < 5.5 → tooth decay.
- Toothpastes (basic) neutralise the acid and protect enamel.
---
Key Formulas
- Acid + Metal → Salt + H2
- Acid + Metal carbonate → Salt + CO2 + H2O
- Acid + Base → Salt + H2O (neutralisation)
- Acid + Metal oxide → Salt + H2O
Common mistakes
- Confusing pH scale: Lower pH = more acidic, higher pH = more basic. pH 3 is more acidic than pH 5.
- Forgetting water of crystallisation: Na2CO3.10H2O has 10 water molecules; these are part of its structure.
- Mixing up indicators: Phenolphthalein is colourless in acid (not red), pink in base.
Summary
Acids produce H+ ions and bases produce OH- ions in water. The pH scale (0-14) measures acidity/basicity. Neutralisation produces salt and water. Important salts like NaHCO3, Na2CO3, and NaOH have wide industrial and household uses. Indicators help us identify acids and bases quickly.