Acids, bases and salts are fundamental to O Level Chemistry. This guide covers everything you need to know. Our chemistry tuition Singapore makes complex topics simple with structured lessons and hands-on practice questions.
Properties of Acids
Acids have a pH less than 7 and:
- Taste sour
- – Turn blue litmus red
- – React with metals to produce hydrogen
- – React with carbonates to produce CO2
- – React with bases to form salts and water
Common acids:
- Hydrochloric acid (HCl)
- – Sulfuric acid (H2SO4)
- – Nitric acid (HNO3)
- – Ethanoic acid (CH3COOH)
Properties of Bases
Bases have a pH greater than 7 and:
- Taste bitter
- – Feel soapy
- – Turn red litmus blue
- – React with acids to form salts and water
Alkalis are bases that dissolve in water.
Common alkalis:
- Sodium hydroxide (NaOH)
- – Potassium hydroxide (KOH)
- – Ammonia solution (NH3)
- – Calcium hydroxide (Ca(OH)2)
The pH Scale
pH measures acidity or alkalinity:
- pH 7: neutral
- – pH < 7: acidic (lower = more acidic)
- – pH > 7: alkaline (higher = more alkaline)
Indicators:
- Litmus: red in acid, blue in alkali
- – Universal indicator: range of colours
- – Methyl orange: red in acid, yellow in alkali
- – Phenolphthalein: colourless in acid, pink in alkali
Neutralisation
Acid + Base → Salt + Water
Example:
HCl + NaOH → NaCl + H2O
General pattern:
H+(aq) + OH-(aq) → H2O(l)
Types of Oxides
Acidic oxides:
- Non-metal oxides
- – Dissolve in water to form acids
- – Example: CO2, SO2, SO3
Basic oxides:
- Metal oxides
- – React with acids
- – Example: Na2O, CaO, CuO
Amphoteric oxides:
- React with both acids and bases
- – Example: ZnO, Al2O3
Neutral oxides:
- Neither acidic nor basic
- – Example: CO, NO, H2O
Types of Salts
Normal salts:
- All H+ ions replaced
- – Example: NaCl, CuSO4
Acid salts:
- Some H+ ions replaced
- – Example: NaHSO4, NaHCO3
Solubility Rules
Soluble salts:
- All sodium, potassium, ammonium salts
- – All nitrates
- – All chlorides (except AgCl, PbCl2)
- – All sulfates (except BaSO4, PbSO4, CaSO4)
Insoluble salts:
- Most carbonates
- – Most hydroxides (except Group 1, Ba, NH4+)
- – Most oxides (except Group 1, Ca, Ba)
Preparing Soluble Salts
Method 1: Acid + Metal
- For metals above hydrogen in reactivity series
- – Add excess metal to acid
- – Filter and crystallise
- – Example: Zn + H2SO4 → ZnSO4 + H2
Method 2: Acid + Insoluble Base
- Add excess base to acid
- – Filter and crystallise
- – Example: CuO + H2SO4 → CuSO4 + H2O
Method 3: Acid + Alkali (Titration)
- For soluble salts of Na, K, NH4+
- – Titrate to find exact proportions
- – Example: NaOH + HCl → NaCl + H2O
Method 4: Acid + Carbonate
- Add excess carbonate to acid
- – Filter and crystallise
- – Example: CuCO3 + H2SO4 → CuSO4 + H2O + CO2
Preparing Insoluble Salts
Precipitation method:
- Mix two solutions
- – Insoluble salt precipitates
- – Filter, wash, dry
Example: AgNO3 + NaCl → AgCl↓ + NaNO3
Common Salt Preparations
Copper(II) sulfate:
CuO + H2SO4 → CuSO4 + H2O
Sodium chloride:
NaOH + HCl → NaCl + H2O
Lead(II) iodide:
Pb(NO3)2 + 2KI → PbI2↓ + 2KNO3
Calcium carbonate:
CaCl2 + Na2CO3 → CaCO3↓ + 2NaCl
Crystallisation
To obtain crystals:
- Prepare saturated solution
- 2. Allow to cool slowly
- 3. Crystals form
- 4. Filter to collect
- 5. Dry between filter papers
Testing for Ions
Cations:
- NaOH: observe precipitate colour and solubility
- – NH3: observe precipitate solubility in excess
Anions:
- Carbonates: add acid, test for CO2 (limewater)
- – Chlorides: add AgNO3, white precipitate
- – Sulfates: add BaCl2, white precipitate
- – Nitrates: add NaOH and Al, test for NH3
Common Mistakes
- Confusing acid and base properties
- 2. Wrong salt preparation method
- 3. Not balancing equations
- 4. Forgetting state symbols
- 5. Wrong formulae for compounds
How Ace Scorers Helps
Our Chemistry programme covers:
- Acids, bases, salts theory
- – Practical preparation methods
- – Equation writing
- – Exam techniques
Contact us for Chemistry tuition.
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