(a) Copper sulphate crystals from a mixture of charcoal and black copper oxide
Method of preparation: Neutralization of insoluble base
Reaction:
CuO + H2SO4 ⟶ CuSO4 + H2O
CuSO4 + 5H2O ⟶ CuSO4.5H2O (blue vitriol)
Procedure:
- Take dilute sulphuric acid in a beaker and heat it on wire gauze.
- Add black cupric oxide in small quantities at a time, with stirring till no more of it dissolves and the excess compound settles to the bottom.
- Filter it hot and collect the filtrate in a china dish. Evaporate the filtrate by heating to the point of crystallization and then allow it to cool and
- Collect the bright blue crystals of copper (II) sulphate pentahydrate CuSO4.5H2O (blue vitriol) and dry the crystals.
(b) Zinc sulphate crystals from Zinc dust
Method of preparation: Displacement — By the action of dilute acid (dil. H2SO4) on an active metal (Zinc, Iron).
Reaction:
Zn(s) + H2SO4(aq) ⟶ ZnSO4 + H2↑
ZnSO4 + 7H2O ⟶ ZnSO4.7H2O (white vitriol)
Procedure:
- Take dilute sulphuric acid in a beaker and heat it on a wire gauze. Add some granulated zinc pieces with constant stirring.
- Effervescence take place because of the liberation of hydrogen gas.
Add zinc till the Zinc settles at the base of the beaker.
When effervescence stops, it indicates that all the acid has been used up. - The excess of zinc is filtered off.
- Collect the solution in a china dish and evaporate the solution to get crystals. Filter, wash them with water and dry them between the folds of a filter paper. The white, needle-shaped crystals are of hydrated Zinc sulphate.
(c) Sodium hydrogen carbonate crystals:
Method of preparation: By passing carbon dioxide gas into a cold solution of sodium carbonate.
Reaction:
Na2CO3 + CO2 + H2O ⟶ 2NaHCO3
Procedure:
- Dissolve 5 grams of anhydrous sodium carbonate in about 25 cm3 of distilled water in a flask.
- Cool the solution by keeping the flask in a freezing mixture.
- Pass carbon dioxide gas in the solution.
- Crystals of sodium bicarbonate will precipitate out after some time.
- Filter the crystals and dry it in the folds of filter paper.
(d) calcium sulphate from calcium carbonate.
Method of preparation: By Double Decomposition (precipitation)
Reaction:
CaCO3 + 2HNO3 ⟶ Ca(NO3)2 + H2O + CO2↑
Ca(NO3)2 + Na2SO4 ⟶ CaSO4↓ + 2NaNO3
Procedure:
- First insoluble Calcium Carbonate [CaCO3] is converted to soluble Calcium Nitrate [Ca(NO3)2] with the help of dilute nitric acid.
- The resulting solution is then treated with Sodium Sulphate [Na2SO4] and a precipitate of Calcium Sulphate is obtained.
Chapter Overview: Acids, Bases and Salts
This chapter covers the properties, definitions, and reactions of acids, bases, and salts. According to the Arrhenius theory, acids produce H+ ions in aqueous solution and bases produce OH− ions. The Bronsted-Lowry theory defines acids as proton donors and bases as proton acceptors. The pH scale (0-14) measures the concentration of H+ ions: pH < 7 is acidic, pH = 7 is neutral, pH > 7 is basic. Students must understand the properties of acids (sour taste, turn blue litmus red, react with metals to produce H2) and bases (bitter taste, soapy feel, turn red litmus blue). Neutralisation is the reaction between an acid and a base to form salt and water. Salts are classified as normal, acidic, or basic salts. Understanding the preparation of salts by neutralisation, direct combination, and double decomposition is essential. The chapter also covers indicators (litmus, phenolphthalein, methyl orange) and their colour changes. Strong acids (HCl, H2SO4, HNO3) are fully ionised; weak acids (CH3COOH, H2CO3) are partially ionised.
Key Definitions & Reactions
| Concept | Details |
|---|---|
| Arrhenius Acid | Substance that produces H+ (H3O+) ions in aqueous solution |
| Arrhenius Base | Substance that produces OH− ions in aqueous solution |
| pH Scale | Measures H+ ion concentration; pH = −log[H+]; range 0 to 14 |
| Neutralisation | Acid + Base → Salt + Water (e.g., HCl + NaOH → NaCl + H2O) |
| Basicity of an Acid | Number of replaceable H+ ions (HCl = monobasic, H2SO4 = dibasic, H3PO4 = tribasic) |
| Acidity of a Base | Number of replaceable OH− ions (NaOH = monoacidic, Ca(OH)2 = diacidic) |
| Normal Salt | All H+ ions replaced by metal ions (e.g., Na2SO4) |
| Acid Salt | Partial replacement of H+ (e.g., NaHSO4) |
Must-Know Concepts
- Acid + Metal → Salt + H2↑ (e.g., 2HCl + Zn → ZnCl2 + H2↑)
- Acid + Metal Oxide → Salt + Water (e.g., 2HCl + CuO → CuCl2 + H2O)
- Acid + Carbonate → Salt + Water + CO2↑ (e.g., 2HCl + CaCO3 → CaCl2 + H2O + CO2↑)
- Strong acids are fully dissociated; weak acids are partially dissociated
- Dilute acids are those with more water; concentrated acids have less water
- Indicators: Litmus (red/blue), Phenolphthalein (colourless/pink), Methyl orange (red/yellow)
- Water of crystallisation: CuSO4·5H2O (blue vitriol), Na2CO3·10H2O (washing soda)
Strong vs Weak Acids
| Property | Strong Acid | Weak Acid |
|---|---|---|
| Ionisation | Complete | Partial |
| pH | Very low (0-2) | Moderately low (3-5) |
| Conductivity | Good conductor | Poor conductor |
| Examples | HCl, H2SO4, HNO3 | CH3COOH, H2CO3 |
Important Diagrams to Practice
- pH scale with common substances marked (lemon juice, vinegar, blood, soap solution)
- Laboratory preparation of HCl gas from NaCl and conc. H2SO4
- Action of dilute acid on metal, carbonate, and base (with gas collection)
Common Mistakes
- Confusing "dilute" with "weak" and "concentrated" with "strong" - these are different concepts
- Writing unbalanced equations for neutralisation reactions
- Forgetting that dry HCl gas does not turn blue litmus red (water is needed for ionisation)
- Mixing up indicator colours: phenolphthalein is colourless in acid (not white)
- Not writing water of crystallisation in salt formulae when asked
Scoring Tips
- Always balance chemical equations - marks are deducted for unbalanced equations
- When asked about "role of water", explain that water helps in ionisation of acids/bases
- Memorise indicator colour changes in tabular form for quick recall
- For salt preparation questions, state the method AND write the balanced equation
Frequently Asked Questions
Why does dry HCl gas not show acidic properties?
Dry HCl does not contain water, so it cannot ionise to produce H+ ions. Acidic properties are due to H+ ions, which form only in the presence of water. Hence, dry HCl does not turn dry blue litmus red.
What is the difference between strong/weak and concentrated/dilute?
Strong/weak refers to the degree of ionisation. A strong acid is fully ionised (HCl), a weak acid is partially ionised (CH3COOH). Concentrated/dilute refers to the amount of acid dissolved in water. A dilute strong acid is still fully ionised.
Why is neutralisation an exothermic reaction?
During neutralisation, H+ ions from the acid combine with OH− ions from the base to form water (H+ + OH− → H2O). The formation of the covalent bond in water releases energy, making the reaction exothermic.