Understanding Bases (Metal Oxides, Hydroxides, and Carbonates)
● Metal Oxides: These are compounds of metals with oxygen (e.g., MgO, CuO). Most are
insoluble in water except for alkali metal oxides (e.g., Na₂O).
● Metal Hydroxides: These are compounds of metals with hydroxide ions (OH⁻). Alkali
metal hydroxides (e.g., NaOH, KOH) are soluble, but most other metal hydroxides (e.g.,
Fe(OH)₃, Cu(OH)₂) are insoluble.
● Metal Carbonates: These are compounds of metals with carbonate ions (CO₃²⁻). Most
carbonates (except those of Group 1 metals and ammonium) are insoluble.
Methods for Preparing Salts
The method used depends on whether the base/carbonate is soluble or insoluble:
1. Titration Method (For Soluble Bases)
Used for preparing salts from soluble bases (alkalis) like NaOH, KOH.
● Why? Because both the acid and alkali are soluble, so adding excess base won't allow
easy separation.
● Example: Making Sodium Chloride (NaCl)
1. Pipette a fixed volume of acid (HCl) into a conical flask.
2. Add a few drops of an indicator (e.g., phenolphthalein or methyl orange).
3. Slowly add NaOH from a burette while swirling until the indicator shows
neutralization.
4. Repeat the experiment without an indicator using the exact volume.
5. Evaporate the water from the solution to obtain pure NaCl crystals.
2. Reacting an Acid with an Insoluble Base
Used for preparing salts from insoluble metal oxides or hydroxides.
● Why? Since the base is insoluble, you add it in excess to ensure all acid reacts and then
filter out the unreacted solid.
● Example: Making Copper(II) Sulfate (CuSO₄)
1. Add excess CuO (black solid) to hot sulfuric acid (H₂SO₄).
2. Stir until no more CuO dissolves (acid is fully reacted).
3. Filter out the excess CuO.
4. Heat the solution to evaporate some water, then allow slow crystallization.
3. Reacting an Acid with an Insoluble Carbonate
Used for preparing salts from insoluble metal carbonates.
● Why? The carbonate will react with the acid to form a salt, water, and CO₂ gas. You add
excess carbonate to ensure all acid reacts and then filter out the unreacted solid.
● Example: Making Calcium Nitrate (Ca(NO₃)₂)
1. Add CaCO₃ (chalk/marble) to nitric acid (HNO₃).
2. Effervescence (CO₂ gas bubbles) occurs, indicating a reaction.
3. Continue adding CaCO₃ until bubbling stops (acid is neutralized).
4. Filter out excess CaCO₃.
5. Evaporate water to obtain Ca(NO₃)₂ crystals.
