Preparing Soluble Salts
Section: 7. Acids, Bases & Salts | Syllabus: Cambridge AS Level Physics 9702
What are Salts? Salts are ionic compounds formed when the hydrogen ions in an acid are replaced by metal ions or ammonium ions. They are produced by neutralization reactions between acids and bases. General equation: Acid + Base → Salt + Water Salts can be classified as: Soluble salts - dissolve in water to form solutions Insoluble salts - do not dissolve in water (precipitates) Naming Salts Salt names come from two parts: First part: from the metal or ammonium ion Second part: from the acid used Example: Copper chloride (copper from metal, chloride from hydrochloric acid) Which Acid Produces Which Salt?
Acid Formula Salt Name Ending Example Salt Hydrochloric acid HCl Chloride Sodium chloride (NaCl) Sulfuric acid H₂SO₄ Sulfate Copper sulfate (CuSO₄) Nitric acid HNO₃ Nitrate Potassium nitrate (KNO₃) Ethanoic acid CH₃COOH Ethanoate Sodium ethanoate (CH₃COONa) Carbonic acid H₂CO₃ Carbonate Calcium carbonate (CaCO₃) Methods for Preparing Soluble Salts There are three main methods for preparing soluble salts, depending on the reactants used: Method 1: Acid + Metal (for reactive metals) Method 2: Acid + Insoluble Base (metal oxide or hydroxide) Method 3: Acid + Alkali (titration method) Choosing the Method The method you choose depends on: The reactivity of the metal The solubility of the base Whether you can see when the reaction is complete Method 1: Acid + Metal This method is used when you have a reactive metal (but not too reactive).
Suitable metals: Magnesium, Zinc, Iron NOT suitable: Very reactive metals (Na, K, Ca) or unreactive metals (Cu, Ag, Au) General Equation Acid + Metal → Salt + Hydrogen Example: Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g) Step-by-Step Procedure Add excess metal to a fixed volume of dilute acid in a beaker Stir the mixture - the metal will react and dissolve Wait until all the acid has reacted (no more bubbles, excess metal remains) Filter to remove excess unreacted metal Heat the filtrate gently to evaporate some water (crystallization) Cool and crystallize to obtain salt crystals Dry the crystals between filter paper or in a warm oven Why Excess Metal?
We add excess metal to ensure all the acid is used up. This means we can easily remove unreacted metal by filtration, leaving a pure salt solution. Example: Preparing Zinc Chloride Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g) Observations: Bubbles of hydrogen gas (effervescence) Zinc metal dissolves Solution may become warm (exothermic) Excess zinc remains at the end Safety Considerations Wear safety goggles (acids are corrosive) Use dilute acids to control the reaction rate Work in a well-ventilated area Test for hydrogen gas carefully (pop test) Method 2: Acid + Insoluble Base This method uses an insoluble base (metal oxide or metal hydroxide) reacted with an acid.
The insoluble base must be added in excess . General Equations With metal oxide: Acid + Metal Oxide → Salt + Water CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) With metal hydroxide: Acid + Metal Hydroxide → Salt + Water Cu(OH)₂(s) + H₂SO₄(aq) → CuSO₄(aq) + 2H₂O(l) Step-by-Step Procedure Add excess insoluble base to dilute acid in a beaker Warm gently and stir to speed up the reaction Continue adding the base until no more dissolves (excess remains) Filter to remove excess unreacted base Evaporate the filtrate to reduce the volume Crystallize by cooling the saturated solution Dry the crystals Example: Preparing Copper(II) Sulfate CuO(s) + H₂SO₄(aq) → CuSO₄(aq) + H₂O(l) black copper oxide + sulfuric acid → blue copper sulfate solution + water Observations: Black copper oxide dissolves Blue solution forms (copper sulfate) No gas produced Excess black oxide remains Why This Method?
Copper is too unreactive to react directly with dilute acids. However, copper oxide (a base) will react with acids in a neutralization reaction. Common Insoluble Bases Used Base Formula Appearance Copper(II) oxide CuO Black powder Zinc oxide ZnO White/yellow powder Iron(II) oxide FeO Black powder Copper(II) hydroxide Cu(OH)₂ Blue precipitate Method 3: Acid + Alkali (Titration) This method is used when preparing a salt from a soluble base (alkali) .
Because both reactants are soluble, we cannot use excess - we must use exact amounts . We use titration to find the exact volumes needed. General Equation Acid + Alkali → Salt + Water Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l) Step-by-Step Procedure Part 1: Finding the Volumes (Titration) Measure a fixed volume of alkali using a pipette into a conical flask Add a few drops of indicator (e.g., methyl orange or phenolphthalein) Fill a burette with the acid Add acid slowly to the alkali, swirling constantly Stop when the indicator just changes color (neutralization point) Record the volume of acid used Part 2: Preparing the Pure Salt Repeat the experiment using the exact volumes found Do NOT add indicator this time (keeps salt pure) Evaporate the solution to reduce volume Crystallize by cooling Dry the crystals Why No Indicator in Part 2?
Indicators are weak acids…
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