Introduction to Salts
(a) Classification of Salts Based on Their Properties
A salt is an ionic compound formed when the hydrogen ions (H⁺) of an acid are fully or partially replaced by a metal or ammonium ion.
Classification:
Normal (Neutral) Salts
Formed by the complete replacement of all replaceable hydrogen ions of an acid by a metal or ammonium ion.
Result in a neutral solution when dissolved in water (pH ≈ 7).
Example: Sodium chloride (NaCl), potassium sulfate (K₂SO₄).
Acidic Salts (Hydrogen Salts):
- Formed by the partial replacement of the replaceable hydrogen ions of a polyprotic acid (an acid with more than one replaceable H⁺) by a metal or ammonium ion.
- Contain remaining replaceable hydrogen ions, making them capable of reacting with bases.
- Result in an acidic solution when dissolved in water (pH < 7).
- Example: Sodium hydrogen sulfate (NaHSO₄), sodium hydrogen carbonate (NaHCO₃).
Basic Salts (Hydroxide Salts):
- Contain hydroxide ions (OH⁻) along with the metal cation and the anion of the acid.
- Formed by the incomplete neutralization of a polyacidic base (a base with more than one OH⁻) by an acid.
- Examples of polyacidic bases are aluminum hydroxide (Al(OH)₃) and iron(III) hydroxide (Fe(OH)₃).
- Result in a basic (alkaline) solution when dissolved in water (pH > 7).
- Example: Magnesium hydroxide chloride (Mg(OH)Cl), lead(II) hydroxide nitrate (Pb(OH)NO₃).
*Double Salts:
- Formed by the combination of two simple salts with different cations but the same anion, or vice versa, which crystallize together in a fixed ratio.
- Dissociate into their constituent ions when dissolved in water.
- Example: Potassium aluminum sulfate (KAl(SO₄)₂
12H₂O) – Alum, Mohr’s salt ((NH₄)₂Fe(SO₄)₂
6H₂O).
- Preparation of Salts Using Appropriate Methods in the Laboratory** General Principle:
The method of preparation depends on the solubility of the desired salt.
Preparation of Soluble Salts:
Reaction between an Acid and a Reactive Metal:
- Applicable for metals above hydrogen in the reactivity series.
General equation:
Acid + Metal → Salt + Hydrogen gas
Example: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)↑
Experiment Suggestion:
React magnesium ribbon with dilute hydrochloric acid in a beaker.
Evaporate the solution to obtain crystals of magnesium chloride.
Safety Note:
Hydrogen gas is flammable, so keep flames away.
- Reaction between an Acid and a Base (Neutralization):
General equation:
Acid + Base → Salt + Water
- Example: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Experiment Suggestion:
Titrate a known volume of sodium hydroxide solution with hydrochloric acid using phenolphthalein as an indicator. Evaporate the resulting solution to obtain crystals of sodium chloride.
Filtrate: The liquid which has been filtered.
Reaction between an Acid and a Metal Carbonate or Hydrogen Carbonate:
General equation:
Acid + Metal Carbonate → Salt + Water + Carbon Dioxide gas Example: H₂SO₄(aq) + CuCO₃(s) → CuSO₄(aq) + H₂O(l) + CO₂(g)↑
Experiment Suggestion:
- Add sulfuric(VI) acid to copper(II) carbonate in a beaker until no more reaction occurs.
- Filter the mixture to remove excess copper(II) carbonate, then evaporate the filtrate to obtain crystals of copper(II) sulfate.
Direct Synthesis (Combination)
Applicable for some salts, especially halides and sulfides of less reactive metals.
General equation: Metal + Nonmetal → Salt Example: Fe(s) + S(s) → FeS(s) (upon heating)
Experiment Suggestion:
Heat iron filings and sulfur powder in a crucible until they react to form iron(II) sulfide.
Safety Note: Perform this in a fume hood, as sulfur fumes can be harmful.
Preparation of Insoluble Salts:
Precipitation Reaction:
By mixing aqueous solutions of two soluble salts containing the desired cation and anion.
General equation:
Soluble Salt 1 + Soluble Salt 2 → Insoluble Salt (Precipitate) + Soluble Salt 3
Example:
AgNO₃(aq) + NaCl(aq) → AgCl(s)↓ + NaNO₃(aq)
Ionic equation:
Ag⁺ (aq) + Cl⁻ (aq) → AgCl(s)
Experiment Suggestion:
- Mix aqueous solutions of silver nitrate and sodium chloride.
- Observe the formation of a white precipitate of silver chloride.
- Filter the precipitate, wash it with distilled water, and dry it to obtain pure silver chloride.
(c) Behaviour of Salts When Exposed to Air
Some salts interact with atmospheric moisture in different ways:
- Hygroscopic Salts: These salts absorb moisture from the air but do not dissolve in it to form a solution. They become damp or sticky.
o Example: Anhydrous copper(II) sulfate (CuSO₄), Calcium chloride (CaCl₂), Sodium hydroxide (NaOH).
- Deliquescent Salts: These salts absorb a large amount of moisture from the air and dissolve in it to form a saturated solution. They eventually turn into a liquid.
Example: Sodium hydroxide (NaOH), Potassium hydroxide (KOH), Magnesium chloride (MgCl₂).
- Efflorescent Salts: These are hydrated salts that lose their water of crystallization when exposed to dry air, forming a powdery layer on their surface.
Example: Washing soda (Na₂CO₃·10H₂O), Hydrated copper(II) sulfate (CuSO₄·5H₂O).
Experiment Suggestion:
Place samples of different salts (e.g., anhydrous copper(II) sulfate, sodium hydroxide pellets, hydrated sodium carbonate crystals) on separate watch glasses and leave them exposed to the air for a day or two. Observe and record any changes in their appearance. (d) Applications of Salts in Day-to-Day Life
Salts have numerous applications in various fields:
- Agriculture: Many salts are used as inorganic fertilizers to provide essential nutrients for plant growth. Examples include ammonium sulfate ((NH₄)₂SO₄), potassium nitrate (KNO₃), and sodium nitrate (NaNO₃).
- Food Industry: Sodium chloride (NaCl) is common table salt used for seasoning and as a food preservative. Other salts are used as food additives and preservatives.
- Medicine: Various salts are used in medicines. For example, magnesium sulfate (MgSO₄) (Epsom salt) is used for muscle relaxation, and sodium chloride (NaCl) is used in saline solutions.
- Paper Industry: Salts like sodium sulfate (Na₂SO₄) are used in the pulping process.
- Paints Industry: Some salts are used as pigments in paints.
- Glass Industry: Salts like sodium carbonate (Na₂CO₃) are used in the manufacture of glass.
- Laundry: Sodium carbonate (Na₂CO₃) (washing soda) is used in laundry detergents to soften water and help remove stains.
(e) Environmental Effects of Inorganic Fertilizers and Mitigation Measures
While inorganic fertilizers increase crop yields, their overuse can have negative environmental impacts:
- Water Pollution (Eutrophication): Excess nitrates and phosphates from fertilizers can leach into rivers and lakes, causing excessive growth of algae (algal blooms). When these algae die and decompose, they deplete the oxygen in the water, harming or killing aquatic organisms.
- Soil Pollution: Overuse of some fertilizers can lead to the build-up of harmful substances in the soil and can affect soil pH and microbial activity.
- Air Pollution: The production and use of some nitrogen-based fertilizers can release greenhouse gases like nitrous oxide (N₂O), contributing to air pollution and climate change.
Mitigation Measures for Sustainable Use:
- Use fertilizers judiciously: Apply fertilizers in the right amounts and at the right time based on soil testing and crop requirements.
- Adopt sustainable farming practices: Crop rotation, cover cropping, and no-till farming can improve soil health and reduce the need for excessive fertilizers.
- Use organic fertilizers: Compost, manure, and other organic materials can provide nutrients to the soil while improving its structure and water retention capacity.
- Improve drainage: Proper drainage can reduce the leaching of fertilizers into water bodies.
- Develop slow-release fertilizers: These fertilizers release nutrients gradually, reducing the risk of nutrient runoff.
- Precision agriculture: Use technology like GPS and sensors to apply fertilizers only where and when needed.
