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Salts and Their Uses

🔄 Quick Recap​

In the previous sections, we learned about:

  • Acids and bases and their chemical properties
  • Neutralization reactions between acids and bases
  • The pH scale and how to measure acidity and alkalinity

Now, let's explore salts - the compounds formed when acids and bases neutralize each other!

📚 What are Salts?​

Salts are ionic compounds formed by the neutralization reaction between an acid and a base. The general formula for a neutralization reaction is:

Acid + Base → Salt + Water

For example:

  • Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
  • HCl + NaOH → NaCl + H₂O

The salt formed (NaCl) is what we commonly know as table salt!

📚 Families of Salts​

Salts can be classified into different families based on the acid or base they're derived from:

Classified by Acid​

  1. Chlorides: Derived from hydrochloric acid (HCl)

    • Sodium chloride (NaCl)
    • Potassium chloride (KCl)
    • Calcium chloride (CaCl₂)

  2. Sulfates: Derived from sulfuric acid (H₂SO₄)

    • Sodium sulfate (Na₂SO₄)
    • Copper sulfate (CuSO₄)
    • Magnesium sulfate (MgSO₄)

  3. Nitrates: Derived from nitric acid (HNO₃)

    • Sodium nitrate (NaNO₃)
    • Potassium nitrate (KNO₃)
    • Ammonium nitrate (NH₄NO₃)

  4. Carbonates: Derived from carbonic acid (H₂CO₃)

    • Sodium carbonate (Na₂CO₃)
    • Calcium carbonate (CaCO₃)
    • Magnesium carbonate (MgCO₃)

Classified by Base​

  1. Sodium salts: Contain sodium (Na⁺) from sodium hydroxide (NaOH)

    • Sodium chloride (NaCl)
    • Sodium sulfate (Na₂SO₄)
    • Sodium carbonate (Na₂CO₃)

  2. Potassium salts: Contain potassium (K⁺) from potassium hydroxide (KOH)

    • Potassium chloride (KCl)
    • Potassium nitrate (KNO₃)
    • Potassium sulfate (K₂SO₄)

  3. Calcium salts: Contain calcium (Ca²⁺) from calcium hydroxide (Ca(OH)₂)

    • Calcium chloride (CaCl₂)
    • Calcium carbonate (CaCO₃)
    • Calcium sulfate (CaSO₄)

📚 pH of Salt Solutions​

Interestingly, not all salt solutions are neutral (pH = 7). The pH of a salt solution depends on the strength of the acid and base that formed it:

  1. Neutral salts: Formed from strong acid + strong base

    • Example: NaCl from HCl (strong acid) + NaOH (strong base)
    • pH = 7

  2. Acidic salts: Formed from strong acid + weak base

    • Example: NH₄Cl from HCl (strong acid) + NH₄OH (weak base)
    • pH < 7

  3. Basic salts: Formed from weak acid + strong base

    • Example: Na₂CO₃ from H₂CO₃ (weak acid) + NaOH (strong base)
    • pH > 7

🖼️ Visual Aid: Salt Formation​

Salt Uses

The image shows common salt (NaCl) and some of its important derivatives.

📚 Water of Crystallization​

Some salts contain water molecules as part of their crystal structure. This water is called "water of crystallization," and salts containing it are called hydrated salts.

For example:

  • Copper sulfate pentahydrate (CuSO₄·5H₂O): Contains 5 water molecules
  • Gypsum (CaSO₄·2H₂O): Contains 2 water molecules
  • Washing soda (Na₂CO₃·10H₂O): Contains 10 water molecules

Anhydrous and Hydrated Salts​

  • Anhydrous salts: Salts without water of crystallization
  • Hydrated salts: Salts with water of crystallization

When hydrated copper sulfate (blue in color) is heated, it loses its water of crystallization and turns white, becoming anhydrous copper sulfate:

CuSO₄·5H₂O → CuSO₄ + 5H₂O
   (Blue)     (White)

If water is added to anhydrous copper sulfate, it turns blue again as it regains its water of crystallization.

🌍 Common Salts and Their Uses​

1. Sodium Chloride (NaCl) - Common Salt​

  • Food preservative and flavoring
  • Production of chlorine and sodium hydroxide
  • De-icing roads in cold countries
  • Water softening

2. Sodium Carbonate (Na₂CO₃·10H₂O) - Washing Soda​

  • Cleaning agent in laundry and dishwashing
  • Water softening (removes hardness)
  • Glass manufacturing
  • Paper manufacturing

3. Sodium Hydrogen Carbonate (NaHCO₃) - Baking Soda​

  • Baking (produces CO₂ that makes cakes rise)
  • Antacid for indigestion
  • Fire extinguishers
  • Tooth cleaning

4. Calcium Sulfate Hemihydrate (CaSO₄·½H₂O) - Plaster of Paris​

  • Making casts for broken bones
  • Making statues and decorative items
  • Making smooth surfaces on walls
  • Dental impressions

5. Calcium Carbonate (CaCO₃) - Limestone/Marble/Chalk​

  • Building material
  • Manufacturing of cement
  • Antacid
  • Source of calcium in animal feed

6. Ammonium Chloride (NH₄Cl) - Sal Ammoniac​

  • Dry cell batteries
  • Soldering flux
  • Cough medicine
  • Fertilizer

🧪 Activity Time! Grow Your Own Salt Crystals​

Materials needed:

  • Table salt (NaCl) or Epsom salt (MgSO₄·7H₂O)
  • Hot water
  • A clean jar
  • A piece of string
  • A pencil or stick

Steps:

  1. Dissolve as much salt as possible in hot water to make a saturated solution
  2. Tie one end of the string to the middle of the pencil/stick
  3. Place the pencil/stick across the mouth of the jar so the string hangs down into the solution
  4. Leave the jar in a place where it won't be disturbed
  5. Observe over several days as water evaporates and crystals form on the string

This works because as water evaporates, the solution becomes supersaturated, causing salt crystals to form on the string.

✅ Solved Example​

Problem: If 5.85 g of sodium chloride is dissolved in water to make 500 mL of solution, what is the molarity of the solution? (Atomic masses: Na = 23, Cl = 35.5)

Solution:

Step 1: Calculate the molar mass of NaCl. Molar mass of NaCl = 23 + 35.5 = 58.5 g/mol

Step 2: Calculate the number of moles of NaCl. Moles of NaCl = Mass / Molar mass = 5.85 g / 58.5 g/mol = 0.1 mol

Step 3: Calculate the molarity. Molarity = Moles / Volume (in liters) = 0.1 mol / 0.5 L = 0.2 M

Therefore, the molarity of the sodium chloride solution is 0.2 M.

🧠 Memory Trick​

To remember the hydrated salts and their colors:

  • Blue vitriol is Blue (Copper sulfate pentahydrate)
  • Green vitriol is Green (Iron(II) sulfate heptahydrate)
  • White vitriol is White (Zinc sulfate heptahydrate)

⚠️ Common Misconceptions​

Misconception 1: All salts are edible​

Not all salts are safe to eat. While sodium chloride (table salt) is used in food, many other salts are toxic or harmful if ingested.

Misconception 2: All salts are neutral​

As we learned, salts can be acidic, basic, or neutral depending on the acid and base they're derived from.

Misconception 3: Salt and sodium are the same thing​

Salt (NaCl) contains sodium (Na), but they are not the same. Sodium is a reactive metal, while salt is a stable compound.

💡 Key Points to Remember​

  • Salts are formed by the neutralization reaction between acids and bases
  • Salts can be classified into families based on the acid or base they're derived from
  • The pH of a salt solution depends on the strengths of the acid and base that formed it
  • Some salts contain water of crystallization as part of their crystal structure
  • Salts have numerous uses in daily life, from cooking to industry to medicine

🤔 Think About It!​

  1. Why does salt melt ice on roads in cold countries?
  2. How would you determine if a salt contains water of crystallization?
  3. Why might doctors recommend reducing sodium chloride (table salt) intake for people with high blood pressure?

🔜 What Next?​

Now that we understand salts and their uses, in the next section we'll explore the important chemicals derived from common salt and their applications in our daily lives.