Hello, Future Chemists! Let's Master Salts!

Welcome to one of the most practical and useful chapters in chemistry: the Preparation of Salts. Salts aren't just the white stuff you sprinkle on your chips; they are essential compounds used in medicine, fertilizers, batteries, and industry!
This chapter will teach you the fundamental techniques chemists use to create pure samples of salts, depending on whether the salt is soluble or insoluble in water.

Don't worry if this seems tricky at first! By breaking down the process into easy, step-by-step methods, you'll be preparing perfect crystals in no time.

1. The Golden Rule: Solubility Comes First

Before you even begin mixing chemicals, you must know whether the salt you want to make is soluble (dissolves in water) or insoluble (does not dissolve in water). The method you choose depends entirely on this!

Quick Review: Essential Solubility Rules

Memorizing a few simple rules saves you a lot of trouble. If a salt contains any of these ions, it is usually soluble (it dissolves):

  • Sodium (\({\rm Na}^{+}\))
  • Potassium (\({\rm K}^{+}\))
  • Ammonium (\({\rm NH}_{4}^{+}\))
  • Nitrates (\({\rm NO}_{3}^{-}\))
  • Most Chlorides (except silver and lead)
  • Most Sulfates (except barium, lead, and calcium)

Memory Aid: Think of "SPAN C(l) S(o)": If the salt has SPA or N, it’s always soluble. The others (Chlorides/Sulfates) are usually soluble, with only a few important exceptions.

Quick Check: If you need to make Copper Sulfate, look up Copper Sulfate. Sulfates are usually soluble, and copper is not an exception.
Therefore, Copper Sulfate is a soluble salt.

2. Method A: Making Soluble Salts using an Insoluble Reactant

(Suitable for making salts from metals, insoluble bases (metal oxides/hydroxides), or carbonates.)

This method works because you can easily separate the excess solid reactant from the salt solution. We use an excess of the solid reactant to ensure that all the acid is completely used up (neutralized).

The Reaction Types (Core Equations):
  1. Acid + Insoluble Base (e.g., Metal Oxide or Hydroxide) \(\rightarrow\) Salt + Water
  2. Acid + Metal Carbonate \(\rightarrow\) Salt + Water + Carbon Dioxide
  3. Acid + Reactive Metal \(\rightarrow\) Salt + Hydrogen (Note: Use metals like Zinc, Magnesium, Iron, but never highly reactive ones like Sodium or Potassium for safety!)
Step-by-Step Preparation (e.g., making Copper Sulfate):

Step 1: Reaction (Neutralization)

Heat the dilute acid (e.g., sulfuric acid) gently in a beaker. Add the insoluble reactant (e.g., copper(II) oxide powder) a little at a time, stirring constantly. Continue adding the powder until some remains undissolved at the bottom. This means the acid has been fully neutralized.

Why is excess important? If we didn't use excess, we would be left with some unreacted acid, and the resulting salt solution would be impure and acidic.

Step 2: Filtration

Filter the mixture. The excess, unreacted solid (the copper oxide) is the residue trapped in the filter paper. The resulting solution, known as the filtrate, is a pure solution of the desired salt (copper sulfate).

Step 3: Heating and Evaporation

Heat the salt solution gently to evaporate some of the water. Stop heating when the solution is saturated (a sample drop forms small crystals when cooled on a watch glass). This is known as the crystallization point.

Step 4: Cooling and Crystallization

Transfer the concentrated solution to an evaporating basin and allow it to cool slowly, ideally at room temperature. Slow cooling encourages the formation of large, high-quality crystals.

Step 5: Drying

Once crystals have formed, separate them from the remaining liquid (mother liquor) by filtration. Dry the crystals either by pressing them gently between filter papers or by placing them in a warm oven (not hot).

Key Takeaway for Method A: This method relies on filtration to remove the solid excess reactant, ensuring the salt solution is pure.

3. Method B: Making Soluble Salts using Soluble Reactants (Titration)

(Suitable for making salts from an acid and an alkali (soluble base), such as Sodium Chloride or Potassium Sulfate.)

If you mix two soluble solutions (like Sodium Hydroxide and Hydrochloric Acid), you cannot use filtration to remove any excess reactant because everything is dissolved! Therefore, we must measure the reactants precisely to ensure they neutralize perfectly.

The Technique: Acid-Alkali Titration

Reaction Type: Acid + Alkali \(\rightarrow\) Salt + Water

Step 1: Preliminary Titration (Finding the exact volume)

  • Accurately measure a volume of the alkali (e.g., sodium hydroxide) into a conical flask using a pipette.
  • Add a few drops of a suitable indicator (e.g., phenolphthalein or methyl orange).
  • Slowly add the acid from a burette, drop by drop, swirling the flask constantly, until the indicator just changes colour. This is the end-point.
  • Record the exact volume of acid used. This volume is needed for perfect neutralization.

Common Mistake: Using indicators with the final batch. Indicators are dyes and will contaminate the final salt product!

Step 2: Repetition (Making the Pure Salt Solution)

Repeat the experiment using the exact measured volumes of acid and alkali (the volume found in Step 1), but this time, DO NOT add the indicator.

The resulting pure solution is the neutral salt solution (e.g., Sodium Chloride solution).

Step 3: Evaporation and Crystallization

Follow Steps 3, 4, and 5 from Method A: Gently heat the pure salt solution to the crystallization point, allow it to cool slowly to form crystals, and then filter and dry them.

Key Takeaway for Method B: This method requires highly precise volume measurements (titration) because filtration cannot separate the soluble reactants.

4. Method C: Making Insoluble Salts (Precipitation)

(Suitable for making salts that are insoluble in water, such as Barium Sulfate or Lead Chloride.)

If you want to make an insoluble salt, you can't dissolve the product in water for crystallization. Instead, we use a reaction that forms the salt as a solid immediately—a precipitate.

The Technique: Precipitation (Double Displacement)

This method involves mixing two different soluble salts together. The ions swap partners, and if one of the new combinations is insoluble, it forms a solid.

Example: Making Barium Sulfate (\({\rm BaSO}_{4}\))

You need one solution containing Barium ions (e.g., Barium Chloride, \({{\rm BaCl}_2}\)) and another containing Sulfate ions (e.g., Sodium Sulfate, \({{\rm Na}_{2}{\rm SO}_4}\)). Both starting materials must be soluble!

$$\rm {BaCl}_{2}(aq) + {\rm Na}_{2}{\rm SO}_{4}(aq) \rightarrow {\rm BaSO}_{4}(s) + 2{\rm NaCl}(aq)$$

The Barium Sulfate (\({\rm BaSO}_{4}\)) is the solid precipitate.

Step-by-Step Preparation:

Step 1: Mixing

Mix measured amounts of the two soluble solutions together in a beaker. An immediate cloudiness forms, which is the precipitate (the desired insoluble salt).

Step 2: Filtration

Filter the mixture. The insoluble salt (the precipitate) is left behind as the residue on the filter paper. The soluble salt (the unwanted by-product) remains in the filtrate.

Step 3: Washing

While the solid is still in the filter paper, wash it thoroughly with distilled water. This is crucial! Washing removes any trace of the soluble salt and any unreacted starting chemicals that might still cling to the solid.

Step 4: Drying

Carefully scrape the washed solid onto a piece of filter paper or watch glass and leave it to dry in a warm place or oven.

Key Takeaway for Method C: This method relies on the salt being insoluble so it can be separated instantly by filtration after mixing the two soluble solutions.

5. Synthesis Summary Table (Accessibility Aid)

Use this table to quickly decide which method you need:

Target Salt Solubility Reactant Types Method Name Separation Key
Soluble Acid + Insoluble solid (Metal, Base, or Carbonate) Method A: Using Excess Reactant Filtration (to remove excess solid)
Soluble Acid + Soluble solid (Alkali) Method B: Titration Precise measurement (Titration to ensure purity)
Insoluble Two Soluble Salt Solutions Method C: Precipitation Filtration (to capture the solid product)

You now know the three core methods used in chemistry labs worldwide to prepare pure salts. Great work!

Did You Know? Barium sulfate, which is made using Method C, is opaque to X-rays and is safely consumed by patients as a "barium meal" to help doctors view their digestive tracts!

Keep practicing those solubility rules—they are the key to unlocking this topic!