Define a hydrated substance as a substance that is chemically combined with water and an anhydrous substance as a substance containing no water

Acids, Bases and Salts – Preparation of Salts ⚗️

In this section we’ll learn how to make salts from acids and bases, and we’ll explore the difference between hydrated and anhydrous substances. Think of a salt as a recipe that mixes two ingredients (an acid and a base) to create a new dish. The “water” in the recipe can stay in the dish (hydrated) or be removed (anhydrous).

Hydrated vs. Anhydrous Substances

Hydrated substance – A chemical that is chemically combined with water molecules.


Example: \$CuSO4 \cdot 5H2O\$ (blue copper(II) sulfate that contains five water molecules per formula unit).


Analogy: Imagine a sponge that has absorbed water – the water is part of the sponge’s structure.

Anhydrous substance – A chemical that contains no water.


Example: \$CuSO_4\$ (dry copper(II) sulfate).


Analogy: A dry sponge that has been squeezed out of all its water.

How to Prepare a Salt

  1. Choose an acid (e.g., \$\ce{HCl}\$) and a base (e.g., \$\ce{NaOH}\$).

  2. Mix the solutions together slowly while stirring. The acid donates a proton (\$\ce{H+}\$) and the base donates a hydroxide ion (\$\ce{OH-}\$).

  3. When the \$\ce{H+}\$ and \$\ce{OH-}\$ meet, they form water: \$\ce{H+ + OH- -> H2O}\$.

  4. The remaining ions combine to form a salt: \$\ce{Na+ + Cl- -> NaCl}\$.

  5. Evaporate the water to obtain the solid salt. If you heat the solid, you may remove water of crystallisation and get the anhydrous form.

Example: Mixing \$\ce{H2SO4}\$ with \$\ce{NaOH}\$ gives \$\ce{Na2SO4}\$ and water.


💡 Tip: Always add acid to base, not the other way round, to avoid splashing.

Hydration Table

CompoundHydrated FormAnhydrous Form
Copper(II) Sulphate\$CuSO4 \cdot 5H2O\$ (blue crystals)\$CuSO_4\$ (white powder)
Sodium Chloride\$NaCl \cdot 2H_2O\$ (rare, white crystals)\$NaCl\$ (table salt)

Exam Tip: When a question asks for the “anhydrous” form of a salt, remember to remove all water of crystallisation. If the question mentions “hydrate,” include the water molecules in the formula.

Analogy Reminder: Think of a hydrated salt as a crystal that’s wearing a water “coat.” Removing the coat (heating) gives you the bare crystal (anhydrous salt).