# Water of crystallization

> Water locked inside crystals


## Objectives

- Explain that some crystals hold a fixed number of water molecules.
- Read hydrate formulas written with a dot, including a number in front.
- State that heating drives off the water of crystallization.

## 1. Iron ores and extraction of iron




## 2. Dry crystals, wet tube

Predict: where did the water come from?

- The crystals were wet on the outside
- The heat made water from copper and sulphur
- Water was locked inside the crystals ✓

The water was locked inside the crystals as water of crystallization. Heating drove it out, so the blue crystals turned white; it cannot come from copper and sulphur, because $\ce{CuSO4}$ contains no hydrogen.

[ar-EG] الميّة كانت محبوسة جوه الـ crystals على هيئة water of crystallization. التسخين طلّعها، عشان كده الـ crystals الزرقا بقت بيضا؛ ومينفعش تكون جاية من الـ copper والـ sulphur، لأن الـ $\ce{CuSO4}$ مفيهوش hydrogen.
## 3. The water in the formula

Water of crystallization A fixed number of water molecules held inside the crystal of a solid. Blue copper(II) sulphate crystals The dot joins the salt and its water. $\ce{5H2O}$ means five water molecules for every one $\ce{CuSO4}$. The number is fixed for each substance.


## 4. Hydrated, then heated

Hydrated Describes a substance that contains water of crystallization. Heating drives the water off as vapour The blue hydrated crystals become a white powder. The solid left has lost all its water of crystallization.


## 5. A number in front

Sometimes a number stands in front of the first part too. Plaster of Paris Read it as: one $\ce{H2O}$ for every two $\ce{CaSO4}$. Each number counts the part right after it. Your turn: match each hydrate to the water it holds.


## 6. Read these hydrates

Which of these iron compounds is hydrated?

- $\ce{FeCO3}$
- $\ce{FeSO4.7H2O}$ ✓
- $\ce{Fe(OH)3}$
- $\ce{Fe2O3}$

$\ce{FeSO4.7H2O}$ is the hydrated compound: after the dot it holds seven water molecules for every $\ce{FeSO4}$. The others have no water part after a dot; $\ce{Fe(OH)3}$ has H and O as OH groups, not water molecules.

[ar-EG] الـ $\ce{FeSO4.7H2O}$ هو الـ hydrated compound: بعد النقطة فيه سبعة water molecules لكل $\ce{FeSO4}$. الباقيين مفيهمش جزء ميّة بعد نقطة؛ والـ $\ce{Fe(OH)3}$ فيه H و O على هيئة OH groups، مش water molecules.
## 7. Summary

**Water of crystallization**: a fixed number of water molecules inside a crystal. It is written after a dot: $\ce{CuSO4.5H2O}$. A number in front counts the part right after it. **Hydrated** = contains water of crystallization. Heating drives this water off. Can you explain why heated blue crystals turn white and leave droplets? Heating drives the water of crystallization off as vapour: $\ce{CuSO4.5H2O(s) ->[\Delta] CuSO4(s) + 5H2O(v)}$. The white powder left has lost all its water, and the water collects as droplets near the cool top of the tube. Next: one iron ore, limonite, is a hydrated oxide.



Chapter: Transition Elements (https://yaani.ai/en/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/en/lesson/chem-s3-en.lesson-chem.water-of-crystallization