What is Water of Crystallization? | Simple Explanation for Class 7

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What is Water of Crystallization (chemistry)?

Grade Level:

Class 7

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Definition

What is it?

Water of crystallization is the fixed number of water molecules chemically bonded within the crystal structure of certain salts. These water molecules are part of the crystal, giving it a specific shape and properties, but they are not 'wet' water.

Simple Example

Quick Example

Imagine you buy a packet of 'Poha' (flattened rice) from the market. It looks dry, but it actually contains a tiny bit of moisture that keeps it from becoming too brittle. Similarly, some salts, even though they look dry and solid, have water molecules 'locked' inside their structure. This 'locked' water is the water of crystallization.

Worked Example

Step-by-Step

Let's say you have 100 grams of copper sulfate pentahydrate (CuSO4.5H2O), which is a blue crystal. You want to find out how much of that 100 grams is just the water of crystallization.
1. First, find the molar mass of water (H2O): H = 1, O = 16. So, H2O = (2 x 1) + 16 = 18 g/mol.
2. Since it's 'pentahydrate', there are 5 water molecules. So, the mass of 5 water molecules = 5 x 18 = 90 g/mol.
3. Next, find the molar mass of anhydrous copper sulfate (CuSO4): Cu = 63.5, S = 32, O = 16. So, CuSO4 = 63.5 + 32 + (4 x 16) = 63.5 + 32 + 64 = 159.5 g/mol.
4. The molar mass of the whole hydrated salt (CuSO4.5H2O) = 159.5 (CuSO4) + 90 (5H2O) = 249.5 g/mol.
5. Now, to find the percentage of water in the 100 grams: (Mass of 5H2O / Molar mass of CuSO4.5H2O) x 100 = (90 / 249.5) x 100 = 36.07%.
6. So, in 100 grams of CuSO4.5H2O, the mass of water of crystallization is 36.07% of 100 grams = 36.07 grams.
ANSWER: 36.07 grams of water of crystallization.

Why It Matters

Understanding water of crystallization is crucial in fields like Material Science and Pharmacy, where chemists design new materials or medicines. It impacts how crystals form, their stability, and how they react. Scientists and engineers use this knowledge to create better products, from electronic components to medicines.

Common Mistakes

MISTAKE: Thinking water of crystallization makes the salt wet or watery. | CORRECTION: Water of crystallization is chemically bonded within the crystal structure, making the salt appear dry and solid, not wet.

MISTAKE: Confusing water of crystallization with absorbed atmospheric moisture. | CORRECTION: Water of crystallization is part of the salt's chemical formula, while absorbed moisture is just surface water that can be easily wiped or dried off.

MISTAKE: Forgetting to include the water molecules when calculating the total molecular weight of a hydrated salt. | CORRECTION: Always add the total mass of the water molecules (e.g., 5H2O) to the mass of the anhydrous salt when finding the molecular weight of the hydrated compound.

Practice Questions

Try It Yourself

QUESTION: What is the main difference between water of crystallization and a drop of water on a salt crystal? | ANSWER: Water of crystallization is chemically bonded inside the crystal, making it part of the crystal's structure, while a drop of water on the surface is just external moisture.

QUESTION: If you heat a blue crystal of copper sulfate pentahydrate (CuSO4.5H2O) strongly, what happens to its color and why? | ANSWER: It turns white because the water of crystallization is driven off as steam, leaving behind anhydrous (water-free) copper sulfate.

QUESTION: A sample of hydrated sodium carbonate (Na2CO3.10H2O) weighs 28.6 grams. If all the water of crystallization is removed, what would be the mass of the anhydrous sodium carbonate left? (Atomic masses: Na=23, C=12, O=16, H=1) | ANSWER: Molar mass of Na2CO3 = (2x23) + 12 + (3x16) = 46 + 12 + 48 = 106 g/mol. Molar mass of 10H2O = 10 x 18 = 180 g/mol. Molar mass of Na2CO3.10H2O = 106 + 180 = 286 g/mol. Ratio of anhydrous Na2CO3 to hydrated Na2CO3.10H2O is 106/286. So, mass of anhydrous Na2CO3 = (106/286) x 28.6 grams = 10.6 grams.

MCQ

Quick Quiz

Which of the following is an example of a salt containing water of crystallization?

Common table salt (NaCl)

Sugar (C12H22O11)

Washing soda (Na2CO3.10H2O)

Pure gold (Au)

The Correct Answer Is:

Washing soda (Na2CO3.10H2O) explicitly shows 10 molecules of water (10H2O) as part of its chemical formula, indicating it contains water of crystallization. The other options are either anhydrous or not salts with water of crystallization.

Real World Connection

In the Real World

You've probably seen those small packets that say 'Silica Gel - Do Not Eat' inside new shoes, electronics, or medicine bottles. Silica gel often contains water of crystallization, which allows it to absorb moisture from the air and keep things dry. This is important in industries like electronics manufacturing and food packaging to prevent damage from humidity, especially in India's varied climate.

Key Vocabulary

Key Terms

Hydrated salt: A salt that contains water of crystallization. | Anhydrous salt: A salt that does not contain water of crystallization. | Efflorescence: The process where a hydrated salt loses its water of crystallization to the atmosphere. | Deliquescence: The process where a substance absorbs so much moisture from the air that it dissolves in it.

What's Next

What to Learn Next

Now that you understand water of crystallization, you can explore concepts like 'Hydrated and Anhydrous Salts' and 'Efflorescence and Deliquescence'. These topics will help you understand how these water molecules affect the physical and chemical properties of compounds, and their uses in everyday life.