Inaugurated by IN-SPACe
ISRO Registered Space Tutor
S6-SA4-0451
What is Crystallisation (Purification Technique)?
Grade Level:
Class 10
AI/ML, Physics, Biotechnology, Space Technology, Chemistry, Engineering, Medicine
Definition
What is it?
Crystallisation is a purification technique used to separate a pure solid in crystal form from a solution. It works by forming crystals of the desired substance while impurities remain dissolved or are removed.
Simple Example
Quick Example
Imagine you have some dirty salt (like the salt from the ocean mixed with sand). To get pure salt, you can dissolve it in water, filter out the sand, and then let the water evaporate slowly. The pure salt will form beautiful crystals as the water disappears, leaving the impurities behind in the remaining liquid.
Worked Example
Step-by-Step
Let's purify some impure copper sulphate powder.
1. Take 50g of impure copper sulphate powder.
---2. Dissolve the powder in the minimum amount of hot water (e.g., 100ml) to create a saturated solution. This means no more solid can dissolve at that temperature.
---3. Filter the hot solution to remove any insoluble impurities (like tiny stones or dirt).
---4. Heat the filtered solution gently to evaporate some of the water. This makes the solution even more concentrated. Stop heating when a thin film of crystals appears on the surface if you blow on it.
---5. Allow the hot, concentrated solution to cool slowly at room temperature without disturbance. As it cools, pure copper sulphate will start to form crystals.
---6. Once crystals have formed, separate them from the remaining liquid (called mother liquor) by filtration.
---7. Wash the crystals with a small amount of cold water to remove any surface impurities.
---8. Dry the pure crystals between filter papers or in a warm oven.
Answer: You now have pure copper sulphate crystals.
Why It Matters
Crystallisation is vital in industries from medicine to space technology. Pharmaceutical companies use it to make pure medicines, and chemical engineers use it to produce high-purity materials. Scientists in biotechnology and advanced materials research also rely on it to get precise compounds.
Common Mistakes
MISTAKE: Heating the solution too quickly or allowing it to cool too fast, leading to very small, impure crystals. | CORRECTION: Always heat gently and allow the solution to cool slowly and undisturbed. This gives time for larger, purer crystals to form.
MISTAKE: Not filtering the hot solution before crystallisation, leaving insoluble impurities in the final product. | CORRECTION: Always filter the hot saturated solution to remove insoluble impurities before allowing it to cool and crystallise.
MISTAKE: Washing the crystals with hot water, causing them to redissolve. | CORRECTION: Always wash the separated crystals with a small amount of cold water to remove surface impurities without dissolving the crystals.
Practice Questions
Try It Yourself
QUESTION: Why is crystallisation considered a better purification technique than simple evaporation for substances like sugar? | ANSWER: Crystallisation prevents decomposition of heat-sensitive substances (like sugar) and removes soluble impurities, which simple evaporation cannot do.
QUESTION: You have a solution of potassium nitrate with some common salt (sodium chloride) as an impurity. How would you use crystallisation to obtain pure potassium nitrate, knowing that potassium nitrate's solubility increases significantly with temperature, while common salt's solubility changes little? | ANSWER: Dissolve the mixture in hot water to create a saturated solution of potassium nitrate. Filter if needed. Cool the solution slowly. As it cools, pure potassium nitrate will crystallise out because its solubility drops sharply, while most of the common salt will remain dissolved in the solution.
QUESTION: A student dissolved 30g of an impure solid in 100ml of hot water. After filtration and slow cooling, 25g of pure crystals were obtained. If the original impurity was insoluble and weighed 2g, what was the percentage purity of the original impure solid? | ANSWER: Mass of pure solid = 25g. Mass of insoluble impurity = 2g. Mass of soluble impurity = Original impure solid - Pure crystals - Insoluble impurity = 30g - 25g - 2g = 3g. Total original pure substance = 25g (crystals) + 3g (soluble impurity, assuming it was the desired substance but stayed dissolved) = 28g. Percentage purity = (Mass of pure solid / Total mass of impure solid) * 100 = (25g / 30g) * 100 = 83.33%.
MCQ
Quick Quiz
Which of the following is a key advantage of crystallisation over simple evaporation for purifying substances?
It is a faster process.
It can separate heat-sensitive substances without decomposition.
It removes both soluble and insoluble impurities effectively.
It requires less equipment.
The Correct Answer Is:
B
Crystallisation allows for the purification of heat-sensitive substances (like sugar) without decomposing them, unlike simple evaporation which involves strong heating. While it removes insoluble impurities by filtration and soluble ones by leaving them in the mother liquor, option B is a unique and significant advantage.
Real World Connection
In the Real World
In India, crystallisation is crucial in the sugar industry, where sugarcane juice is processed to obtain pure sugar crystals for our chai and mithai. It's also used in making pure common salt from seawater in coastal areas, ensuring the salt we use in our food is clean and safe.
Key Vocabulary
Key Terms
SATURATED SOLUTION: A solution containing the maximum amount of solute that can be dissolved at a given temperature. | SOLUBILITY: The maximum amount of a substance that can dissolve in a given amount of solvent at a specific temperature. | MOTHER LIQUOR: The remaining solution after crystals have formed and been separated. | IMPURITIES: Undesired substances mixed with the main substance.
What's Next
What to Learn Next
Great job understanding crystallisation! Next, you can explore other separation techniques like distillation and chromatography. These build on the ideas of separating mixtures and are used in different situations, like purifying water or separating colours in ink.
