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Mixtures and their Separation - Chromatography-advanced

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Chromatography is a separation technique used to separate solutes that dissolve in the same solvent but have different solubilities and rates of movement.

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The name is derived from the Greek word 'chroma', meaning color, as it was initially used for separating plant pigments.

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The process involves two phases: the Stationary Phase (e.g., filter paper) and the Mobile Phase (the solvent, usually water or alcohol, that moves along the stationary phase).

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Principle: Separation occurs because components that are more soluble in the mobile phase rise faster and travel a greater distance than components that are less soluble or have a higher affinity for the stationary phase.

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The RfR_f value (Retention Factor) is a characteristic property of a substance. It is defined as the ratio of the distance traveled by the solute to the distance traveled by the solvent front.

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Advanced Applications: Separating colors in a dye, pigments from natural colors (like chlorophyll from leaves), and detecting/separating drugs from blood samples in forensic science.

📐Formulae

Rf=Distance travelled by the component from the baselineDistance travelled by the solvent front from the baselineR_f = \frac{\text{Distance travelled by the component from the baseline}}{\text{Distance travelled by the solvent front from the baseline}}

0<Rf<10 < R_f < 1

💡Examples

Problem 1:

In an experiment to separate the pigments of black ink using paper chromatography, the solvent (water) moved a distance of 12 cm12 \text{ cm} from the baseline. A blue component of the ink moved a distance of 9 cm9 \text{ cm}. Calculate the RfR_f value for the blue component.

Solution:

Given: Distance travelled by the component (d1d_1) = 9 cm9 \text{ cm} Distance travelled by the solvent (d2d_2) = 12 cm12 \text{ cm}

Using the formula: Rf=d1d2R_f = \frac{d_1}{d_2} Rf=912R_f = \frac{9}{12} Rf=0.75R_f = 0.75

Explanation:

The RfR_f value is a dimensionless ratio. A value of 0.750.75 indicates that the blue pigment is relatively soluble in the solvent and moved 75%75\% of the distance covered by the solvent front.

Problem 2:

A student performs chromatography on a mixture of three dyes: AA, BB, and CC. After 2020 minutes, dye AA has an Rf=0.2R_f = 0.2, dye BB has an Rf=0.5R_f = 0.5, and dye CC has an Rf=0.8R_f = 0.8. Which dye is the most soluble in the solvent?

Solution:

Dye CC is the most soluble.

Explanation:

In chromatography, the component with the highest RfR_f value (0.80.8 in this case) has travelled the furthest distance relative to the solvent. This indicates it has the highest solubility in the mobile phase and the least affinity for the stationary phase.