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Matter and Chemical Change - Separating Mixtures (Filtration, Distillation, Chromatography)

Grade 7IB

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

🔑Concepts

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A mixture consists of two or more substances that are physically combined but not chemically bonded, meaning they can be separated using physical methods.

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Filtration is used to separate an insoluble solid (the residueresidue) from a liquid (the filtratefiltrate) by passing the mixture through a porous material like filter paper.

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Distillation is a technique used to separate a solvent from a solution or to separate two liquids with different boiling points. It involves the processes of evaporation followed by condensation.

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In distillation, the liquid with the lower boiling point evaporates first, travels through a condenser (cooled by H2OH_2O), and is collected as the distillatedistillate.

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Paper Chromatography separates substances (like pigments in ink) based on their solubility in a mobile phase (the solvent) and their affinity for the stationary phase (the paper).

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The Retention Factor (RfR_f) is a ratio used in chromatography to identify substances. It is always a value between 00 and 11.

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Fractional distillation is a more precise version of distillation used when the boiling points of the liquids in the mixture are close together, such as separating ethanol (C2H5OHC_2H_5OH, boiling point ≈78∘C\approx 78^\circ C) from water (H2OH_2O, boiling point 100∘C100^\circ C).

📐Formulae

Rf=distance traveled by the substancedistance traveled by the solvent frontR_f = \frac{\text{distance traveled by the substance}}{\text{distance traveled by the solvent front}}

Concentration=mass of solute (g)volume of solvent (L)\text{Concentration} = \frac{\text{mass of solute (g)}}{\text{volume of solvent (L)}}

💡Examples

Problem 1:

A student performs paper chromatography on a sample of green food coloring. The yellow component travels 5.6 cm5.6\text{ cm} from the baseline, while the solvent front travels 8.0 cm8.0\text{ cm}. Calculate the RfR_f value for the yellow dye.

Solution:

Rf=5.68.0=0.7R_f = \frac{5.6}{8.0} = 0.7

Explanation:

The RfR_f value is calculated by dividing the distance moved by the solute by the distance moved by the solvent. Note that RfR_f values have no units because they are a ratio.

Problem 2:

Explain how to separate a mixture containing sand, salt, and water to obtain pure sand and pure salt.

Solution:

  1. Use Filtration to remove the sand (insoluble solid). The sand remains as the residueresidue. 2. Use Evaporation or Distillation on the resulting salt-water filtratefiltrate to remove the H2OH_2O and leave behind the NaClNaCl (salt) crystals.

Explanation:

Filtration separates substances based on particle size (sand is too large for the filter paper pores), while distillation/evaporation separates substances based on boiling points (water evaporates at 100∘C100^\circ C while salt has a much higher melting/boiling point).

Problem 3:

In a distillation setup, why is it important that cold water (H2OH_2O) enters the bottom of the Liebig condenser and leaves from the top?

Solution:

This ensures the condenser is completely filled with cold water, providing maximum cooling efficiency for the vapor.

Explanation:

Gravity would pull water out too quickly if it entered from the top; filling from the bottom ensures the entire jacket surrounding the inner tube is chilled, facilitating the phase change from gas back to liquid.