Chemistry (0620) | Separation and purification

Welcome to the World of Separation and Purification!

Hello future chemist! This chapter is incredibly practical and important. In the real world, substances are rarely found perfectly pure. Everything from the water you drink to the metals used in your phone has gone through a process of separation and purification.

Here, we will learn the essential techniques chemists use to take messy mixtures and separate them into their clean, useful components. Think of it as learning how to 'un-mix' things! These skills are tested heavily in practical examinations (Paper 5 and 6), so pay close attention to the apparatus and procedures.

1. Assessing Purity: How Do We Know It's Clean?

Before we separate anything, we need a reliable way to check if a substance is pure or contaminated (a mixture). We use physical properties like melting point (M.P.) and boiling point (B.P.) for this.

The Key Difference between Pure and Impure Substances

• Pure Substance: A substance that consists of only one element or one compound.
  Example: Perfectly clean distilled water or a crystal of pure salt.
• Impure Substance (Mixture): Contains two or more elements or compounds not chemically bonded together.
  Example: Tap water (which contains dissolved minerals) or muddy water.

Purity Check using Melting and Boiling Points

The presence of impurities changes these fixed points:

1. Melting Point:

• Pure: Melts sharply at a single, fixed temperature (e.g., pure water melts exactly at 0 °C).
• Impure: Melts over a range of temperatures, and the melting point is depressed (lowered).

2. Boiling Point:

• Pure: Boils sharply at a single, fixed temperature (e.g., pure water boils exactly at 100 °C at standard pressure).
• Impure: Boils over a range of temperatures, and the boiling point is elevated (raised).

Quick Analogy: Think of a fixed B.P. like a precise alarm clock. For a pure substance, the alarm (boiling) goes off exactly at the set time (temperature). For an impure substance, the alarm is faulty—it goes off early (depressed melting point) or late (elevated boiling point) and sounds for a long time (a range of temperatures).

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2. Separating Mixtures Involving Solids and Liquids

The choice of separation method depends entirely on the nature of the components (solid or liquid) and whether they are soluble or insoluble.

Filtration (12.4.1b)

Purpose: To separate an insoluble solid from a liquid.
Example: Separating sand (insoluble solid) from water (liquid).

Process Steps:

1. Pour the mixture through a filter funnel lined with filter paper.
2. The solid particles are too large to pass through the tiny holes in the paper and remain behind as the residue.
3. The liquid component passes through and is collected below as the filtrate.

Common Mistake to Avoid: Never fill the filter paper above the top edge; otherwise, the liquid runs down the side of the filter paper and contaminates the filtrate.

Crystallisation (12.4.1c)

Purpose: To obtain a pure, soluble solid from a solution (often an aqueous solution).
Example: Obtaining pure copper(II) sulfate crystals from a copper(II) sulfate solution.

Process Steps:

1. Evaporate the solvent (usually by gentle heating) until the solution is saturated and crystals just start to form (the "point of crystallisation").
2. Remove the heat and allow the hot, saturated solution to cool slowly, typically at room temperature. Slow cooling allows large, pure crystals to form.
3. Collect the crystals by filtration.
4. Wash the crystals with a small amount of cold solvent to remove any remaining impurities, and then dry them (e.g., using a warm oven or blotting paper).

Key Takeaway: Crystallisation is preferred over complete evaporation because it results in a purer solid with a defined crystalline structure.

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3. Separating Liquids (Distillation)

Distillation methods rely on the fact that liquids have different boiling points.

Simple Distillation (12.4.1d)

Purpose: To separate a solvent from a solution (where the solvent is collected, and the solute is often left as the residue).
Example: Obtaining pure water (distilled water) from salty water.

Apparatus Used:

• A distilling flask containing the solution, connected to a condenser.
• A thermometer placed so that its bulb is level with the side arm leading to the condenser (to measure the boiling point of the vapour).
• The condenser cools the hot vapour, turning it back into a liquid (the distillate), which is collected.

Process Summary: The liquid with the lowest boiling point boils, turns into gas (vapour), is cooled in the condenser, and collected as a pure liquid. The higher-boiling solute remains in the flask.

Did You Know? Distilled water is used in laboratories (10.1.3) because it contains fewer chemical impurities than tap water, preventing unwanted reactions or inaccurate measurements.

Fractional Distillation (12.4.1e)

Purpose: To separate two or more miscible liquids (liquids that mix completely) that have boiling points that are relatively close together.
Example: Separating ethanol (B.P. 78 °C) from water (B.P. 100 °C), or separating crude oil into useful fractions (11.3.5).

Key Apparatus: A fractionating column is placed vertically above the distillation flask and below the condenser. This column is usually packed with glass beads or metal rings to increase the surface area.

How the Fractionating Column Works:

The mixture is heated, and vapours rise up the column.

• As the vapours rise, they repeatedly condense and re-evaporate on the cooler surfaces of the beads.
• Each condensation/re-evaporation cycle is like a tiny, continuous simple distillation.
• By the time the vapour reaches the top of the column and enters the condenser, it is almost entirely the liquid component with the lowest boiling point (the most volatile component).

Key Takeaway: Simple distillation works best when B.P. differences are large (>25°C). Fractional distillation is necessary when the B.P. differences are smaller.

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4. Chromatography: Separating Components of a Solution

Chromatography is a powerful technique used to separate and identify substances in a mixture, often used for pigments, dyes, or ink.

Paper Chromatography Basics (12.3.1)

This method works because different substances have different solubilities in the solvent and different attractions to the paper.

The Principle of Separation:

• Stationary Phase: The filter paper (the material that does not move).
• Mobile Phase: The solvent (the liquid that moves up the paper, carrying the sample).

The substances in the mixture separate because they travel at different speeds:
→ Components that are more soluble in the solvent and less attracted to the paper travel faster and further up the paper.
→ Components that are less soluble in the solvent and more attracted to the paper travel slower and stay closer to the baseline.

The Procedure (Step-by-Step):

1. Draw a baseline (using pencil, as pen ink would separate) near the bottom of the filter paper.
2. Place a small spot of the mixture (e.g., ink) on the baseline.
3. Place the paper in a container with the solvent, ensuring the solvent level is below the baseline.
4. Allow the solvent to soak up the paper until it nearly reaches the top (the solvent front).
5. Remove the paper and let it dry. The resulting pattern of separated spots is called a chromatogram.

Interpreting Chromatograms (12.3.2)

Chromatograms are used for two main purposes:

1. Identifying Pure vs. Impure Substances

• A pure substance shows only one spot on the chromatogram (because it contains only one component).
• An impure substance (a mixture) shows two or more spots.

2. Identifying Unknown Substances

You can identify an unknown spot by running known standard substances next to it.

• If an unknown spot travels the exact same distance as a known substance, they are likely the same compound.
• If the known substance has been added to the unknown and only one spot appears, it confirms that the unknown is pure and matches the known substance.

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*Supplement Content for Extended Students*

Identifying Colourless Substances (12.3.3)

Not all substances are naturally coloured (like food dyes). If you are separating a mixture of colourless compounds (like amino acids), the chromatogram will look blank once dried.

To make these colourless spots visible, a locating agent is used.
A locating agent is a chemical that is sprayed onto the dried chromatogram and then often gently heated. This agent reacts chemically with the separated components to produce coloured spots, allowing them to be measured and identified.

Calculating the R_f Value (12.3.4)

The Retention Factor (\( R_f \)) is a ratio used to identify compounds, as it is a constant value for a specific substance under specific conditions (same temperature, same solvent, same paper).

The formula for the \( R_f \) value is:

\[ R_f = \frac{distance\ travelled\ by\ substance}{distance\ travelled\ by\ solvent} \]

Important Notes about \( R_f \):

• The \( R_f \) value must always be less than 1, because the substance cannot travel further than the solvent front.
• The distance travelled by the substance is measured from the baseline to the centre of the spot.
• The distance travelled by the solvent is measured from the baseline to the solvent front.

Memory Aid: \( R_f \) value = *Runner's* distance / *Finish Line* distance.

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5. Choosing the Right Separation Technique (12.4.2)

In exams, you must be able to suggest the best method based on the properties of the mixture. Here is a simple guide:

• If you have an insoluble solid and a liquid: Use Filtration. (To get the pure solid, dry the residue).
• If you have a soluble solid and a liquid, and you want the solid: Use Crystallisation (if purity is important and you want crystals) or simple Evaporation (if the solid decomposes on heating, or crystals aren't needed).
• If you have a soluble solid and a liquid, and you want the liquid (solvent): Use Simple Distillation.
• If you have two or more miscible liquids: Use Fractional Distillation.
• If you have a mixture of dyes or pigments (coloured solutes in a solvent): Use Chromatography.
• If you have two immiscible liquids (liquids that don't mix, like oil and water): Use a separating funnel (not explicitly detailed in separation methods but good context knowledge).

Final Key Takeaway: Separation techniques are physical methods that do not create new chemical substances. They simply sort components based on differences in physical properties like particle size (filtration), solubility (crystallisation/chromatography), or boiling point (distillation).