Chemistry (9202) | Purity and chromatography

🧪 International GCSE Chemistry (9202) Study Notes

Chapter: Purity and Chromatography

Welcome to the fascinating world of Chemical Analysis! In this chapter, we learn how chemists check if a substance is "clean" and how they separate mixtures into their individual components. Understanding purity is vital—it affects everything from the medicines we take to the food we eat. Don't worry if this seems tricky at first; we will break down the concepts into easy steps!

Section 1: What is a Pure Substance?

In everyday life, "pure" might mean something natural or clean. In Chemistry, the definition is much stricter.

1.1 Defining Pure Substances and Mixtures

A. Pure Substance

• A pure substance is a single element OR a single compound.
• It contains only one type of chemical particle (atom or molecule) throughout.
• Example: Distilled water (\(H_2O\)) is a pure compound. Oxygen gas (\(O_2\)) is a pure element.

B. Mixture

• A mixture consists of two or more different elements or compounds that are not chemically bonded together.
• The substances in a mixture can be separated using physical methods (like filtering or boiling).
• Example: Salt water (salt + water), air (nitrogen + oxygen + others), or colored ink (many dyes + solvent).

Analogy: Imagine a box of Lego bricks. A pure substance is a box filled with only identical red 2x4 bricks. A mixture is a box filled with red, blue, and yellow bricks all mixed up.

Section 2: Testing Purity using Physical Properties

Since we can't always see the molecules, how do chemists prove that a substance is pure? They measure its physical properties, specifically its melting and boiling points (MP and BP).

2.1 Melting and Boiling Points of Pure Substances

A crucial property of a pure substance is that it melts or boils at a sharp, fixed temperature.

• Pure Water melts exactly at 0°C and boils exactly at 100°C (at standard pressure).
• The temperature remains constant during the phase change (melting or boiling).

2.2 The Effect of Impurities on MP and BP

If a substance contains impurities (is a mixture), its physical properties are dramatically affected. Impurities disrupt the orderly arrangement of the molecules, making the phase changes messy.

When measuring the MP or BP of a mixture:

1. The substance will melt or boil over a range of temperatures, not a fixed point.
2. The Melting Point (MP) is lowered (or depressed).
3. The Boiling Point (BP) is raised (or elevated).

Example: When you add salt (an impurity) to water, it lowers the freezing point (useful for making roads less icy!) and raises the boiling point (useful for cooking pasta a little faster!).

Section 3: Chromatography – Separating Mixtures

If we discover we have a mixture, how do we separate the individual components? We use a technique called Chromatography.

3.1 What is Chromatography?

Chromatography is a powerful analytical technique used to separate components of a mixture (like dyes, pigments, or amino acids). It works by separating substances based on their different degrees of solubility and attraction to surfaces.

Analogy: A Race Track
Imagine a group of runners (the components of your mixture) starting at the same line. The track (the paper) and the air/wind (the solvent) affect how fast each runner moves. Some runners are strongly attracted to the track surface and move slowly; others are easily carried by the wind and move quickly. Chromatography separates them based on these differing speeds.

3.2 The Two Phases

All types of chromatography involve two phases:

1. The Stationary Phase: This part does not move. In paper chromatography, this is the filter paper.
2. The Mobile Phase: This part moves and carries the mixture along with it. This is usually a solvent (like water or ethanol).

Section 4: Paper Chromatography - The Process

Paper chromatography is the simplest type and is commonly used in school laboratories.

4.1 Step-by-Step Procedure

Follow these steps carefully to perform a successful chromatography experiment:

1. Prepare the Paper: Draw a faint pencil line (the baseline or origin) near the bottom of a piece of chromatography paper. (Why pencil? Ink will run and ruin the experiment, but pencil lead is insoluble in the solvent!)
2. Apply the Sample: Place a tiny spot of the mixture (e.g., ink) onto the pencil line. Allow the spot to dry completely.
3. Prepare the Solvent: Place a small volume of solvent (the mobile phase) into a beaker or jar.
4. Set Up the Experiment: Carefully stand the paper upright in the beaker so that the solvent level is BELOW the pencil line. (Common Mistake: If the solvent is above the line, the spot will just wash straight into the solvent at the bottom.)
5. Observe the Run: The solvent moves up the paper (stationary phase) by capillary action, carrying the components of the mixture at different speeds.
6. Stop the Run: Remove the paper when the solvent is near the top edge. Immediately mark the highest point the solvent reached with a pencil. This is the solvent front.
7. Dry and Analyze: Allow the chromatogram (the separated paper) to dry.

Result Interpretation:

• If the substance is pure, it will only show one spot on the final chromatogram.
• If the substance is a mixture, it will separate into multiple spots (one for each component).

Section 5: Calculating the Retention Factor (\(R_f\) Value)

The \(R_f\) value (Retention Factor) is a specific measurement that helps identify an unknown substance by comparing how far it traveled relative to the solvent.

5.1 The \(R_f\) Formula

The \(R_f\) value is the ratio of the distance travelled by the substance (the spot) to the distance travelled by the solvent front, both measured from the original baseline.

The calculation is:

$$R_f = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent front}}$$

We use MathJax for the formula:

\(R_f = \frac{\text{Distance travelled by solute}}{\text{Distance travelled by solvent front}}\)

5.2 Measuring and Using \(R_f\)

• Distances must be measured in the same unit (usually cm or mm).
• The distance travelled by the substance is measured from the baseline to the centre of the spot.
• Since the substance can never travel further than the solvent, the \(R_f\) value will always be between 0 and 1.
• The \(R_f\) value for a specific substance is a fixed value under standard conditions (same temperature, same solvent, same paper).
• Chemists can compare the \(R_f\) value of an unknown spot against known values in databases to identify the substance.

Example: If a dye travels 4 cm, and the solvent front travels 8 cm, the \(R_f\) value is:

\(R_f = \frac{4 \text{ cm}}{8 \text{ cm}} = 0.5\)

Good luck with your revision! Remember, practice interpreting chromatograms and calculating \(R_f\) values.